is the name given in our language to the Definition, visible moist vapour which arises from all bodies which contain juices easily expelled from them by heats not sufficient for their combustion. Thus we say, the steam of boiling water, of malt, of a tan-bed, &c. It is distinguished from smoke by its not having been produced by combustion, by not containing any foot, and by its being condensible by cold into water, oil, inflammable spirits, or liquids composed of these.

We see it rise in great abundance from bodies when they are heated, forming a white cloud, which diffuses like itself and disappears at no very great distance from the body from which it was produced. In this case the surrounding air is found loaded with the water or other juices which seem to have produced it, and the steam seems to be completely soluble in air, as salt is in water, composing while thus united a transparent elastic fluid.

But in order to its appearance in the form of an when opaque white cloud, the mixture with or dissemination eliminated in air seems absolutely necessary. If a tea-kettle boils violently, so that the steam is formed at the spout in great abundance, it may be observed, that the visible cloud is not formed at the very mouth of the spout, but at a small distance before it, and that the vapour is perfectly transparent at its first emission. This is rendered still more evident by fitting to the spout of the tea-kettle tea-kettle a glass pipe of any length, and of as large a diameter as we please. The steam is produced as copiously as without this pipe, but the vapour is transparent through the whole length of the pipe. Nay, if this pipe communicate with a glass vessel terminating in another pipe, and if the vessel be kept sufficiently hot, the steam will be as abundantly produced at the mouth of this second pipe as before, and the vessel will be quite transparent. The visibility therefore of the matter which constitutes the steam is an accidental or extraneous circumstance, and requires the admixture with air; yet this quality again leaves it when united with air by solution. It appears therefore to require a diffusion in the air. The appearances are quite agreeable to this notion: for we know that one perfectly transparent body, when minutely divided and diffused among the parts of another transparent body, but not dissolved in it, makes a mass which is visible. Thus oil beaten up with water makes a white opaque mass.

In the mean time, as steam is produced, the water gradually wastes in the tea kettle, and will soon be totally expended, if we continue it on the fire. It is reasonable therefore to suppose, that this steam is nothing but water changed by heat into an elastic form. If so, we should expect that the privation of this heat would leave it in the form of water again. Accordingly this is fully verified by experiment; for if the pipe fitted to the spout of the tea kettle be surrounded with cold water, no steam will issue, but water will continually trickle from it in drops: and if the process be conducted with the proper precautions, the water which we thus obtain from the pipe will be found equal in quantity to that which disappears from the tea-kettle.

This is evidently the common process for distilling; and the whole appearances may be explained by saying, that the water is converted by heat into an elastic vapour, and that this, meeting with colder air, imparts to it the heat which it carried off as it arose from the heated water, and being deprived of its heat it is again water. The particles of this water being vastly more remote from each other than when they were in the tea-kettle, and thus being disseminated in the air, become visible, by reflecting light from their anterior and posterior surfaces, in the same manner as a transparent salt becomes visible when reduced to a fine powder. This disseminated water being presented to the air in a very extended surface, is quickly diffused by it, as pounded salt is in water, and again becomes a transparent fluid, but of a different nature from what it was before, being no longer convertible into water by depriving it of its heat.

Accordingly this opinion, or something very like it, has been long entertained. Muthenbroeck expressly says, that the water in the form of vapour carries off with it all the heat which is continually thrown in by the fuel. But Dr Black was the first who attended minutely to the whole phenomena, and enabled us to form distinct notions of the subject. He had discovered that it was not sufficient for converting ice into water that it be raised to that temperature in which it can no longer remain in the form of ice. A piece of ice of the temperature 32° Fahrenheit's thermometer will remain a very long while in air of the temperature 50° before it be all melted, remaining all the while of the temperature 32°, and therefore continually absorbing heat from the surrounding air. By comparing the time in which the ice had its temperature changed from 28° to 32° with the subsequent time of its complete liquefaction, he found that it absorbed about 130 or 140 times as much heat as would raise its temperature one degree; and he found that one pound of ice, when mixed with one pound of water 140 degrees warmer, was just melted, but without rising in its temperature above 32°. Hence he justly concluded, that water differed from ice of the same temperature by containing, as a constituent ingredient, a great quantity of fire, or of the cause of heat, united with it in such a way as not to quit it for another colder body, and therefore so as not to go into the liquor of the thermometer and expand it. Considered therefore as the possible cause of heat, it was latent, which Dr Black expressed by the abbreviated term LATENT HEAT. If any more heat was added to the water it was not latent, but would readily quit it for the thermometer, and, by expanding the thermometer, would show what is the degree of this redundant heat, while fluidity alone is the indication of the combined and latent heat.

Dr Black, in like manner, concluded, that in order to convert water into an elastic vapour, it was necessary, not only to increase its uncombined heat till its temperature is 212°, in which state it is just ready to become elastic; but also to pour into it a great quantity of fire, or the cause of heat, which combines with every particle of it, so as to make it repel, or to recede from, its adjoining particles, and thus to make it a particle of an elastic fluid. He supposed that this additional heat might be combined with it so as not to quit it for the thermometer; and therefore so as to be in a latent state, having elastic fluidity for its sole indication.

This opinion was very consistent with the phenomenon of boiling off a quantity of water. The application of heat to it causes it gradually to rise in its temperature till it reaches the temperature 212°. It then begins to send off elastic vapour, and is slowly expended in this way, continuing all the while of the same temperature. The steam also is of no higher temperature, as appears by holding a thermometer in it. We must conclude that this steam contains all the heat which is expended in its formation. Accordingly the foiling power of steam is well known; but it is extremely difficult to obtain precise measures of the quantity of heat absorbed by water during its conversion into steam. Dr Black endeavoured to ascertain this point, by comparing the time of raising its temperature a certain number of degrees with the time of boiling it off by the same external heat; and he found that the heat latent in steam, which balanced the pressure of the atmosphere, was not less than 800 degrees. He also directed Dr Irvine of Glasgow to the form of an experiment for measuring the heat actually extracted from such steam during its condensation in the refrigeratory of a still, which was found to be not less than 774 degrees. Dr Black was afterwards informed by Mr Watt, that a course of experiments, which he had made in each of these ways with great precision, determined the latent heat of steam under the ordinary pressure of the atmosphere to be about 948 or 950 degrees. Mr Watt also found that water would distil with great ease. Steam in vacuo when of the temperature 70°; and that in this case the latent heat of the steam is not less than 1200 or 1300 degrees: and a train of experiments, which he had made by distilling in different temperatures, made him conclude that the sum of the sensible and latent heats is a constant quantity. This is a curious and not an improbable circumstance; but we have no information of the particulars of these experiments. The conclusion evidently presupposes a knowledge of that particular temperature in which the water has no heat; but this is a point which is still subject to debate.

This conversion of liquids (for it is not confined to water, but obtains also in ardent spirits, oils, mercury, &c.) is the cause of their boiling. The heat is applied to the bottom and sides of the vessel, and gradually accumulates in the fluid, in a sensible state, uncombined, and ready to quit it and enter into any body that is colder, and to diffuse itself between them. Thus it enters into the fluid of a thermometer, expands it, and thus gives us the indication of the degree in which it has been accumulated in the water; for the thermometer swells as long as it continues to absorb sensible heat from the water: and when the sensible heat in both is in equilibrium, in a proportion depending on the nature of the two fluids, the thermometer rises no more, because it absorbs no more heat or fire from the water; for the particles of water which are in immediate contact with the bottom, are now (by this gradual expansion of liquidity) at such distance from each other, that their laws of attraction for each other and for heat are totally changed. Each particle either no longer attracts, or perhaps it repels its adjoining particle, and now accumulates round itself a great number of the particles of heat, and forms a particle of elastic fluid, so related to the adjoining new formed particles, as to repel them to a distance at least a hundred times greater than their distances in the state of water. Thus a mass of elastic vapour of sensible magnitude is formed. Being at least ten thousand times lighter than an equal bulk of water, it must rise up through it, as a cork would do, in form of a transparent ball or bubble, and getting to the top, it dissipates, filling the upper part of the vessel with vapour or steam. Thus, by boiling the liquid into bubbles, which are produced all over the bottom and sides of the vessel, it produces the phenomenon of ebullition or boiling. Observe, that during its passage up through the water, it is not changed or condensed; for the surrounding water is already so hot that the sensible or uncombined heat in it, is in equilibrium with that in the vapour, and therefore it is not disposed to absorb any of that heat which is combined as an ingredient of this vapour, and gives it its elasticity. For this reason, it happens that water will not boil till its whole mass be heated up to 212°; for if the upper part be colder, it robs the rising bubble of that heat which is necessary for its elasticity, so that it immediately collapses again, and the surface of the water remains still. This may be perceived by holding water in a Florence flask over a lamp or chuffer. It will be observed, some time before the real ebullition, that some bubbles are formed at the bottom, and get up a very little way, and then disappear. The distances which they reach before collapsing increase as the water continues to warm farther up the mass, till at last it breaks out into boiling. If the handle of a tea-kettle be grasped with the hand, a tremor will be felt for some little time before boiling, arising from the little succussions which are produced by the collapsing of the bubbles of vapour. This is much more violent, and is really a remarkable phenomenon, if we suddenly plunge a lump of red hot iron into a vessel of cold water, taking care that no red part be near the surface. If the hand be now applied to the side of the vessel, a most violent tremor is felt, and sometimes strong thumps: these arise from the collapsing of very large bubbles. If the upper part of the iron be too hot, it warms the surrounding water so much, that the bubbles from below come up through it uncondensed, and produce ebullition without this succussion. The great resemblance of this tremor to the feeling which we have during the shock of an earthquake has led many to suppose that these last are produced in the same way, and their hypothesis, notwithstanding the objections which we have elsewhere stated to it, is by no means unfounded.

It is owing to a similar cause that violent thumps are sometimes felt on the bottom of a tea-kettle, especially observed in one which has been long in use. Such are frequently the boiling crusts on the bottom with a stony concretion. This crust is sometimes detached in little scales. When one of them adheres by one end to the bottom, the water gets between them in a thin film. Hence it may be heated considerably above the boiling temperature, and it suddenly rises up in a large bubble, which collapses immediately. A smooth shell lying on the bottom will produce this appearance very violently, or a thimble with the mouth down.

In order to make water boil, the fire must be applied to the bottom or sides of the vessel. If the heat be applied at the top of the water, it will waste away without boiling; for the very superficial particles to the bottom are first supplied with the heat necessary for rendering them elastic, and they fly off without agitating the rest of the vessel.

Since this disengagement of vapour is the effect of its

(A) We explained the opaque and cloudy appearance of steam, by saying that the vapour is condensed by coming into contact with the cooler air. There is something in the form of this cloud which is very inexplicable. The particles of it are sometimes very distinguishable by the eye; but they have not the smart star-like brilliancy of very small drops of water, but give the fainter reflection of a very thin film or vehicle like a soap bubble. If we attend also to their motion, we see them descending very slowly in comparison with the descent of a solid drop; and this vesicular constitution is established beyond a doubt by looking at a candle through a cloud of steam. It is seen surrounded by a faint halo with prismatic colours, precisely such as we can demonstrate by optical laws to belong to a collection of vehicles, but totally different from the halo which would be produced by a collection of solid drops. It is very difficult to conceive how these vehicles can be formed of watery particles, each of which was furthered... Steam. Its elasticity, and since this elasticity is a determined force when the temperature is given, it follows, that fluids cannot boil till the elasticity of the vapour overcomes the pressure of the incumbent fluid and of the atmosphere. Therefore, when this pressure is removed or diminished, the fluids must sooner overcome what remains, and boil at a lower temperature. Accordingly it is observed that water will boil in an exhausted receiver when of the heat of the human body. If two glass balls A and B (fig. 1.) be connected by a flender tube, and one of them A be filled with water (a small opening or pipe b being left at top of the other), and this be made to boil, the vapour produced from it will drive all the air out of the other, and will at last come out itself, producing steam at the mouth of the pipe. When the ball B is observed to be occupied by transparent vapour, we may conclude that the air is completely expelled. Now that the pipe by sticking it into a piece of tallow or bees-wax; the vapour in B will soon condense, and there will be a vacuum. The flame of a lamp and blow-pipe being directed to the little pipe, will cause it immediately to close and seal hermetically. We now have a pretty instrument or toy called a Pulse Glass. Grasp the ball A in the hollow of the hand; the heat of the hand will immediately expand the bubble of vapour which may be in it, and this vapour will drive the water into B, and then will blow up through it for a long while, keeping it in a state of violent ebullition, as long as there remains a drop or film of water in A. But care must be taken that B is all the while kept cold, that it may condense the vapour as fast as it rises through the water. Touching B with the hand, or breathing warm on it, will immediately stop the ebullition in it. When the water in A has thus been distipated, grasp B in the hand; the water will be driven into A, and the ebullition will take place there as it did in B. Putting one of the balls into the mouth will make the ebullition more violent in the other, and the one in the mouth will feel very cold. This is a pretty illustration of the rapid absorption of the heat by the particles of water which are thus converted into elastic vapour. We have seen this little toy suspended by the middle of the tube like a balance, and thus placed in the inside of a window, having two holes a and b cut in the pane, in such a situation that when A is full of water and preponderates, B is opposite to the hole b. Whenever the room became sufficiently warm, the vapour was formed in A, and immediately drove the water into B, which was kept cool by the air coming into the room through the hole b. By this means B was made to preponderate in its turn, and A was then opposite to the hole a, and the process was now repeated in the opposite direction; and this amusement continued as long as the room was warm enough.

We know that liquors differ exceedingly in the temperatures necessary for their ebullition. This forms the great chemical distinction between volatile and fixed bodies. But the difference of temperature in which they boil, or are converted into permanently elastic vapour, under the pressure of the atmosphere, is not a certain measure of their differences of volatility. The natural boiling point of a body is that in which it will be converted into elastic vapour under no pressure, or in vacuo. The boiling point in the open air depends on the law of the elasticity of the vapour in relation to its heat. A fluid A may be less volatile, that is, may require more heat to make it boil in vacuo, than a fluid B: But if the elasticity of the vapour of A be more increased by an increase of temperature than that of the vapour of B, A may boil at as low, or even at a lower temperature, in the open air, than B does; for the increased elasticity of the vapour of A may sooner overcome the pressure of the atmosphere. Few experiments have been made on the relation between the temperature and the elasticity of different vapours. So long ago as the year 1765, we had occasion to examine the boiling points of all such liquors as we could manage in an air-pump; that is, such as did not produce vapours which destroyed the valves and the leathers of the pistons: and we thought that the experiments gave us reason to conclude, that the elasticity of all the vapours was affected by heat nearly in the same degree. For we found that the difference between their boiling points in the air and in vacuo was nearly the same in all, namely, about 120 degrees Fahrenheit's thermometer. It is exceedingly difficult to make experiments of this kind: The vapours are so condensible, and change their elasticity so prodigiously by a trifling change of temperature, that it is almost impossible to examine this point with precision. It is, however, as we shall see by and bye, a subject of considerable practical importance in the mechanic arts; and an accurate knowledge of the relation would be of great use also to the distiller: and it would be no less important to discover the relation of their elasticity and density, by examining their compressibility, in the same manner as we have ascertained the relation in the case of what we call aerial fluids, that is, such as we have never observed in the form of liquids or solids, except in consequence of their union with each other or with other bodies. In the article Pneumatics we took notice of it as something like a natural law, that all these airs, or gases as they are now called, had their elasticity very nearly, if not exactly proportional to their density. This appears from the experiments of Achard, of Fontana, and others, on vital air, inflammable air, fixed air, and some others. It gives us some presumption to suppose that it holds in all elastic vapours whatever, and that it is connected with their elasticity; and it renders it somewhat probable that they are all elastic, only because the cause of heat (the matter of fire if you will) is elastic, and that their law of elasticity, in respect of density, is the same with that of fire. But it must be

rounded with many particles of fire, now communicated to the air, and how each of these vesicles shall include within it a ball of air; but we cannot refuse the fact. We know, that if, while linseed oil is boiling or nearly boiling, the surface be obliquely struck with the ladle, it will be dashed into a prodigious number of exceedingly small vesicles, which will float about in the air for a long while. Mr Sauflure was (we think) the first who distinctly observed this vesicular form of mists and clouds; and he makes considerable use of it in explaining several phenomena of the atmosphere. be observed, that although we thus assign the elasticity of fire as the immediate cause of the elasticity of vapour, in the same way, and on the same grounds, that we ascribe the fluidity of brine to the fluidity of the water which holds the solid salt in solution, it does not follow that this is owing, as is commonly supposed, to a repulsion or tendency to recede from each other exerted by the particles of fire. We are as much entitled to infer a repulsion of unlimited extent between the particles of water; for we see that by its means a single particle of sea-salt becomes disseminated through the whole of a very large vessel. If water had not been a visible and palpable substance, and the salt only had been visible and palpable, we might have formed a similar notion of chemical solution. But we, on the contrary, have considered the quaquaversum motion or expansion of the salt as a dissemination among the particles of water; and we have ascribed it to the strong attraction of the atoms of salt for the atoms of water, and the attraction of these salt for each other, thinking that each atom of salt accumulates round itself a multitude of watery atoms, and by so doing must recede from the other saline atoms. Nay, we farther see, that by forces which we naturally consider as attractions, an expansion may be produced of the whole mass, which will act against external mechanical forces. It is thus that wood swells with almost insuperable force by imbibing moisture; it is thus that a sponge immersed in water becomes really an elastic compressible body; resembling a blown bladder; and there are appearances which warrant us to apply this mode of conception to elastic fluids.—When air is suddenly compressed, a thermometer included in it shows a rise of temperature; that is, an appearance of heat now redundant which was formerly combined. The heat seems to be squeezed out as the water from the sponge.

Accordingly this opinion, that the elasticity of steam and other vapours is owing merely to the attraction for fire, and the consequent dissemination of their particles through the whole mass of fire, has been entertained by many naturalists, and it has been ascribed entirely to attraction. We by no means pretend to decide; but we think the analogy by far too slight to found any confident opinion on it. The aim is to solve phenomena by attraction only, as if it were of more easy conception than repulsion. Considered merely as facts, they are quite on a par. The appearances of nature in which we observe actual recesses of the parts of body from each other, are as distinct, and as frequent and familiar, as the appearances of actual reproach. And if we attempt to go farther in our contemplation, and to conceive the way and the forces by which either the approximation or recesses of the atoms are produced, we must acknowledge that we have no conception of the matter; and we can only say, that there is a cause of these motions, and we call it a force, as in every case of the production of motion. We call it attraction or repulsion just as we happen to contemplate an access or a recess. But the analogy here is not only slight, but imperfect, and fails most in those cases which are most simple, and where we should expect it to be most complete. We can squeeze water out of a sponge, it is true, or out of a piece of green wood; but when the white of an egg, the tremella, or some gums, swell to a hundred times their dry dimensions by imbibing water, we cannot squeeze out a particle. If fluidity (for the reasoning must equally apply to this as to vapourousness) be owing to an accumulation of the extended matter of fire, which gradually expanded the solid by its very minute additions; and if the accumulation round a particle of ice, which is necessary for making it a particle of water, be so great in comparison of what gives it the expansion of one degree, as experiment obliges us to conclude—it seems an inevitable consequence that all fluids should be many times rarer than the solids from which they are produced. But we know that the difference is trifling in all cases, and in some (water, for instance, and iron) the solid is rarer than the fluid. Many other arguments, (each of them perhaps of little weight when taken alone, but which are all systematically connected) concur in rendering it much more probable that the matter of fire, in causing elasticity, mutual reacts immediately by its own elasticity, which we cannot but conceive in any other way than as a mutual tendency between its particles to receive from each other; and we doubt not but that, if it could be obtained alone, we should find it an elastic fluid like air. We even think that there are cases in which it is observed in this state. The elastic force of gunpowder is very much beyond the elasticity of all the vapours which are produced in its deflagration, each of them being expanded as much as we can reasonably suppose by the great heat to which they are exposed. The writer of this article exploded some gunpowder mixed with a considerable portion of finely powdered quartz, and another parcel mixed with fine filings of copper. The elasticity was measured by the penetration of the ball which was discharged, and was great in the degree now mentioned. The experiment was so conducted, that much of the quartz and copper was collected; none of the quartz had been melted, and some of the copper was not melted. The heat, therefore, could not be such as to explain the elasticity by expansion of the vapours; and it became not improbable that fire was acting here as a detached chemical fluid by its own elasticity. But to return to our subject.

There is one circumstance in which we think our own experiments show a remarkable difference (at least probably in degree) between the condensible and incondensible vapours. It is well known, that when air is very suddenly expanded, cold is produced, and heat when it is suddenly condensed. When making experiments with and in the hopes of discovering the connection between the condensible elasticity and density of the vapours of boiling water, pours; and also of boiling spirits of turpentine, we found the change of density accompanied by a change of temperature vastly greater than in the case of incondensible gases. When the vapour of boiling water was suddenly allowed to expand into five times its bulk, we observed the depression of a large and sensitive air thermometer to be at least four or five times greater than in a similar expansion of common air of the same temperature. The chemical reader will readily see reasons for expecting, on the contrary, a smaller alteration of temperature, both on account of the much greater rarity of the fluid, and on account of a partial condensation of its water and the consequent disengagement of combined heat.

This difference in the quantity of fire which is combined in vapours and gases is so considerable, as to authorize us to suppose that there is some difference in the constitution of vapours and gases, and that the tution of connection vapour. connection between the specific bases of the vapour and the fire which it contains, is not the same in air, for instance, as in the vapour of boiling water; and this difference may be the reason why the one is easily condensable by cold, while the other has never been exhibited in a liquid or solid form, except by means of its chemical union with other substances. In this particular instance we know that there is an essential difference—that in vital or atmospheric air there is not only a prodigious quantity of fire which is not in the vapour of water, but that it also contains light, or the cause of light, in a combined state. This is fully evinced by the great discovery of Mr Cavendish of the composition of water. Here we are taught that water (and consequently its vapour) consists of air from which the light and greatest part of the fire have been separated. And the subsequent discoveries of the celebrated Lavoisier show, that almost all the condensible gases with which we are acquainted consist either of airs which have already lost much of their fire (and perhaps light too), or of matters in which we have no evidence of fire or light being combined in this manner.

This consideration may go far in explaining this difference in the condensibility of these different species of aerial fluids, the gases and the vapours; and it is with this qualification only that we are disposed to allow that all bodies are condensible into liquids or solids by abstracting the heat. In order that vital air may become liquid or solid, we hold that it is not sufficient that a body be presented to it which shall simply abstract its heat. This would only abstract its uncombined fire. But another and much larger portion remains chemically combined by means of light. A chemical affinity must be brought into action which may abstract, not the fire from the oxygen (to speak the language of Mr Lavoisier), but the oxygen from the fire and light. And our production is not the detached basis of air, but detached heat and light, and the formation of an oxide of some kind.

To prosecute the chemical consideration of STEAMS farther than these general observations, which are applicable to all, would be almost to write a treatise of chemistry, and would be a repetition of many things which have been treated of in sufficient detail in other articles of this work. We shall therefore conclude this article with some other observations, which are also general, with respect to the different kinds of coercible vapours, but which have a particular relation to the following article.

Steam or vapour is an elastic fluid, whose elasticity balances the pressure of the atmosphere; and it has been produced from a solid or liquid body raised to a sufficient temperature for giving it this elasticity; that is, for causing the fluid to boil. This temperature must vary with the pressure of the air. Accordingly it is found, that when the air is light (indicated by the barometer being low), the fluid will boil sooner. When the barometer stands at 30 inches, water boils at the temperature 212°. If it stands so low as 28 inches, water will boil at 208°. In the plains of Quito, or at Guadalajara in Mexico, where the barometer stands at about 21 inches, water will boil at 195°. Highly rectified alcohol will boil at 160°, and vitriolic ether will boil at 88° or 89°. This is a temperature by no means uncommon in these places; nay, the air is frequently warmer. Vitriolic ether, therefore, is a liquor which can hardly be known in those countries. It is hardly possible to preserve it in that form. If a phial have not its stopper firmly tied down, it will be blown out, and the liquor will boil and be dissipated in steam. On the top of Chimborazo, the human blood must be disposed to give out air-bubbles.

We said some time ago, that we had concluded, from some experiments made in the receiver of an air-pump, that fluids boil in vacuo at a temperature nearly 120° lower than that necessary for their boiling in the open air. But we now see that this must have been a gross approximation; for in these experiments from them, the fluids were boiling under the pressure of the vapour which they produced, and which could not be abstracted from them by working the pump. It appears from the experiments of Lord Charles Cavendish, mentioned in the article Pneumatics, that water of the temperature 75° only was converted into elastic vapour, which balanced a pressure of 4/5ths of an inch of mercury, and in this state it occupied the receiver, and did not allow the mercury in the gauge to sink to the level. As fast as this was abstracted by working the air-pump, more of it was produced from the surface of the water, so that the pressure continued the same, and the water did not boil. Had it been possible to produce a vacuum above this water, it would have boiled for a moment, and would even have continued to boil, if the receiver could have been kept very cold.

Upon reading these experiments, and some very curious ones of Mr Nairne, in the Phil. Trans. vol. lxvii., the writer of this article was induced to examine morements to particularly the relation between the temperature of the determine vapour and its elasticity, in the following manner:

ABCD (fig. 2.) is the section of a small digester made of copper. Its lid, which is fastened to the body of the vessel with screws, is pierced with three holes, each of which vapour and had a small pipe soldered into it. The first hole was furnished with a brass safety-valve V, nicely fitted to it by grinding. The area of this valve was exactly 4/5th of an inch. There reposed on the flasks at top of this valve the arm of a steelyard carrying a sliding weight. This arm had a scale of equal parts, so adjusted to the weight that the number on the scale corresponded to the inches of mercury, whose pressure on the under surface of the valve is equal to that of the steelyard on its top; so that when the weight was at the division 10, the pressure of the steelyard on the valve was just equal to that of a column of mercury 10 inches high, and 4/5th of an inch base. The middle hole contained a thermometer T firmly fixed into it, so that no vapour could escape by its sides. The ball of this thermometer was but a little way below the lid. The third hole received occasionally the end of a glass pipe SGE, whose descending leg was about 36 inches long. When this syphon was not used, the hole was properly shut with a plug.

The vessel was half filled with distilled water which had been purged of air by boiling. The lid was then fixed on, having the third hole S plugged up. A lamp being placed under the vessel, the water boiled, and the steam issued copiously by the safety-valve. The thermometer stood at 213°, and a barometer in the room at 29.9 inches. The weight was then put on the fifth division. The thermometer immediately began to rise; and when it was at 225°, the steam issued by the sides... Steam of the valve. The weight was removed to the 10th division; but before the thermometer could be distinctly observed, the steam was issuing at the valve. The lamp was removed farther from the bottom of the vessel, that the progress of heating might be more moderate; and when the steam ceased to issue from the valve, the thermometer was at 227°. The weight was now shifted to 15°; and by gradually approaching the lamp, the steam again issued; and the thermometer was at 132°. This mode of trial was continued all the way to the 75th division of the scale. The experiments were then repeated in the contrary order; that is, the weight being suspended at the 75th division, and the steam issuing strongly at the valve, the lamp was withdrawn, and the moment the steam ceased to come out, the thermometer was observed. The same was done at the 70th, 65th, division, &c. These experiments were several times repeated both ways; and the means of all the results for each division are expressed in the following table, where column 1st expresses the elasticity of the steam, being the sum of 29.9, and the division of the fleelyard; column 2d expresses the temperature of the steam corresponding to this elasticity.

| I. | II. | |----|-----| | 35 inches. | 219° | | 40 | 226 | | 45 | 232 | | 50 | 237 | | 55 | 242 | | 60 | 247 | | 65 | 251 | | 70 | 255 | | 75 | 259 | | 80 | 263 | | 85 | 267 | | 90 | 270½ | | 95 | 274½ | | 100 | 278 | | 105 | 281 |

A very different process was necessary for ascertaining the elasticity of the steam in lower temperatures, and consequently under smaller pressures than that of the atmosphere. The glass syphon SGF was now fixed into its hole in the lid of the digester. The water was made to boil smartly for some time, and the steam issued copiously both at the valve and at the syphon. The lower end of the syphon was now immersed into a broad faucer of mercury, and the lamp instantly removed, and everything was allowed to grow cold. By this the steam was gradually condensed, and the mercury rose in the syphon, without sensible sinking in the faucer. The valve and all the joints were smeared with a thick clammy cement, composed of oil, tallow, and rosin, which effectually prevented all ingress of air. The weather was clear and frosty, and the barometer standing at 29.84, and the thermometer in the vessel at 42°. The mercury in the syphon stood at 29.7, or somewhat higher, thus showing a very complete condensation. The whole vessel was surrounded with pounded ice, of the temperature 32°. This made no sensible change in the height of the mercury. A mark was now made at the surface of the mercury. One observer was stationed at the thermometer, with instructions to call out as the thermometer reached the divisions 42, 47, 52, 57, and so on by every five degrees till it should attain the boiling heat. Another observer noted the corresponding descents of the mercury by a scale of inches, which had its beginning placed at 29.84 from the surface of the mercury in the faucer.

The pounded ice was now removed, and the lamp placed at a considerable distance below the vessel, so as to warm its contents very slowly. These observations being very easily made, were several times repeated, and their mean results are set down in the following table: Only observe, that it was found difficult to note down the descents for every fifth degree, because they succeeded each other too fast. Every 10th was judged sufficient for establishing the law of variation. The first column of the table contains the temperature, and the second the descent (in inches) of the mercury from the mark 29.84.

| Temperature | Descent | |-------------|---------| | 32° | 8 | | 40 | 0.1 | | 50 | 0.2 | | 60 | 0.35 | | 70 | 0.53 | | 80 | 0.82 | | 90 | 1.18 | | 100 | 1.61 | | 110 | 2.25 | | 120 | 3.00 | | 130 | 3.95 | | 140 | 5.15 | | 150 | 6.72 | | 160 | 8.65 | | 170 | 11.05 | | 180 | 14.05 | | 190 | 17.85 | | 200 | 22.62 | | 210 | 28.65 |

Four or five numbers at the top of the column of elasticities are not so accurate as the others, because the mercury passed pretty quickly through these points. But the progress was extremely regular through the remaining points; so that the elasticities corresponding to temperatures above 70° may be considered as very accurately ascertained.

Not being altogether satisfied with the method employed for measuring the elasticity in temperatures above that of boiling water, a better form of experiment was adopted. (Indeed it was the want of other apparatus which made it necessary to employ the former.) A glass tube was procured of the form represented in fig. 3, having a little cistern L, from the top and bottom of which proceeded the syphons K and MN. The cistern contained mercury, and the tube MN was of a slender bore, and was about six feet two inches long. The end K was firmly fixed in the third hole of the lid, and the long leg of the syphon was furnished with a scale of inches, and firmly fastened to an upright post.

The lamp was now applied at such a distance from the vessel as to warm it slowly, and make the water boil, the steam escaping for some time through the safety-valve. A heavy weight was then suspended on the fleelyard; such as it was known that the vessel would support, and at the same time, such as would not allow the steam to force the mercury out of the long tube. The thermometer began immediately to rise, as also the mercury Steam mercury in the tube MN. Their correspondent stations are marked in the following table:

| Temperature | Elasticity | |-------------|------------| | 212° | 8.0 | | 220 | 5.9 | | 230 | 14.6 | | 240 | 25.0 | | 250 | 30.9 | | 260 | 50.4 | | 270 | 64.2 | | 280 | 106.0 |

This form of the experiment is much more susceptible of accuracy than the other, and the measures of elasticity are more to be depended on. In repeating the experiment, they were found much more constant; whereas, in the former method, differences occurred of two inches and upwards.

We may now connect the two sets of experiments into one table, by adding to the numbers in this last table the constant height 29.9, which was the height of the mercury in the barometer during the last set of observations.

| Temperature | Elasticity | |-------------|------------| | 32° | 0.0 | | 40 | 0.1 | | 50 | 0.1 | | 60 | 0.35 | | 70 | 0.55 | | 80 | 0.82 | | 90 | 1.25 | | 100 | 1.6 | | 110 | 2.25 | | 120 | 3.0 | | 130 | 3.95 | | 140 | 5.15 | | 150 | 6.72 | | 160 | 8.65 | | 170 | 11.05 | | 180 | 14.05 | | 190 | 17.85 | | 200 | 22.62 | | 210 | 28.65 | | 220 | 33.8 | | 230 | 44.7 | | 240 | 54.9 | | 250 | 66.8 | | 260 | 80.3 | | 270 | 94.1 | | 280 | 105.9 |

In the memoirs of the Royal Academy of Berlin for 1782, there is an account of some experiments made by Mr Achard on the elastic force of steam, from the temperature 32° to 212°. They agree extremely well with those mentioned here, rarely differing more than two or three tenths of an inch. He also examined the elasticity of the vapour produced from alcohol, and found, that when the elasticity was equal to that of the vapour of water, the temperature was about 35° lower. Thus, when the elasticity of both was measured by 28.1 inches of mercury, the temperature of the watery vapour was 209°, and that of the spirituous vapour was 173°. When the elasticity was 18.5, the temperature of the water was 189.5, and that of the alcohol 154.6. When the elasticity was 11.05, the water was 168°, and the alcohol 13.04. Observing the difference between the temperatures of equally elastic vapours of water and alcohol not to be constant, but gradually to diminish, in Mr Achard's experiments, along with the elasticity, it became interesting to discover whether and at what temperature this difference would vanish altogether. Experiments were accordingly made by the writer of this article, similar to those made with water. They were not made with the same scrupulous care, nor repeated as they deserved, but they furnished rather an unexpected result. The following table will give the reader a distinct notion of them:

| Temperature | Elasticity | |-------------|------------| | 32° | 0.0 | | 40 | 0.1 | | 60 | 0.8 | | 80 | 0.8 | | 100 | 3.9 | | 120 | 6.9 | | 140 | 12.2 | | 160 | 21.3 | | 180 | 34.4 | | 200 | 52.4 | | 220 | 78.5 | | 240 | 115.5 |

We say that the result was unexpected; for as the natural boiling point seemed by former experiments to be fault in common all fluids about 120° or more below their boiling point in the ordinary pressure of the atmosphere, it was reasonable to expect that the temperature at which they cease to emit sensibly elastic steam would have some quality elastic relation to their temperatures when emitting steam of any determinate elasticity. Now as the vapour of alcohol has its temperature about 36° lower than the temperature of water equally elastic, it was to be expected that the temperature at which it ceased to be sensibly affected would be several degrees lower than 32°. It is evident, however, that this is not the case. But this is a point that deserves more attention, because it is closely connected with the chemical relation between the element (if such there be) of fire and the bodies into whose composition it seems to enter as a constituent part. What is the temperature 32° to make it peculiarly connected with elasticity? It is a temperature assumed by us for our own convenience, on account of the familiarity of water in our experiments. Ether, we know, boils in a temperature far below this, as appears from Dr Cullen's experiments narrated in the Essays Physical and Literary of Edinburgh. On the faith of former experiments, we may be pretty certain that it will boil in vacuo at the temperature —14°, because in the air it boils at +106°. Therefore we may be certain, that the steam or vapour of ether, when of the temperature 32°, will be very sensibly elastic. Indeed Mr Lavoirier says, that if it be exposed in an exhausted receiver in winter, its vapour will support mercury at the height of 10 inches. A series of experiments on this vapour similar to the above would be very instructive. We even wish that those on alcohol were more carefully repeated. If we draw a curve line, of which the abscissa is the line of temperatures, and the ordinates are the corresponding heights of the mercury in these experiments on water and alcohol, we shall observe, that although they both sensibly coincide at $32^\circ$, and have the abscissa for their common tangent, a very small error of observation may be the cause of this, and the curve which expresses the elasticity of spirituous vapour may really intersect the other, and go backwards considerably beyond $32^\circ$.

This range of experiments gives rise to some curious and important reflections. We now see that no particular temperature is necessary for water affuming the form of permanently elastic vapour; and that it is highly probable that it affumes this form even at the temperature $32^\circ$; only its elasticity is too small to afford us any sensible measure. It is well known that even ice evaporates (see experiments to this purpose by Mr Wilson in the Philosophical Transactions, when a piece of polished metal covered with hoar-frost became perfectly clear by exposing it to a dry frosty wind).

Even mercury evaporates, or is converted into elastic vapour, when all external pressure is removed. The dim film which may frequently be observed in the upper part of a barometer which stands near a stream of air, is found to be small globules of mercury sticking to the inside of the tube. They may be seen by the help of a magnifying glass, and are the best test of a well made barometer. They will be entirely removed by causing the mercury to rise along the tube. It will lick them all up. They consist of mercury which had evaporated in the void space, and was afterwards condensed by the cold glass. But the elasticity is too small to occasion a sensible depression of the column, even when considerably warmed by a candle.

Many philosophers accordingly imagine, that spontaneous evaporation in low temperatures is produced in this way. But we cannot be of this opinion, and must still think that this kind of evaporation is produced by the dissolving power of the air. When moist air is suddenly rarefied, there is always a precipitation of water. This is most distinctly seen when we work an air-pump briskly. A mist is produced, which we see plainly fall to the bottom of the receiver. But by this new doctrine the very contrary should happen, because the tendency of water to appear in the elastic form is promoted by removing the external pressure; and we really imagine that more of it now actually becomes simple elastic watery vapour. But the mist or precipitation shows incontrovertibly, that there had been a previous solution. Solution is performed by forces which act in the way of attraction; or, to express it more safely, solutions are accompanied by the mutual approaches of the particles of the menstrum and solvent; all such tendencies are observed to increase by a diminution of distance. Hence it must follow, that air of double density will dissolve more than twice as much water. Therefore when we suddenly rarefy saturated air (even though its heat should not diminish) some water must be let go. What may be its quantity we know not; but it may be more than what would now become elastic by this diminution of surrounding pressure; and it is not unlikely but this may have some effect in producing the vehicles which we found so difficult to explain.

These may be filled with pure watery vapour, and be floating in a fluid composed of water dissolved in air. An experiment of Fontana's seems to put this matter out of doubt. A distilling apparatus AB (fig. 4.) was so contrived, that the heat was applied above the surface of the water in the alembic A. This was done by inclosing it in another vessel CC, filled with hot water. In the receiver B there was a sort of barometer D, with an open cistern, in order to see what pressure there was on the surface of the fluid. While the receiver and alembic contained air, the heat applied at A produced no sensible distillation during several hours: But on opening a cock E in the receiver at its bottom, and making the water in the alembic to boil, steam was produced which soon expelled all the air, and followed it through the cock. The cock was now shut, and the whole allowed to grow cold by removing the fire, and applying cold water to the alembic. The barometer fell to a level nearly. Then warm water was allowed to get into the outer vessel CC. The barometer rose a little, and the distillation went on briskly without the smallest ebullition in the alembic. The conclusion is obvious: while there was air in the receiver and communicating pipe, the distillation proceeded entirely by the dissolving power of this air. Above the water in the alembic it was quickly saturated; and this saturation proceeded slowly along the fill air in the communicating pipe, and at last might take place through the whole of the receiver. The sides of the receiver being kept cold, should condense part of the water dissolved in the air in contact with them, and this should trickle down the sides and be collected. But any person who has observed how long a crystal of blue vitriol will lie at the bottom of a glass of still water before the tinge will reach the surface, will see that it must be next to impossible for distillation to go on in these circumstances; and accordingly none was observed. But when the upper part of the apparatus was filled with pure watery vapour, it was supplied from the alembic as fast as it was condensed in the receiver, just as in the pulle glass.

Another inference which may be drawn from these experiments is, that Nature seems to affect a certain law in the dilatation of aeriform fluids by heat. They seem to be dilatable nearly in proportion of their present dilatation. For if we suppose that the vapours resemble air, in having their elasticity in any given temperature proportional to their density, we must suppose that if steam of the elasticity 60, that is, supporting 60 inches of mercury, were subjected to a pressure of 30 inches, it would expand into twice its present bulk. The augmentation of elasticity therefore is the measure of the bulk into which it would expand in order to acquire its former elasticity. Taking the increase of elasticity therefore as a measure of the bulk into which it would expand under one constant pressure, we see that equal increments of temperature produce nearly equal multiplications of bulk. Thus if a certain diminution of temperature diminishes its bulk $\frac{1}{4}$th, another equal diminution of temperature will diminish this new bulk $\frac{1}{4}$th very nearly. Thus in our experiments, the temperatures $110^\circ$, $140^\circ$, $170^\circ$, $200^\circ$, $230^\circ$, are in arithmetical progression, having equal differences; and we see that the corresponding elasticities $2.25$, $5.15$, $11.35$, $22.62$, $44.7$, are very nearly in the continued proportion of $1$ to $2$. The elasticity corresponding to the temperature $260$ deviates considerably from this law, which would give $88$ or $89$ instead of $80$; and the deviation Steam deviation increases in the higher temperatures. But still we feel that there is a considerable approximation to this law; and it will frequently assist us to recollect, that whatever be the present temperature, an increase of 30 degrees doubles the elasticity and the bulk of watery vapour.

That $4^\circ$ will increase the elasticity from 1 to $1 + \frac{1}{2}$

| Temperature | Elasticity | |-------------|------------| | 8 | 1 | | 10 | 1 | | 12\(\frac{1}{2}\) | 1 | | 18 | 1 | | 22 | 1 | | 24 | 1 | | 26 | 1 |

This is sufficiently exact for most practical purposes. Thus an engineer finds that the injection cools the cylinder of a steam-engine to $192^\circ$. It therefore leaves a steam whose elasticity is three-fifths of its full elasticity, = 18 inches \(^2\). But it is better at all times to have recourse to the table. Observe, too, that in the lower temperatures, i.e. below $110^\circ$, this increment of temperature does more than double the elasticity.

This law obtains more remarkably in the incalculable vapours; such as vital air, atmospheric air, fixed air, &c., all of which have also their elasticity proportional to their bulk inversely; and perhaps the deviation from the law in steams is connected with their chemical difference of constitution. If the bulk were always augmented in the same proportion by equal augmentations of temperature, the elasticities would be accurately represented by the ordinates of a logarithmic curve, of which the temperatures are the corresponding abscissae; and we might contrive such a scale for our thermometer, that the temperatures would be the common logarithms of the elasticities, or of the bulks having equal elasticity; or, with our present scale, we may find such a multiplier \(m\) for the number \(x\) of degrees of our thermometer (above that temperature where the elasticity is equal to unity), that this multiple shall be the common logarithm of the elasticity \(y\); so that \(mx = \log y\).

But our experiments are not sufficiently accurate for determining the temperature where the elasticity is measured by 1 inch; because in these temperatures the elasticities vary by exceedingly small quantities. But if we take 11.24 for the unit of elasticity, and number our temperature from $170^\circ$, and make \(m = 0.00035\), we shall find the product \(mx\) to be very nearly the logarithm of the elasticity. The deviations, however, from this law, are too great to make this equation of any use. But it is very practicable to frame an equation which shall correspond with the experiments to any degree of accuracy; and it has been done for air in a translation of General Roy's Measurement of the Bale at Hounslow Heath into French by Mr Prony. It is as follows: Let \(x\) be the degrees of Reaumur's thermometer; let \(y\) be the expansion of 10,000 parts of air; let \(e = 10\), \(m = 2.7979\), \(n = 0.01768\); then \(y = e^{m + nx} - 627.5\). Now \(e\) being = 10, it is plain that \(e^{m + nx}\) is the number, of which \(m + nx\) is the common logarithm. This formula is very exact as far as the temperature $60^\circ$; but beyond this it needs a correction; because air, like the vapour of water, does not expand in the exact proportion of its bulk.

We observe this law considerably approximated to in And is confirmed by the augmentation of the bulk or elasticity of elastic vapours; that is, it is a fact that a given increment of approximate temperature makes very nearly the same proportional augmentation of bulk and elasticity. This gives us some notion of the manner in which the supposed expanding of the bulk causes produces the effect. When vapour of the bulk or elasticity \(4\) is expanded into a bulk \(5\) by an addition of 10 degrees of elastic vapours, a certain quantity of fire goes into it, and is accumulated round each particle, in such a manner that the temperature of each, which formerly was \(m\), is now \(m + 10\). Let it now receive another equal augmentation of temperature. This is now \(m + 20\), and the bulk is \(5 \times 5\) or \(64\), and the arithmetical increase of bulk is \(1\). The absolute quantity of fire which has entered it is greater than the former, both on account of the greater augmentation of space and the greater temperature. Consequently if this vapour be compressed into the bulk \(5\), there must be heat or fire in it which is not necessary for the temperature \(m + 20\), far less for the temperature \(m + 10\). It must therefore emerge, and be disposed to enter a thermometer which has already the temperature \(m + 20\); that is, the vapour must grow hotter by compression; not by squeezing out the heat, like water out of a sponge, but because the law of attraction for heat is deranged. It would be a very valuable acquisition to our knowledge to learn with precision the quantity of sensible heat produced in this way; but no satisfactory experiments have yet been made. M. Lavoisier, with his chemical friends and colleagues, were busily employed in this inquiry; but the wickedness of their countrymen deprived the world of this and many other important additions which we might have expected from this celebrated and unfortunate philosopher. He had made, in conjunction with M. de la Place, a numerous train of accurate and expensive experiments for measuring the quantity of latent or combined heat in elastic vapours. This is evidently a very important point to the distiller and practical chemist. This heat must all come from the fuel; and it is greatly worth while to know whether any saving may be made of this article. Thus we know that distillation will go on either under the pressure of the air, or in an alembic and receiver from which the air has been expelled by steam; and we know that this last may be conducted in a very low temperature, even not exceeding that of the human body. But it is uncertain whether this may not employ even a greater quantity of fuel, as well as occasion a great expense of time. We are disposed to think, that when there is no air in the apparatus, and when the condensation can be speedily performed, the proportion of fuel expended to the fluid which comes over will diminish continually as the heat, and consequently the density of the steam, is augmented; because in this case the quantity of combined heat must be less. In the mean time, we earnestly recommend the trial of this mode of distillation in vessels cleared of air. It is undoubtedly of great advantage to be able to work with smaller fires; and it would secure us against all accidents of blowing off the Steam, the head of the hill, often attended with terrible consequences (B).

We must not conclude this article without taking notice of some natural phenomena which seem to owe their origin to the action of elastic steam.

We have already taken notice of the resemblance of the tremor and succussions observed in the shocks of many earthquakes to those which may be felt in a vessel where water is made to boil internally, while the breaking out of the ebullition is stifled by the cold of the upper parts; and we have likewise stated the objections which are usually made to this theory of earthquakes. We may perhaps refine the subject under the article Volcano; but in the meantime we do not hesitate to say, that the wonderful appearances of the Geyser spring in Iceland (see Huer; and Iceland, No. 3—5.) are undoubtedly produced by the expansion of steam in ignited caverns. Of these appearances we suppose the whole train to be produced as follows.

A cavern may be supposed of a shape analogous to CBDEF (fig. 5.), having a perpendicular funnel AB issuing from a depressed part of the roof. The part F may be lower than the rest, remote, and red-hot. Such places we know to be frequent in Iceland. Water may be continually trickling into the part CD. It will fill it up to B, and even up to E, and then trickle slowly along into F. As soon as any gets into contact with an ignited part, it expands into elastic steam, and is partly condensed by the cold sides of the cavern, which it gradually warms, till it condenses no more. This production of steam hinders not in the smallest degree the trickling of more water into F, and the continual production of more steam. This now presses on the surface of the water in CD, and causes it to rise gradually in the funnel BA; but slowly, because its cold surface is condensing an immense quantity of steam. We may easily suppose that the water trickles faster into F than it is expended in the production of steam; so that it reaches farther into the ignited part, and may even fall in a stream into some deeper pit highly ignited. It will now produce steam in vast abundance, and of prodigious elasticity; and at once push up the water through the funnel in a solid jet, and to a great height. This must continue till the surface of the water sinks to BD. If the lower end of the funnel have any inequalities or notches, as is most likely, the steam will get admission along with the water, which in this particular place is boiling hot, being superficial, and will get to the mouth of the funnel, while water is still pressed in below. At last the steam gets in at B on all sides; and as it is converging to B, along the surface of the water, with prodigious velocity it sweeps along with it much water, and blows it up through the funnel with great force. When this is over, the remaining steam blows out unmixed with water, growing weaker as it is expended, till the bottom of the funnel is again stopped by the water increasing in the cavern CBD. All the phenomena above ground are perfectly conformable to the necessary consequences of this very probable construction of the cavern. The feeling of being lifted up, immediately before the jet, in all probability is owing to a real heaving up of the whole roof of the cavern by the first expansion of the great body of steam. We had an accurate description of the phenomena from persons well qualified to judge of these matters who visited these celebrated springs in 1789.

STEAM-Engine, is the name of a machine which derives its moving power from the elasticity and condensibility of the steam of boiling water. It is the most valuable present which the arts of life have ever received from the philosopher. The mariner's compass, the telescope, gunpowder, and other most useful servants to human weaknesses and ingenuity, were the productions of chance, and we do not exactly know to whom we are indebted for them; but the steam-engine was, in the very beginning, the result of reflection, and the production of a very ingenious mind; and every improvement it has received, and every alteration in its construction and principles, were also the results of philosophical study.

The steam-engine was beyond all doubt invented by Steam on the marquis of Worcester during the reign of Charles II., sine invent. This nobleman published in 1663 a small book entitled marquis of A Century of Inventions; giving some obscure and Worcester's enigmatical account of a hundred discoveries or contrivances of his own, which he extols as of great importance to the public. He appears to have been a person of much knowledge and great ingenuity; but his description or accounts of these inventions seem not so much intended to instruct the public, as to raise wonder; and his encomiums on their utility and importance

(B) We earnestly recommend this subject to the consideration of the philosopher. The laws which regulate the formation of elastic vapour, or the general phenomena which it exhibits, give us that link which connects chemistry with mechanical philosophy. Here we see chemical affinities and mechanical forces set in immediate opposition to each other, and the one made the indication, characteristic, and measure of the other. We have not the least doubt that they make but one science, the Science of Universal Mechanics; nor do we despair of seeing the phenomena of solution, precipitation, crystallization, fermentation, nay animal and vegetable secretion and assimilation, successfully investigated, as cases of local motion, and explained by the agency of central forces. Something of this kind, and that not inconsiderable, was done when Dr Cullen first showed how the double affinities might be illustrated by the affinities of numbers. Dr Black gave to this hint (for it was little more) that elegant precision which characterizes all his views. Mr Kirwan has greatly promoted this study by his numerous and ingenious examples of its application; and the most valuable passages of the writings of Mr Lavoisier, are those where he traces with logical precision the balancings of force which appear in the chemical phenomena. It is from the similar balancings and consequent measurements, which may be observed and obtained in the present case, that we are to hope for admission into this almost unbounded science of contemplation. We have another link equally interesting and promising, viz. the production of heat by friction. This also highly deserves the consideration of the mathematical philosopher. ance are to a great degree extravagant, resembling more the puff of an advertising tradesman than the patriotic communications of a gentleman. The marquis of Worcester was indeed a projector, and very importunate and mysterious within his applications for public encouragement. His account, however, of the steam-engine, although by no means fit to give us any distinct notions of its structure and operation, is exact as far as it goes, agreeing precisely with what we now know of the subject. It is No. 68. of his inventions. His words are as follow: "This admirable method which I propose of raising water by the force of fire has no bounds if the vessels be strong enough: for I have taken a cannon, and having filled it three-fourths full of water, and shut up its muzzle and touch-hole, and exposed it to the fire for 24 hours, it burst with a great explosion. Having afterwards discovered a method of fortifying vessels internally, and combined them in such a way that they filled and acted alternately, I have made the water spout in an uninterrupted stream 40 feet high; and one vessel of rarefied water raised 40 of cold water. The person who conducted the operation had nothing to do but turn two cocks; so that one vessel of water being consumed, another begins to force, and then to fill itself with cold water, and so on in succession."

It does not appear that the noble inventor could ever interest the public by these accounts. His character as a projector, and the many failures which persons of this turn of mind daily experience, probably prejudiced people against him, and prevented all attention to his projects. It was not till towards the end of the century, when experimental philosophy was prosecuted all over Europe with uncommon ardour, that these notions again engaged attention. Captain Savary, a person also of great ingenuity and ardent mind, saw the reality and practicability of the marquis of Worcester's project. He knew the great expansive power of steam, and had discovered the inconceivable rapidity with which it is converted into water by cold; and he soon contrived a machine for raising water, in which both of these properties were employed. He says, that it was entirely his own invention. Dr Defaguliers insists that he only copied the marquis's invention, and charges him with gross plagiarism, and with having bought up and burned the copies of the marquis's book, in order to secure the honour of the discovery to himself. This is a very grievous charge, and should have been substantiated by very distinct evidence. Defaguliers produces none such; and he was much too late to know what happened at that time. The argument which he gives is a very foolish one, and gave him no title to consider Savary's experiment as a falsehood; for it might have happened precisely as Savary relates, and not as it happened to Defaguliers. The fact is, that Savary obtained his patent of invention after a hearing of objections, among which the discovery of the marquis of Worcester was not mentioned; and it is certain that the account given in the Century of Inventions could instruct no person who was not sufficiently acquainted with the properties of steam to be able to invent the machine himself.

Captain Savary obtained his patent after having actually erected several machines, of which he gave a description in a book intitled The Miner's Friend, published in 1696, and in another work published in 1699. Much about this time Dr Papin, a Frenchman and fellow of the Royal Society, invented a method of dissolving bones and other animal solids in water, by confining them in clothe vessels, which he called digesters, so as to acquire a great degree of heat. For it must be observed in this place, that it had been discovered long before (in 1684) by Dr Hooke, the most inquisitive experimental philosopher of that inquisitive age, that water could not be made to acquire above a certain temperature in the open air; and that as soon as it begins to boil, its temperature remains fixed, and an increase of heat only produces a more violent ebullition, and a more rapid waste. But Papin's experiments made the elastic power of steam very familiar to him; and when he left England and settled as professor of mathematics at Marburgh, he made many awkward attempts to employ this force in mechanics, and even for raising water. It appears that he had made experiments with this view in 1698, by order of Charles, landgrave of Hesse. For this reason the French affect to consider him as the inventor of the steam-engine. He indeed published some account of his invention in 1707; but he acknowledges that Captain Savary had also, and without any communication with him, invented the same thing. Whoever will take the trouble of looking at the description which he has given of these inventions, which are to be seen in the Acta Eruditorum Lipsiae, and in Leupold's Theatrum Machinarum, will see that they are most awkward, absurd, and impracticable. His conceptions of natural operations were always vague and imperfect, and he was neither philosopher nor mechanician.

We are thus anxious about the claim of those gentlemen, because a most respectable French author, Mr Bofaut, says in his Hydrodynamique, that the first notion of the steam-engine was certainly owing to Dr Papin, who had not only invented the digester, but had in 1695 published a little performance describing a machine for raising water, in which the pistons are moved by the vapour of boiling water alternately dilated and condensed. Now the fact is, that Papin's first publication was in 1707, and his piston is nothing more than a floater on the surface of the water, to prevent the waste of steam by condensation; and the return of the piston is not produced, as in the steam-engine, by the condensation of the steam, but by admitting the air and a column of water to press it back into its place. The whole contrivance is so awkward, and so unlike any distinct notions of the subject, that it cannot do credit to any person. We may add, that much about the same time Mr Amontons contrived a very ingenious but intricate machine, which he called the fire-wheel. It consisted of a number of buckets placed in the circumference of a wheel, and communicating with each other by very intricate circuitous passages. One part of this circumference was exposed to the heat of a furnace, and another to a stream or cistern of cold water. The communications were so disposed, that the steam produced in the buckets on one side of the wheel drove the water into buckets on the other side, so that one side of the wheel was always much heavier than the other; and it must therefore turn round, and may execute some work. The death of the inventor, and the intricacy of the machine, caused it to be neglected. Another member of the Parisian academy of sciences (Mr Deffandes) also presented to the academy a project of a steam-wheel, where the impulsive force of the va- pour was employed; but it met with no encouragement. The English engineers had by this time so much improved Savary's first invention, that it supplanted all others. We have therefore no hesitation in giving the honour of the first and complete invention to the marquis of Worcester; and we are not disposed to refuse Captain Savary's claim to originality as to the construction of the machine, and even think it probable that his own experiments made him see the whole, independent of the marquis's account.

Captain Savary's engine, as improved and simplified by himself, is as follows.

A (fig. 6.) represents a strong copper boiler properly built up in a furnace. There proceeds from its top a large steam-pipe B, which enters into the top of another strong vessel R called the receiver. This pipe has a cock at C called the steam-cock. In the bottom of the receiver is a pipe F, which communicates sidewise with the rising pipe KGH. The lower end H of this pipe is immersed in the water of the pit or well, and its upper part K opens into the cistern into which the water is to be delivered. Immediately below the pipe of communication F there is a valve G, opening when pressed from below, and shutting when pressed downwards. A similar valve is placed at I, immediately above the pipe of communication. Lastly, there is a pipe ED which branches off from the rising pipe, and enters into the top of the receiver. This pipe has a cock D called the injection-cock. The mouth of the pipe ED has a nozzle pierced with small holes, pointing from a centre in every direction. The keys of the two cocks C and D are united, and the handle g is called the regulator.

Let the regulator be so placed that the steam-cock C is open and the injection-cock D is shut; put water into the boiler A, and make it boil strongly. The steam coming from it will enter the receiver, and gradually warm it, much steam being condensed in producing this effect. When it has been warmed so as to condense no more, the steam proceeds into the rising pipe; the valve G remains shut by its weight; the steam lifts the valve I, and gets into the rising pipe, and gradually warms it. When the workman feels this to be the case, or hears the rattling of the valve I, he immediately turns the steam-cock so as to shut it, the injection-cock still remaining shut (at least we may suppose this for the present.) The apparatus must now cool, and the steam in the receiver collapses into water. There is nothing now to balance the pressure of the atmosphere; the valve I remains shut by its weight; but the air incumbent on the water in the pit presses up this water through the suction-pipe HG, and causes it to lift the valve G, and flow into the receiver R, and fill it to the top, if not more than 20 or 25 feet above the surface of the pit water.

The steam-cock is now opened. The steam which, during the cooling of the receiver, has been accumulating in the boiler, and acquiring a great elasticity by the action of the fire, now rushes in with great violence, and, pressing on the surface of the water in the receiver, causes it to shut the valve G and open the valve I by its weight alone, and it now flows into the rising pipe, and would stand on a level if the elasticity of the steam were no more than what would balance the atmospheric pressure. But it is much more than this, and therefore it presses the water out of the receiver into the rising pipe, and will even cause it to come out at K, if the elasticity of the steam is sufficiently great. In order to ensure this, the boiler has another pipe in its top, covered with a safety-valve V, which is kept down by a weight W suspended on a steel-yard LM. This weight is so adjusted that its pressure on the safety-valve is somewhat greater than the pressure of a column of water VK as high as the point of discharge K. The fire is so regulated that the steam is always issuing a little by the loaded valve V. The workman keeps the steam-valve open till he hears the valve I rattle. This tells him that the water is all forced out of the receiver, and that the steam is now following it. He immediately turns the regulator which shuts the steam-cock, and now, for the first time, opens the injection-cock. The cold water trickles at first through the holes of the nozzle f, and falling down through the steam, begins to condense it; and then its elasticity being less than the pressure of the water in the pipe KEDf, the cold water spouts in all directions through the nozzle, and, quick as thought, produces a complete condensation. The valve G now opens again by the pressure of the atmosphere on the water of the pit, and the receiver is soon filled with cold water. The injection-cock is now shut, and the steam-cock opened, and the whole operation is now repeated; and so on continually.

This is the simple account of the process, and will serve to give the reader an introductory notion of the operation; but a more minute attention must be paid to many particulars before we can see the properties and defects of this ingenious machine.

The water is driven along the rising pipe by the defects of elasticity of the steam. This must in the boiler, and this makes every part of the machine, exert a pressure on every square inch of the vessels equal to that of the upright column of water. Suppose the water to be raised 100 feet, about 25 of this may be done in the suction-pipe; that is, the upper part of the receiver may be about 25 feet above the surface of the pit-water. The remaining 75 must be done by forcing, and every square inch of the boiler will be squeezed out by a pressure of more than 30 pounds. This very moderate height therefore requires very strong vessels; and the marquis of Worcester was well aware of the danger of their bursting. A copper boiler of six feet diameter must be nine-tenths of an inch thick to be just in equilibrium with this pressure; and the foldered joint will not be able to withstand it, especially in the high temperature to which the water must be heated in order to produce steam of sufficient elasticity. By consulting the table of the elasticity of steam deduced from our experiments mentioned in the preceding article, we see that this temperature must be at least 280° of Fahrenheit's thermometer. In this heat soft folder is just ready to melt, and has no tenacity; even sterner folder is considerably weakened by it. Accordingly, in a machine erected by Dr Desaguilliers, the workman having loaded the safety-valve a little more than usual to make the engine work more briskly, the boiler burst with a dreadful explosion, and blew up the furnace and adjoining parts of the building as if it had been gunpowder. Mr Savary succeeded pretty well in raising moderate quantities of water to small heights, but could make nothing of deep mines. Many attempts were made, on the mor- quis's principle, to strengthen the vessels from within by radiated bars and by hoops, but in vain. Very small boilers or evaporators were then tried, kept red hot, or nearly so, and supplied with a slender stream of water trickling into them; but this afforded no opportunity of making a collection of steam during the refrigeration of the receiver, so as to have a magazine of steam in readiness for the next forcing operation; and the working of such machines was always an employment of great danger and anxiety.

The only situation in which this machine could be employed with perfect safety, and with some effect, was where the whole lift did not exceed 30 or 35 feet. In this case the greatest part of it was performed by the suction-pipe, and a very manageable pressure was sufficient for the rest. Several machines of this kind were erected in England about the beginning of this century. A very large one was erected at a salt-work in the south of France. Here the water was to be raised no more than 18 feet. The receiver was capacious, and it was occasionally supplied with steam from a small salt-pan constructed on purpose with a cover. The entry of the steam into the receiver merely allowed the water to run out of it by a large valve, which was opened by the hand, and the condensation was produced by the help of a small forcing pump also worked by the hand. In no particular situation as this (and many such may occur in the endless variety of human wants), this is a very powerful engine; and having few moving and rubbing parts, it must be of great durability. This circumstance has occasioned much attention to be given to this first form of the engine, even long after it was supplanted by those of a much better construction. A very ingenious attempt was made very lately to adapt this construction to the uses of the miners. The whole depth of the pit was divided into lifts of 15 feet, in the same manner as is frequently done in pump-machines. In each of these was a suction-pipe 14 feet long, having above it a small receiver like R, about a foot high, and its capacity somewhat greater than that of the pipe. This receiver had a valve at the head of the suction-pipe, and another opening outwards into the little cistern, into which the next suction-pipe above dipped to take in water. Each of these receivers sent up a pipe from its top, which all met in the cover of a large vessel above ground, which was of double the capacity of all the receivers and pipes. This vessel was close on all sides. Another vessel of equal capacity was placed immediately above it, with a pipe from its bottom passing through the cover of the lower vessel and reaching near to its bottom. This upper vessel communicates with the boiler, and constitutes the receiver of the steam-engine. The operation is as follows: The lower vessel is full of water. Steam is admitted into the upper vessel, which expels the air by a valve, and fills the vessel. It is then condensed by cold water. The pressure of the atmosphere would cause it to enter by all the suction-pipes of the different lifts, and press on the surface of the water in the lower receiver, and force it into the upper one. But because each suction-pipe dips in a cistern of water, the air presses this water before it, raises it into each of the little receivers which it fills, and allows the spring of the air (which was formerly in them, but which now passes up into the lower receiver) to force the water out of the lower receiver into the upper one. When this has been completed, the steam is again admitted into the upper receiver. This allows the water to run back into the lower receiver, and the air returns into the small receivers in the pit, and allows the water to run out of each into its proper cistern. By this means the water of each pipe has been raised 15 feet. The operation may thus be repeated continually.

The contrivance is ingenious, and similar to those which are to be met with in the hydraulics of Schottus, Sturmius, and other German writers. But the operation must be exceedingly slow; and we imagine that the expense of steam must be great, because it must fill a very large and very cold vessel, which must waste a great portion of it by condensation. We see by some late publications of the very ingenious Mr Blackey, that he is still attempting to maintain the reputation of this machine by some contrivances of this kind; but we imagine that they will be ineffectual, except in some very particular situations.

For the great defect of the machine, even when we can secure it against all risk of bursting, is the prodigious waste of steam, and consequently of fuel. Daily experience shows, that a few scattered drops of cold water are sufficient for producing an almost instantaneous condensation of a great quantity of steam. Therefore when the steam is admitted into the receiver of Savary's engine, and comes into contact with the cold top and cold water, it is condensed with great rapidity; and the water does not begin to bubble till its surface has become so hot that it condenses no more steam. It may now begin to yield to the pressure of the incumbent steam; but as soon as it descends a little, more of the cold surface of the receiver comes into contact with the steam, and condenses more of it, and the water can descend no farther till this addition of cold surface is heated up to the state of evaporation. This rapid condensation goes on all the while the water is descending. By some experiments frequently repeated by the writer of this article, it appears that no less than $\frac{1}{2}$ths of the whole steam is useless condensed in this manner, and not more than $\frac{1}{4}$th is employed in allowing the water to descend by its own weight; and he has reason to think that the portion thus wasted will be considerably greater, if the steam be employed to force the water out of the receiver to any considerable height.

Observe, too, that all this waste must be repeated in every succeeding stroke; for the whole receiver must be cooled again in order to fill itself with water.

Many attempts have been made to diminish this waste; but all to little purpose, because the very filling of the receiver with cold water occasions its sides made to condense a prodigious quantity of steam in the face of this waste exceeding stroke. Mr Blackey has attempted to lessen this waste by using two receivers. In the first was oil; and into this only the steam was admitted. This oil passed to and fro between the two receivers, and never touched the water except in a small surface. But this hardly produced a sensible diminution of the waste: for it must now be observed, that there is a necessity for the first cylinder's being cooled to a considerable degree below the boiling point; otherwise, though it will condense much steam, and allow the water to rise into the receiver, there will be a great diminution of the height of suction, unless the vessel be much cooled. This appears plainly by inspecting the table of elasticity. Thus, if the vessel be cooled no lower than $180^\circ$, we should lose one half of the pressure of the atmosphere; if cooled to $120^\circ$, we should still lose $\frac{3}{5}$th. The inspection of this table is of great use for understanding and improving this noble machine; and without a constant recollection of the elasticity of steam corresponding to its actual heat, we shall never have a notion of the niceties of its operation.

The rapidity with which the steam is condensed is really astonishing. Experiments have been made on steam-vessels of five feet in diameter and seven feet high; and it has been found, that about four ounces of water, as warm as the human blood, will produce a complete condensation in less than a second; that is, will produce all the condensation that it is capable of producing, leaving an elasticity about one-fifth of the elasticity of the air. In another experiment with the same steam-vessel, no cold water was allowed to get into it, but it was made to communicate by a long pipe four inches in diameter with another vessel immersed in cold water. The condensation was so rapid that the time could not be measured: it certainly did not exceed half a second. Now this condensation was performed by a very trifling surface of contact. Perhaps we may explain it a little in this way: When a mass of steam, in immediate contact with the cold water, is condensed, it leaves a void, into which the adjoining steam instantly expands; and by this expansion its capacity for heat is increased, or it grows cold, that is, abstracts the heat from the steam situated immediately beyond it. And in this expansion and refrigeration it is itself partly condensed or converted into water, and leaves a void, into which the circumjacent steam immediately expands, and produces the same effect on the steam beyond it. And thus it may happen that the abstraction of a small quantity of heat from an inconsiderable mass of steam may produce a condensation which may be very extensive. Did we know the change made in the capacity of steam for heat by a given change of bulk, we should be able to tell exactly what would be the effect of this local actual condensation. But experiment has not yet given us any precise notions on this subject. We think that this rapid condensation to a great distance by a very moderate actual abstraction of heat is a proof that the capacity of steam for heat is prodigiously increased by expansion. We say a very moderate actual abstraction of heat, because very little heat is necessary to raise four ounces of blood-warm water to a boiling temperature, which will unfit it for condensing steam. The remarkable phenomenon of snow and ice produced in the Hungarian machine, when the air condensed in the receiver is allowed to blow through the cock (see PNEUMATICS), shows this to be the case in moist air, that is, in air holding water in a state of chemical solution. We see something very like it in a thunder-storm. A small black cloud sometimes appears in a particular spot, and in a very few seconds spreads over many hundred acres of sky, that is, a precipitation of water goes on with that rapid diffusion. We imagine that this increase of capacity or demand for heat, and the condensation that must ensue if this demand is not supplied, is much more remarkable in pure watery vapours, and that this is a capital distinction of their constitution from vapours dissolved in air.

The reader must now be so well acquainted with what passes in the steam-vessel, and with the exterior results from it, as readily to comprehend the propriety of the changes which we shall now describe as having been made in the construction and principle of the steam engine.

Of all places in England the tin-mines of Cornwall attempts stood most in need of hydraulic affluence; and Mr Savary was much engaged in projects for draining them by his steam-engine. This made its construction and principles well known among the machinists and engineers of that neighbourhood. Among these were Mr Newcomen, an ironmonger or blacksmith, and Mr Cawley a glazier at Dartmouth in Devonshire, who had dabbled much with this machine. Newcomen was a person of some reading, and was in particular acquainted with the person, writings, and projects of his countryman Dr Hooke. There are to be found among Hooke's papers, in the possession of the Royal Society, some notes of observations, for the use of Newcomen his countryman, on Papin's boasted method of transmitting to a great distance the action of a mill by means of pipes. Papin's project was to employ the mill to work two air-pumps of great diameter. The cylinders of these pumps were to communicate by means of pipes with equal cylinders furnished with pistons, in the neighbourhood of a distant mine. These pistons were to be connected, by means of levers, with the piston-rods of the mine. Therefore, when the piston of the air-pump at the mill was drawn up by the mill, the corresponding piston at the side of the mine would be pressed down by the atmosphere, and thus would raise the piston-rod in the mine, and draw the water. It would appear from these notes, that Dr Hooke had diffused Mr Newcomen from erecting a machine on this principle, of which he had exposed the fallacy in several discourses before the Royal Society. One passage is remarkable. "Could he (meaning Papin) make a speedy vacuum under your second piston, your work is done."

It is highly probable that, in the course of this speculation, it occurred to Mr Newcomen that the vacuum he so much wanted might be produced by steam, and that this gave rise to his new principle and construction of the steam-engine. The specific deferaturum was in Newcomen's mind; and therefore, when Savary's engine appeared, and became known in his neighbourhood many years after, he would readily catch at the help which it promised.

Savary, however, claims the invention as his own; but Switzer, who was personally acquainted with both, is positive that Newcomen was the inventor. By his principles (as a Quaker) being adverse from contention, he was contented to share the honour and the profits with Savary, whose acquaintance at court enabled him to procure the patent in 1705, in which all the three were associated. Posterity has done justice to the modest inventor, and the machine is universally called NEWCOMEN'S.

(A) But if it has been found that the condensation requires more cold water than what is allowed above, and it is suspected that the rapidity of condensing a large volume of steam by the cold surface of a vessel is overrated. The water in the boiler being supposed to be in a state of strong ebullition, and the steam issuing by the safety-valve, let us consider the machine in a state of rest, having both the steam-cock and injection-cock shut. How the resting position or attitude of the machine must be such as appears in sketch, the pump rods preponderating, and the great piston being drawn up to the top of the cylinder. Now open the steam-cock by turning the handle T of the regulator. The steam from the work-boiler will immediately rush in, and flying all over the cylinder, will mix with the air. Much of it will be condensed by the cold surface of the cylinder and piston, and the water produced from it will trickle down the sides, and run off by the eduction-pipe. This condensation and waste of steam will continue till the whole cylinder and piston are made as hot as boiling water. When this happens, the steam will begin to open the snifting valve f, and issue through the pipe; slowly at first and very cloudy, being mixed with much air. The blast at f will grow stronger by degrees, and more transparent, having already carried off the greatest part of the common air which filled the cylinder. We supposed that the water was boiling briskly, so that the steam was issuing by the safety-valve which is in the top of the boiler, and through every crevice. The opening of the steam-cock puts an end to this at once, and it has sometimes happened that the cold cylinder abstracts the steam from the boiler with such astonishing rapidity, that the pressure of the atmosphere has burst up the bottom of the boiler. We may here mention an accident of which we were witnesses, which also shows the immense rapidity of the condensation. The boiler was in a frail shed at the side of the engine-house; a shout of snow from the top of the house fell down and broke through the roof of the shed, and was scattered over the head of the boiler, which was of an oblong or oval shape. In an instant the sides of it were squeezed together by the pressure of the atmosphere.

When the manager of the engine perceives that not only the blast at the snifting valve is strong and steady, but that the boiler is now fully supplied with steam of proper strength, appearing by the renewal of the discharge at the safety-valve, he shuts the steam-cock, and opens the injection-cock S by turning its handle V. The pressure of the column of water in the injection-pipe ZS immediately forces some water through the spout R. This coming in contact with the pure vapour which now fills the cylinder, condenses it, and thus makes a partial void, into which the more dilute steam immediately expands, and by expanding collapses (as has been already observed). What remains in the cylinder no longer balances the atmospheric pressure on the surface of the water in the injection cistern, and therefore the water pours rapidly through the hole R by the joint action of the column ZS, and the unbalanced pressure of the atmosphere; at the same time the snifting-valve f, and the eduction-valve h, are shut by the unbalanced pressure of the atmosphere. The velocity of the injection water must therefore rapidly increase, and the jet will dash (if single) against the bottom of the piston, and be scattered through the whole capacity of the cylinder. In a very short space of time, therefore, the condensation of the steam becomes universal, and the elasticity of what remains is almost nothing. The whole pressure... pressure of the atmosphere is exerted in the upper surface of the piston, while there is hardly any on its under side. Therefore, if the load on the outer end E of the working beam is inferior to this pressure, it must yield to it. The piston P must descend, and the pump piston L must ascend, bringing along with it the water of the mine, and the motion must continue till the great piston reaches the bottom of the cylinder; for it is not like the motion which would take place in a cylinder of air rarefied to the same degree. In this last case, the impelling force would be continually diminished, because the capacity of the cylinder is diminished by the descent of the piston, and the air in it is continually becoming more dense and elastic. The piston would stop at a certain height, where the elasticity of the included air, together with the load at E, would balance the atmospheric pressure on the piston. But when the contents of the cylinder are pure vapour, and the continued stream of injected cold water keeps down its temperature to the same pitch as at the beginning, the elasticity of the remaining steam can never increase by the descent of the piston, nor exceed what corresponds to this temperature. The impelling or accelerating force therefore remains the same, and the descent of the piston will be uniformly accelerated, if there is not an increase of resistance arising from the nature of the work performed by the other end of the beam. This circumstance will come under consideration afterwards, and we need not attend to it at present. It is enough for our present purpose to see, that if the cylinder has been completely purged of common air before the steam-cock was shut, and if none has entered since, the piston will descend to the very bottom of the cylinder. And this may be frequently observed in a good steam-engine, where every part is air-tight. It sometimes happens, by the pit-pump drawing air, or some part of the communication between the two strains giving way, that the piston comes down with such violence as to knock out the bottom of the cylinder with the blow.

The only observation which remains to be made on the motion of the piston in descending is, that it does not begin at the instant the injection is made. The piston was kept at the top by the preponderancy of the outer end of the working beam, and it must remain there till the difference between the elasticity of the steam below it and the pressure of the atmosphere exceeds this preponderance. There must therefore be a small space of time between the beginning of the condensation and the beginning of the motion. This is very small, not exceeding the third or the fourth part of a second; but it may be very distinctly observed by an attentive spectator. He will see, that the instant the injection cock is opened, the cylinder will sensibly rise upwards a little by the pressure of the air on its bottom. Its whole weight is not nearly equal to this pressure; and instead of its being necessary to support it by a strong floor, we must keep it down by strong jolts loaded by heavy walls. It is usual to frame these joints into the pots which carry the axis of the working-beam, and are therefore loaded with the whole strain of the machine. This rising of the cylinder shows the instantaneous commencement of the condensation; and it is not till after this has been distinctly observed that the piston is seen to start, and begin to descend.

When the manager feels the piston as low as he thinks proper, he shuts the injection-cock, and opens the steam-cock. The steam has been accumulating above the water in the boiler during the whole time of the piston's descent, and is now rushing violently through the pipe. The moment, therefore, that the steam-cumstances cock is opened, it rushes violently into the cylinder, having an elasticity greater than that of the air. It therefore immediately blows open the stuffing valve, and allows (at least) the water which had come in by the former injection, and what arose from the condensed steam, to descend by its own weight through the eduction pipe d e g h to open the valve A, and to run out into the well. And we must easily see that this water is boiling hot; for while lying in the bottom of the cylinder, it will condense steam till it acquires this temperature, and therefore cannot run down till it condenses no more. There is still a waft of steam at its first admission, in order to heat the inside of the cylinder and the injected water to the boiling temperature; but the space being small, and the whole being already very warm, this is very soon done; and when things are properly constructed, little more steam is wanted than what will warm the cylinder; for the eduction pipe receives the injection water even during the descent of the piston, and it is therefore removed pretty much out of the way of the steam.

This first puff of the entering steam is of great service; it drives out of the cylinder the vapour which it finds there. This is seldom pure watery vapour: all puff of enter water contains a quantity of air in a state of chemical union. The union is but feeble, and a boiling heat is sufficient for disengaging the greatest part of it by increasing its elasticity. It may also be disengaged by simply removing the external pressure of the atmosphere. This is clearly seen when we expose a glass of water in an exhausted receiver. Therefore the small space below the piston contains watery vapour mixed with all the air which had been disengaged from the water in the boiler by ebullition, and all that was separated from the injection water by the diminution of external pressures. All this is blown out of the cylinder by the first puff of steam. We may observe in this place, that waters differ exceedingly in the quantity of air which they hold in a state of solution. All spring water contains much of it; and water newly brought up from deep mines contains a great deal more, because the solution was aided in these situations by great pressures. Such waters sparkle when poured into a glass. It is therefore of great consequence to the good performance of a steam-engine to use water containing little air, both in the engine and in the injection-cistern. The water of running brooks is preferable to all others, and the freer it is from any saline impregnation it generally contains steam-leafs air. Such engines as are so unfortunately situated, that they are obliged to employ the very water which employed they have brought up from great depths, are found contain greatly inferior in their performance to others. The little air collected below the piston greatly diminishes the accelerating force, and the expulsion of such a quantity requires a long-continued blast of the best steam at the beginning of every stroke. It is advisable to keep such water in a large shallow pond for a long while before using it.

Let us now consider the state of the piston. It is evident that it will start or begin to rise the moment piston rises. the steam-cock is opened; for at that instant the excess of atmospheric pressure, by which it was kept down in opposition to the preponderance of the outer end of the beam, is diminished. The piston is therefore dragged upwards, and it will rise even although the steam which is admitted be not so elastic as common air. Suppose the mercury in the barometer to stand at 30 inches, and that the preponderance at the outer end of the beam is \( \frac{1}{5} \)th of the pressure of the air on the piston, the piston will not rise if the elasticity of the steam is not equal to \( \frac{3}{5} - \frac{1}{5} \), that is, to 26.7 inches nearly; but if it is just this quantity, the piston will rise as fast as this steam can be supplied through the steam-pipe, and the velocity of its ascent depends entirely on the velocity of this supply. This observation is of great importance; and it does not seem to have occurred to the mathematicians, who have paid most attention to the mechanism of the motion of this engine. In the mean time, we may clearly see that the entry of the steam depends chiefly on the counter weight at E: for suppose there was none, steam no stronger than air would not enter the cylinder at all; and if the steam be stronger, it will enter only by the excess of its strength. Writers on the steam-engine (and even some of great reputation) familiarly speak of the steam giving the piston a push: But this is scarcely possible. During the rise of the piston the stuffing valve is never observed to blow; and we have not heard any well-attested accounts of the piston-chains ever being slackened by the upward pressure of the steam, even at the very beginning of the stroke. During the rising of the piston the steam is (according to the common conception and manner of speaking) sucked in, in the same way that air is sucked into a common syringe or pump when we draw up the piston; for in the steam-engine the piston is really drawn up by the counter weight. But it is still more sucked in, and requires a more copious supply, for another reason. As the piston descended only in consequence of the inside of the cylinder's being sufficiently cooled to condense the steam, this cooled surface must again be presented to the steam during the rise of the piston, and must condense steam a second time. The piston cannot rise another inch till the part of the cylinder which the piston has already quitted has been warmed up to the boiling point, and steam must be expended in this warming. The inner surface of the cylinder is not only of the heat of boiling water while the piston rises, but is also perfectly dry; for the film of water left on it by the ascending piston must be completely evaporated, otherwise it will be condensing steam. That the quantity thus wasted is considerable, appears by the experiments of Mr Beighton. He found that five pints of water were boiled off in a minute, and produced 16 strokes of an engine whose cylinder contained 113 gallons of 282 inches each; and he thence concluded that steam was 2886 times rarer than water. But in no experiment made with scrupulous care on the expansion of boiling water does it appear that the density of steam exceeds \( \frac{1}{10,000} \)th of the density of water. Desaguliers says that it is above 14,000 times rarer than water. We have frequently attempted to measure the weight of steam which filled a very light vessel, which held 12,600 grains of water, and found it always less than one grain; so that we have no doubt of its being much more than 10,000 times rarer than water. This being the case, we may safely suppose that the number of gallons of steam, instead of being 16 times 113, were nearly five times as much; and that only \( \frac{1}{5} \)th was employed in allowing the piston to rise, and the remaining \( \frac{4}{5} \)ths were employed to warm the cylinder. But no direct experiment shows so great an expansion of water when converted into steam at 212°. Mr Watt never found it under the pressure of the air more than 1800 times rarer than water.

The moving force during the ascent of the piston must be considered as resulting chiefly, if not solely, from the preponderating weight of the piton-rods. The office of this is to return the steam-piston to the top of the cylinder, where it may again be pressed down by the air, and make another working stroke by raising the pump rods. But the counter-weight at E has another service to perform in this use of the engine; namely, to return the pump pistons into their places at the bottom of their respective working barrels, in order that they also may make a working stroke. This requires force independent of the friction and inertia of the moving parts; for each piston must be pushed down through the water in the barrel, which must rise through the piston with a velocity whose proportion to the velocity of the piston is the same with that of the bulk of the piston to the bulk of the perforation through which the water rises through the piston. It is enough at present to mention this in general terms: we shall consider it more particularly afterwards, when we come to calculate the performance of the engine, and to deduce from our acquired knowledge maxims of construction and improvement.

From this general consideration of the ascent of the piston, we may see that the motion differs greatly from that of the descent. It can hardly be supposed to accelerate, even if the steam in the cylinder were in a moment annihilated. For the resistance to the descent of the piston is the same with the weight of the column of water, which would cause it to flow through the box of the pump piston with the velocity with which it really rises through it, and must therefore increase as the square of that velocity increases; that is, as the square of the velocity of the piston increases. Independent of friction, therefore, the velocity of descent through the water must soon become a maximum, and the motion become uniform. We shall see by and by, that in such a pump as is generally used this will happen in less than the tenth part of a second. The friction of the pump will diminish this velocity a little, and retard the time of its attaining uniformity. But, on the other hand, the supply of steam which is necessary for this motion, being susceptible of no acceleration from its previous motion, and depending entirely on the briefness of the ebullition, an almost instantaneous stop is put to acceleration.

Accordingly, any person who observes with attention the working of a steam-engine, will see that the rise of the piston and descent of the pump-rods is extremely uniform, whereas the working stroke is very sensibly accelerated. Before quitting this part of the subject, and lest it should afterwards escape our recollection, we may observe, that the counter-weight is different during the two motions of the pump-rods. While the machine is making a working stroke, it is lifting not only the lower pump-rods, lumn of water in the pump, but the absolute weight of the pistons and piston-rods also; but while the pump-rods are descending, there is a diminution of the counter-weight by the whole weight lost by the immersion of the rod in water. The wooden rods which are generally used, soaked in water, and joined by iron straps, are heavier, and but a little heavier, than water, and they are generally about one-third of the bulk of the water in the pumps.

These two motions complete the period of the operation; and the whole may be repeated by shutting the steam-cock and opening the injection-cock whenever the piston has attained the proper height. We have been very minute in our attention to the different circumstances, that the reader may have a distinct notion of the state of the moving forces in every period of the operation. It is by no means sufficient that we know in general that the injection of cold water makes a void which allows the air to press down the piston, and that the readmission of the steam allows the piston to rise again. This lumping and slovenly way of viewing it has long prevented even the philosopher from seeing the defects of the construction, and the methods of removing them.

We now see the great difference between Savary's and Newcomen's engine, in respect of principle. Savary's was really an engine which raised water by the force of steam; but Newcomen's raises water entirely by the pressure of the atmosphere, and steam is employed merely as the most expeditious method of producing a void, into which the atmospheric pressure may impel the first mover of his machine. The elasticity of the steam is not the first mover.

We see also the great superiority of this new machine. We have no need of steam of great and dangerous elasticity; and we operate by means of very moderate heats, and consequently with much smaller quantities of fuel; and there is no bounds to the power of this machine. How deep soever a mine may be, a cylinder may be employed of such dimensions that the pressure of the air on its piston may exceed in any degree the weight of the column of water to be raised. And lastly, this form of the machine renders it applicable to almost every mechanical purpose; because a skilful mechanic can readily find a method of converting the reciprocating motion of the working beam into a motion of any kind which may suit his purpose. Savary's engine could hardly admit of such an immediate application, and seems almost restricted to raising water.

Inventions improve by degrees. This engine was first offered to the public in 1705. But many difficulties occurred in the execution, which were removed one by one; and it was not till 1712 that the engine seemed to give confidence in its efficacy. The most exact and unremitting attention of the manager was required to the precise moment of opening and shutting the cocks; and neglect might frequently be ruinous, by beating out the bottom of the cylinder, or allowing the piston to be wholly drawn out of it. Stops were contrived to prevent both of these accidents; then strings were used to connect the handles of the cocks with the beam, so that they should be turned whenever it was in certain positions. These were gradually changed and improved into detents and catches of different shapes; at last, in 1717, Mr Beighton, a very ingenious and well-informed artist, simplified the whole of these subordinate movements, and brought the machine into the form in which it has continued, without the smallest material change, to the present day. We shall now describe one of these improved engines, copying almost exactly the drawings and description given by Boulton in his Hydrodynamique; these being by far the most accurate and perspicuous of any that have been published.

Fig. 8. No. 1, is a perspective view of the boiler cylinder, and all the parts necessary for turning the cocks. Fig. 8. No. 2, is a vertical section of the same; and the same pieces of both are marked with the same letters of reference.

The rod X of the piston P is suspended from the arch of the working-beam, as was represented in the preceding sketch (fig. 7). An upright bar of timber FG is also seen hanging by a chain. This is suspended from a concentric arch of the beam, as may be seen also in the sketch at φ. The bar is called the plug-beam; and it must rise and fall with the piston, but with a slower motion. The use of this plug-beam is to give motion to the different pieces which turn the cocks.

The steam-pipe K is of one piece with the bottom of the cylinder, and rises within it an inch or two, to prevent any of the cold injection water from falling into the boiler. The lower extremity Z of the steam-pipe penetrates the head of the boiler, projecting a little way. A flat plate of brass, in shape resembling a racket or battledore, called the regulator, applies itself exactly to the whole circumference of the steam-pipe, and completely excludes the steam from the cylinder. Being moveable round an upright axis, which is represented by the dotted lines at the side of the steam-pipe in the profile, it may be turned aside by the handle i, no. 1. The profile shows in the section of this plate a protuberance in the middle. This rests on a strong flat spring, which is fixed below it athwart the mouth of the steam-pipe. This spring presses it strongly towards the steam-pipe, causing it to apply very close; and this knob slides along the spring, while the regulator turns to the right or left.

We have said that the injection-water is furnished from a cistern placed above the cylinder. When the cistern cannot be supplied by pipes from some more elevated source, its water is raised by the machine itself. A small lifting pump i k (fig. 7), called the jack-head or jaquette, is worked by a rod y, suspended from a concentric arch γ near the outer end of the working beam. This forces a small portion of the pit water along the rising pipe i LM into the injection cistern.

In figure 8. No. 1, and 2, the letters Q M 3' represent the pipe which brings down the water from the injection cistern. This pipe has a cock at R to open or shut the passage of this water. It sprouts through the jet 3', and dashing against the bottom of the piston, it is dispersed into drops, and scattered through the whole capacity of the cylinder, so as to produce a rapid condensation of the steam.

An upright post A may be observed in the perspective view of the cylinder, &c. This supports one end B of a horizontal iron axis BC. The end C is supported by a similar post, of which the place only is marked by the dotted lines Δ, that the pieces connected... ted with this axis may not be hid by it. A kind of stirrup \(a b c d\) hangs from this axis, supported by the hooks \(a\) and \(d\). This stirrup is crossed near the bottom by a round bolt or bar \(e\), which passes through the eyes or rings that are at the ends of the horizontal fork \(h f g\), whose long tail \(h\) is double, receiving between its branches the handle \(i\) of the regulator. It is plain from this construction, that when the stirrup is made to vibrate round the horizontal axis \(BC\), on which it hangs freely by its hooks, the bolt \(e\) must pull or push the long fork \(h f g\) backwards and forwards horizontally, and by so doing will move the regulator round its axis by means of the handle \(i\). Both the tail of the fork and the handle of the regulator are pierced with several holes, and a pin is put through them which unites them by a joint. The motion of the handle may be increased or diminished by choosing for the joint a hole near to the axis or remote from it; and the exact position at which the regulator is to stop on both sides is determined by pins stuck in the horizontal bar on which the end of the handle appears to rest.

This alternate motion of the regulator to the right and left is produced as follows: There is fixed to the axis \(BC\) a piece of iron \(k l\), called the \(Y\), on account of its resemblance to that letter of the alphabet inverted. The stalk \(o\) carries a heavy lump \(p\) of lead or iron; and a long leather strap \(q p r\) is fastened to \(p\) by the middle, and the two ends are fastened to the beam above it, in such a manner that the lump may be alternately raised and held up to the right and left of the perpendicular. By adjusting the length of the two parts of the strap, the \(Y\) may be stopped in any desired position. The two claws \(k\) and \(l\) extend out from each other, and from the line of the stalk, and they are of such length as to reach the horizontal bolt \(e\), which crosses the stirrup below, but not to reach the bottom of the fork \(h f g\). Now suppose the stirrup hanging perpendicularly, and the stalk of the \(Y\) also held perpendicular; carry it a little outward from the cylinder, and then let it go. It will tumble farther out by its weight, without affecting the stirrup till the claw \(l\) strikes the horizontal bolt \(e\), and then it pushes the stirrup and the fork towards the cylinder, and opens the regulator. It sets it in motion with a smart jerk, which is an effectual way of overcoming the cohesion and friction of the regulator with the mouth of the steam-pipe. This push is adjusted to a proper length by the strap \(q p\), which stops the \(Y\) when it has gone far enough. If we now take hold of the stalk of the \(Y\), and move it up to the perpendicular, the width between its claws is such as to permit this motion, and something more, without affecting the stirrup. But when pushed still nearer to the cylinder, it tumbles towards it by its own weight, and then the claw \(k\) strikes the bolt \(e\), and drives the stirrup and fork in the opposite direction, till the lump \(p\) is caught by the strap \(r p\), now stretched to its full length, while \(q p\) hangs slack. Thus by the motion of the \(Y\) the regulator is opened and shut. Let us now see how the motion of the \(Y\) is produced by the machine itself. To the horizontal axis \(BC\) are attached two spanners or handles \(m\) and \(n\). The spanner \(m\) passes through a long slit in the plug-beam, and is at liberty to move upwards or downwards by its motion round the axis \(BC\). A pin \(z\) which goes through the plug-beam catches hold of \(m\) when the beam rises along with the piston; and the pin is so placed, that when the beam is within an inch or two of its highest rise, the pin has lifted \(m\) and thrown the stalk of the \(Y\) past the perpendicular. It therefore tumbles over with great force, and gives a smart blow to the fork, and immediately shuts the regulator. By this motion the spanner \(m\) is removed out of the neighbourhood of the plug-beam. But the spanner \(n\), moving along with it in the same direction, now comes into the way of the pins of the plug-beam. Therefore, when the piston descends again by the condensation of the steam in the cylinder, a pin marked \(\omega\) in the side of the plug-beam catches hold of the tail of the spanner \(n\), and by pressing it down raises the lump on the stalk of the \(Y\) till it passes the perpendicular, and it then falls down, outwards from the cylinder, and the claw \(l\) again drives the fork in the direction \(h i\), and opens the steam valve. This opening and shutting of the steam valve is executed in the precise moment that is proper, by placing the pins \(\pi\) and \(\omega\) at a proper height of the plug-beam. For this reason, it is pierced through with a great number of holes, that the places of these pins may be varied at pleasure. This, and a proper curvature of the spanners \(m\) and \(n\), make the adjustment as nice as we please.

The injection cock \(R\) is managed in a similar manner. On its key may be observed a forked arm \(s t\), like a crab's claw; at a little distance above it is the gudgeon or axis \(u\) of a piece \(y u z\), called the hammer or the \(F\), from its resemblance to that letter. It has a lump of metal \(y\) at one end, and a spear \(u s\) projects from its middle, and passes between the claws \(s\) and \(t\) of the arm of the injection-cock. The hammer \(y\) is held up by a notch in the under side of a wooden lever \(DE\), moveable round the center \(D\), and supported at a proper height by a string \(E\), made fast to the joint above it.

Suppose the injection-cock shut, and the hammer in the position represented in the figure. A pin \(\beta\) of the plug-frame rises along with the piston, and catching hold of the detent \(DE\), raises it, and disengages the hammer \(y\) from its notch. This immediately falls down, and strikes a board \(L\) put in the way to stop it. The spear \(u s\) takes hold of the claw \(t\), and forces it aside towards \(x\), and opens the injection-cock. The piston immediately descends, and along with it the plug-frame. During its descent the pin \(\beta\) meets with the tail \(u z\) of the hammer, which is now raised considerably above the level, and brings it down along with it, raising the lump \(y\), and gradually shutting the injection-cock, because the spear takes hold of the claw \(s\) of its arm. When the beam has come to its lowest situation, the hammer is again engaged in the notch of the detent \(DE\), and supported by it till the piston again reaches the top of the cylinder.

In this manner the motions of the injection cock are also adjusted to the precise moment that is proper for them. The different pins are so placed in the plug-frame, that the steam-cock may be completely shut before the injection-cock is opened. The inherent motion of the machine will give a small addition to the ascent of the piston without expending steam all the while; and by leaving the steam rather less elastic than before, the subsequent descent of the piston is promoted. There was a considerable propriety in the gradual shutting... ting of the injection-cock. For after the first dash of the cold water against the bottom of the piston, the condensation is nearly complete, and very little more water is needed; but a continual accession of some is absolutely necessary for completing the condensation, as the capacity of the cylinder diminishes, and the water warms which is already injected.

In this manner the motion of the machine will be repeated as long as there is a supply of steam from the boiler, and of water from the injection cistern, and a discharge procured for what has been injected. We proceed to consider how far these conditions also are provided by the machine itself.

The injection cistern is supplied with water by the jackhead pump, as we have already observed. From this source all the parts of the machine receive their respective supplies. In the first place, a small branch 13, 13, is taken off from the injection-pipe immediately below the cistern, and conducted to the top of the cylinder, where it is furnished with a cock. The spout is so adjusted, that no more runs from it than what will keep a constant supply of a foot of water above the piston to keep it tight. Every time the piston comes to the top of the cylinder, it brings this water along with it, and the surplus of its evaporation and leakage runs off by a waste-pipe 14, 14. This water necessarily becomes almost boiling hot, and it was thought proper to employ its surplus for supplying the waste of the boiler. This was accordingly practised for some time. But Mr Beighton improved this economical thought, by supplying the boiler from the education-pipe, 2, 2, the water of which must be still hotter than that above the piston. This contrivance required attention to many circumstances, which the reader will understand by considering the perspective and profile. The education-pipe comes out of the bottom of the cylinder at 1 with a perpendicular part, which bends sidewise below, and is shut at the extremity 1. A deep cup 5 communicates with it, holding a metal valve nicely fitted to it by grinding, like the key of a cock. To secure its being always air-tight, a slender stream of water trickles into it from a branch 6 of the waste-pipe from the top of the cylinder. The education-pipe branches off at 2, and goes down to the hot well, where it turns up, and is covered with a valve. In the perspective view may be observed an upright pipe 4, 4, which goes through the head of the boiler, and reaches to within a few inches of its bottom. This pipe is called the feeder, and rises about three or four feet above the boiler. It is open at both ends, and has a branch 3, 3, communicating with the bottom of the cup 5, immediately above the metal valve, and also a few inches below the level of the entry 2 of the education-pipe. This communicating branch has a cock by which its passage may be diminished at pleasure. Now suppose the steam in the boiler to be very strong, it will cause the boiling water to rise in the feeding pipe above 3, and coming along this branch, to rise also in the cup 5, and run over. But the height of this cup above the surface of the water in the boiler is such, that the steam is never strong enough to produce this effect. Therefore, on the contrary, any water that may be in the cup 5 will run off by the branch 3, 3, and go down into the boiler by the feeding pipe.

These things being understood, let us suppose a quantity of injected water lying at the bottom of the Steama-cylinder. It will run into the education-pipe, fill the crooked branch 1, 1, and open the valve in the bottom of the cup (its weight being supported by a wire hanging from a flender spring), and it will fill the cup to the lowest con-level of the entry 2 of the education-pipe, and then trivance flow along 3, 3, and supply the boiler by the feeder 4, 4. What more water runs in at 1 will now go along the education pipe 2, 2, to the hot well. By properly adjusting the cock on the branch 3, 3, the boiler may be supplied as fast as the waste in steam requires. This is a most ingenious contrivance, and does great honour to Mr Beighton. It is not, however, of much importance. The small quantity which the boiler requires may be immediately taken even from a cold cistern, without sensibly diminishing the production of steam: for the quantity of heat necessary for raising the sensible heat of cold water to the boiling temperature is small, when compared with the quantity of heat which must then be combined with it in order to convert the water into steam. For the heat expended in boiling off a cubic foot of water is about five times as much as would bring it to a boiling heat from the temperature of 55°. No difference can be observed in the performance of such engines, and of those which have their boilers supplied from a brook. It has, however, the advantage of being purged of air; and when an engine must derive all its supplies from pit water, the water from the education-pipe is vastly preferable to that from the top of the cylinder.

We may here observe, that many writers (among them the Abbé Boffet), in their descriptions of the steam-engine, have drawn the branch of communication 3, 3, from the feeding-pipe to a part of the crooked pipe 1, 1, lying below the valve in the cup 5. But this is quite erroneous; for, in this case, when the injection is made into the cylinder, and a vacuum produced, the water from the boiler would immediately rush up through the pipes 4, 3, and spout up into the cylinder: so would the external air coming in at the top of the feeder.

This contrivance has also enabled us to form some judgement of the internal state of the engine during the absence of performance. Mr Beighton paid a minute attention to form some the situation of the water in the feeders and education, judgement of the internal state of the pipe of an engine, which seems to have been one of the internal state which has yet been erected. It was lifting a co of the column of water whose weight was four-sevenths of the fine during prelude of the air on its piston, and made 16 strokes, of the performance, fix feet each, in a minute. This is acknowledged by all to be a very great performance of an engine of this form. He concluded that the elasticity of the steam in the cylinder was never more than one-tenth greater or less than the elasticity of the air. The water in the feeder never rose more than three feet and a half above the surface of the boiling water, even though it was now lighter by 1/7th than cold water. The education-pipe was only four feet and a half long (vertically), and yet it always discharged the injection water completely; and allowed some to pass into the feeder. This could not be if the steam was much more than one-tenth weaker than air. By grasping this pipe in his hand during the rise of the piston, he could guess very well whereabouts the surface of the hot water in it rested during the motion, and he never found it supported so high as four feet. Therefore the steam in the cylinder had at least eight-ninths of of the elasticity of the air. Mr Buat, in his examination of an engine which is erected at Montrelais, in France, by an English engineer, and has always been considered as the pattern in that country, finds it necessary to suppose a much greater variation in the strength of the steam, and says, that it must have been one-fifth stronger and one-fifth weaker than common air. But this engine has not been nearly so perfect. Its lift was not more than one-half of the pressure of the atmosphere, and it made but nine strokes in a minute.—At W is a valve covering the mouth of a small pipe, and surrounded with a cup containing water to keep it air-tight. This allows the air to escape which had been extricated from the water of half injection. It is driven out by the first strong puff of steam which is admitted into the cylinder, and makes a noise in its exit. The valve is therefore called the fitting-valve.

To finish our description, we observe, that besides the safety valve g (called the PUPPET CLACK), which is loaded with about 3 pounds on the square inch (though the engine will work very well with a load of 1 or 2 pounds), there is another DISCHARGER 10,10, having a clack at its extremity supported by a cord. Its use is to discharge the steam without doors, when the machine gives over working. There is also a pipe SI near the bottom of the boiler, by which it may be emptied when it needs repairs or cleaning.

There are two small pipes II,II, and 12,12, with cocks called GAGE-PIPES. The first descends to within two inches of the surface of the water in the boiler, and the second goes about 2 inches below that surface. If both cocks emit steam, the water is too low, and requires a recruit. If neither give steam, it is too high, and there is not sufficient room above it for a collection of steam. Lastly, there is a filling pipe Q, by which the boiler may be filled when the machine is to be let to work.

The engine has continued in this form for many years. The only remarkable change introduced has been the manner of placing the boiler. It is no longer placed below the cylinder, but at one side, and the steam is introduced by a pipe from the top of the boiler into a flat box immediately below the cylinder. The use of this box is merely to lodge the regulator, and give room for its motions. This has been a very considerable improvement. It has greatly reduced the height of the building. This was formerly a tower. The wall which supported the beam could hardly be built with sufficient strength for withstanding the violent shocks which were repeated without ceasing; and the buildings seldom lasted more than a very few years. But the boiler is now set up in an adjoining shed, and the gudgeons of the main beam rest on the top of upright pots, which are framed into the joints which support the cylinder. Thus the whole moving parts of the machine are contained in one compact frame of carpentry, and have little or no connection with the flight walls of the building, which is merely a cage to hold the machine, and protect it from the weather.

It is now time to inquire what is to be expected from this machine, and to ascertain the most advantageous proportion between the moving power and the load that is to be laid on the machine.

It may be considered as a great pulley, and is indeed sometimes so constructed, the arches at the ends of the working beam being completed to a circle. It must be unequally loaded that it may move. It is loaded, during the working stroke, by the pressure of the atmosphere on the piston side, and by the column of water to be raised and the pump-gear on the pump side.—During the returning stroke it is loaded, on the piston side, by a small part of the atmospheric pressure, and on the pump side by the pump gear acting as a counter weight. The load during the working stroke must therefore consist of the column of water to be raised and this counter weight. The performance of the machine is to be measured only by the quantity of water raised in a given time to a given height. It varies, therefore, in the joint proportion of the weight of the column of water in the pumps, and the number of strokes made by the machine in a minute. Each stroke consists of two parts, which we have called the working and the returning stroke. It does not, therefore, depend simply on the velocity of the working stroke and the quantity of water raised by it. If this were all that is to be attended to, we know that the weight of the column of water should be nearly 3/4ths of the pressure of the atmosphere, this being the proportion which gives the maximum in the common pulley. But the time of the returning stroke is a necessary part of the whole time elapsed, and therefore the velocity of the returning stroke equally merits attention. This is regulated by the counter weight. The number of strokes per minute does not give an immediate proof of the goodness of the engine. A small load of water and a great counter weight will ensure this, because these conditions will produce a brisk motion in both directions.—The proper adjustment of the pressure of the atmosphere on the piston, the column of water to be raised, and the counter weight, is a problem of very great difficulty; and mathematicians have not turned much of their attention to the subject, although it is certainly the most interesting question that practical mechanics affords them.

Mr Boulton has solved it very shortly and simply, upon Mr Boulton's supposition, that the working and returning stroke should be made in equal times. This, indeed, is generally aimed at in the erection of these machines, and they are not reckoned to be well arranged if it be otherwise. We doubt of the propriety of the maxim. Supposing, however, this condition for the present, we may compute the loadings of the two ends of the beam as follows. Let \(a\) be the length of the inner arm of the working beam, or that by which the great piston is supported. Let \(b\) be the outer arm carrying the pump rods, and let \(W\) be a weight equivalent to all the load which is laid on the machine. Let \(c^2\) be the area of the piston; let \(H\) be the height of a column of water having \(c^2\) for its base, and being equal in weight to the pressure exerted by the steam on the under side of the piston; and let \(h\) be the pressure of the atmosphere on the same area, or the height of a column of water of equal weight. It is evident that both strokes will be performed in equal times, if \(h \cdot c^2 - W \cdot b\) be equal to \((h - H) \cdot c^2 \cdot a + W \cdot b\). The first of these quantities is the energy of the machine during the working stroke, and the second expresses the similar energy during the returning stroke. This equation gives us

\[ W = \frac{2 \cdot h \cdot c^2 \cdot a - H \cdot c^2 \cdot a}{2 \cdot b} = \frac{(2 \cdot h - H) \cdot c^2 \cdot a}{2 \cdot b}. \]

If we suppose the arms of the lever equal and $H = h$, we have $W = \frac{h}{2}$; that is, the whole weight of the outer end of the beam should be half the pressure of the air on the great piston. This is nearly the usual practice; and the engineers express it by saying, that the engine is loaded with seven or eight pounds on the square inch. This has been found to be nearly the most advantageous load. This way of expressing the matter would do well enough, if the maxim were not founded on erroneous notions, which hinder us from seeing the state of the machine, and the circumstances on which its improvement depends. The piston bears a pressure of 15 pounds, it is said, on the square inch, if the vacuum below it be perfect; but as this is far from being the case, we must not load it above the power of its vacuum, which very little exceeds eight pounds. But this is very far from the truth. When the cylinder is tight, the vacuum is not more than $\frac{1}{15}$th deficient, when the cylinder is cooled by the injection to the degree that is every day practicable, and the piston really bears during its descent a pressure very near to 14 pounds on the inch. The load must be diminished, not on account of the imperfect vacuum, but to give the machine a reasonable motion. We must consider not only the moving force, but also the quantity of matter to be put in motion. This is so great in the steam-engine, that even if it were balanced, that is, if there were suspended on the piston arm a weight equal to the whole column of water and the counter weight, the full pressure of the atmosphere on the steam piston would not make it move twice as fast as it does.

This equation by Mr Boffin is moreover essentially faulty in another respect. The W in the first member is not the same with the W in the second. In the first it is the column of water to be raised, together with the counter weight. In the second it is the counter weight only. Nor is the quantity H the same in both cases, as is most evident. The proper equation for ensuring the equal duration of the two strokes may be had in the following manner. Let it be determined by experiment what portion of the atmospheric pressure is exerted on the great piston during its descent. This depends on the remaining elasticity of the steam. Suppose it $\frac{c}{a}$ths: this we may express by $a \cdot b$, $a$ being $\frac{c}{a}$ths. Let it also be determined by experiment what portion of the atmospheric pressure on the piston remains unbalanced by the steam below it during its ascent. Suppose this $\frac{c}{a}$ths, we may express this by $b \cdot h$. Then let W be the weight of the column of water to be raised, and c the counter weight. Then, if the arms of the beam are equal, we have the energy during the working stroke $= a \cdot h - W - c$, and during the returning stroke it is $= c - b \cdot h$. Therefore $c - b \cdot h = a \cdot h - W - c$; and $c = \frac{h(a + b) - W}{2}$; which, on the above supposition of the values of $a$ and $b$, gives us $c = \frac{h - W}{2}$. We shall make some use of this equation afterwards; but it affords us no information concerning the most advantageous proportion of $h$ and W, which is the material point.

We must consider this matter in another way: And furthering the that we may not involve ourselves in unnecessary difficulties, let us make the case as simple as possible, and suppose the arms of the working-beam to be of equal length.

We shall first consider the adjustment of things at the outer end of the beam.

Since the sole use of the steam is to give room for the adjustment of the atmospheric pressure by its rapid condensation, it is admitted into the cylinder only to allow the things at the piston to rise again, but without giving it any impetus at the end of the pulse. The pump-rods must therefore be returned to beam con- the bottom of the working barrels by means of a presidered ponderancy at the outer end of the beam. It may be the weight of the pump-rods themselves, or may be considered as making part of this weight. A weight at the end of the beam will not operate on the rods which are suspended there by chains, and it must therefore be attached to the rods themselves, but above their respective pump-barrels, so that it may not lose part of its efficacy by immersion in the water. We may consider the whole under the notion of the pump-gear, and call it p. Its office is to depress the pump-rods with sufficient velocity, by overcoming the resistances arising from the following causes:

1. From the inertia of the beams and all the parts of the apparatus which are in motion during the descent of the pump-rods. 2. From the loss of weight sustained by the immersion of the pump-rods in water. 3. From the friction of all the pistons and the weight of the plug-frame. 4. From the resistance to the piston's motion, arising from the velocity which must be generated in the water in passing through the descending pistons.

The sum of all these resistances is equal to the pressure of some weight (as yet unknown), which we may call m.

When the pump-rods are brought up again, they bring along with them a column of water, whose weight we may call w.

It is evident that the load which must be overcome by the pressure of the atmosphere on the steam piston consists of w and p. Let this load be called L, and the pressure of the air be called P.

If p be L, no water will be raised; if p be 0, the rods will not descend: therefore there is some intermediate value of p which will produce the greatest effect.

In order to discover this, let g be the fall of a heavy body in a second.

The descending mass is p: but it does not descend with its full weight; because it is overcoming a set of resistances which are equivalent to a weight m, and the moving force is $p - m$. In order to discover the space through which the rods will descend in a second, when urged by the force $p - m$ (supposed constant, notwithstanding the increase of velocity, and consequently of m), we must institute this proportion $p : p - m = g : g(p - m)$.

The fourth term of this analogy is the space required.

Let t be the whole time of the descent in seconds. Then $t^2 : t^2 = \frac{g(p - m)}{p} : \frac{t^2 g(p - m)}{p}$. This last term is is the whole descent or length of the stroke accomplished in the time \( t \).

The weight of the column of water, which has now got above the piston, is \( w = L - p \). This must be lifted in the next working stroke through the space \( \frac{t^2 g (p - m)}{p} \). Therefore the performance of the engine must be \( \frac{t^2 g (p - m) (L - p)}{p} \).

That this may be the greatest possible, we must consider \( p \) as the variable quantity, and make the fluxion of the fraction \( \frac{p - m \times L - p}{p} = 0 \).

This will be found to give us \( p = \sqrt{Lm} \); that is, the counter weight or preponderancy of the outer end of the beam is \( \frac{\rho}{\sqrt{Lm}} \).

This gives us a method of determining \( m \) experimentally. We can discover by actual measurement the quantity \( L \) in any engine, it being equal to the unbalanced weights on the beam and the weight of the water in the pumps. Then \( m = \frac{\rho^2}{L} \).

Also we have the weight of the column of water \( = L - p = L - \sqrt{Lm} \).

When therefore we have determined the load which is to be on the outer end of the beam during the working stroke, it must be distributed into two parts, which have the proportion of \( \sqrt{Lm} \) to \( L - \sqrt{Lm} \). The first is the counter weight, and the second is the weight of the column of water.

If \( m \) is a fraction of \( L \), such as an aliquot part of it; that is, if

\[ m = \frac{L}{1}, \frac{L}{2}, \frac{L}{3}, \frac{L}{4}, \frac{L}{5}, \ldots \]

The circumstance which is commonly objected to us by local considerations is the quantity of water, and the depth from which it is to be raised; that is, \( w \); and it will be convenient to determine everything in conformity to this.

We saw that \( w = L - \sqrt{Lm} \). This gives us \( L = \frac{w + m^2}{4} + \frac{m}{2} + w \), and the counter weight \( p = \frac{w + m^2}{4} + \frac{m}{2} \).

Having thus ascertained that distribution of the load on the outer end of the beam which produces the greatest effect, we come now to consider what proportion of moving force we must apply, so that it may be employed to the best advantage, or so that any expense of power may produce the greatest performance. It will be so much the greater as the work done is greater, and the power employed is less; and will therefore be properly measured by the quotient of the work done divided by the power employed.

The work immediately done is the lifting up the weight \( L \). In order to accomplish this, we must employ a pressure \( P \), which is greater than \( L \). Let it be \( L + y \); also let \( s \) be the length of the stroke.

If the mass \( L \) were urged along the space \( s \) by the force \( L + y \), it would acquire a certain velocity, which we may express by \( \sqrt{s} \); but it is impelled only by the force \( y \), the rest of \( P \) being employed in balancing \( L \). The velocities which different forces generate by impelling a body along the same space are as the square roots of the forces. Therefore \( \sqrt{L + y} : \sqrt{y} = \sqrt{s} : \sqrt{\frac{s}{L + y}} \).

The fourth term of this analogy expresses the velocity of the piston at the end of the stroke. The quantity of motion produced will be had by multiplying this velocity by the mass \( L \). This gives \( \frac{L \times \sqrt{s}}{\sqrt{L + y}} \); and this divided by the power expended, or by \( L + y \), gives us the measure of the performance; namely,

\[ \frac{L \times \sqrt{s}}{L + y \times \sqrt{L + y}} \]

That this may be a maximum, consider \( y \) as the variable quantity, and make the fluxion of this formula \( = 0 \). This will give us \( y = \frac{L}{2} \).

Now \( P = L + y = L + \frac{L}{2} = \frac{3}{2}L \). Therefore the whole load on the outer end of the beam, consisting of the water and the counter weight, must be two-thirds of the pressure of the atmosphere on the steam piston.

We have here supposed that the expenditure is the atmospheric pressure; and so it is if we consider it mechanically. But the expenditure of which we are sensible, and which we are anxious to employ to the best advantage, is fuel. Supposing this to be employed with the same judgement in all cases, we are almost intitled, by what we now know of the production of steam, to say that the steam produced is proportional to the fuel expended. But the steam requisite for merely filling the cylinder is proportional to the area of the piston, and therefore to the atmospheric pressure. The result of our investigation therefore is still just; but the steam wasted by condensation on the sides of the cylinder does not follow this ratio, and this is more than what is necessary for merely filling it. This deranges our calculations, and is in favour of large cylinders; but this advantage must be in a great measure compensated by a similar variation in the production of the steam; for in similar boilers of greater dimensions the fuel is less advantageously employed, because the surface to which the fuel is applied does not increase in the ratio of the capacity, just as the surface of the cylinder which wastes the steam. The rule may therefore be confided in as pretty exact.

It is a satisfactory thing to observe these results agree very well with the most successful practice. By many facts agreed changes and trials engineers have established maxims of construction, which are probably not very far from the best. It is a pretty general maxim, that the load of practice, water should be one-half of the atmospheric pressure. They call this loading the engine with \( \frac{7}{8} \) pounds on the inch, and they say that so small a load is necessary on account of the imperfect vacuum. But we have now seen that it is necessary for giving a reasonable velocity of motion. Since, in this practice, \( w \) is made \( \frac{1}{2} \) or \( \frac{1}{3} \)ths of \( P \), and \( L \) should be \( \frac{1}{2} \)ths of \( P \), and \( L \) is \( = w + p \); it follows, that the counter weight should be \( \frac{1}{2} \)th. STE

and we have found this to be nearly the case in several very good engines.

It must be remarked, that in the preceding investigation we introduced a quantity M to express the resistances to the motion of the engine. This was done in order to avoid a very troublesome investigation. The resistances are of such a nature as to vary with the velocity, and most of them as the square of the velocity. This is the case with the resistance arising from the motion of the water through the pistons of the pumps, and that arising from the friction in the long lift during the working stroke. Had we taken the direct method, which is similar to the determination of the motion through a medium which resists in the duplicate ratio of the velocity, we must have used a very intricate exponential calculus, which few of our readers would have the patience to look at.

But the greatest part of the quantity m supposes a motion already known, and its determination depends on this motion. We must now show how its different component parts may be computed.

1. What arises from the inertia of the moving parts is by far the most considerable portion of it. To obtain it, we must find a quantity of matter which, when placed at the end of the beam, will have the same momentum of inertia with that of the whole moving parts in their natural places. Therefore (in the returning stroke) add together the weight of the great piston with its rod and chains; the pit pump-rods, chains, and any weight that is attached to them; the arch-heads and iron-work at the ends of the beam, and 4ths of the weight of the beam itself; also the plug-beam with its arch-head and chain, multiplied by the square of its distance from the axis, and divided by the square of half the length of the beam; also the jack-head pump-rod, chain, and arch-head, multiplied by the square of its distance from the axis, and divided by the square of the half length of the beam. These articles added into one sum may be called M, and may be supposed to move with the velocity of the end of the beam. Suppose this beam to have made a six-foot stroke in two seconds, with an uniformly accelerated motion. In one second it would have moved 3 feet, and would have acquired the velocity of three feet per second. But in one second gravity would have produced a velocity of 3 feet in the same mass. Therefore the accelerating force, which has produced the velocity of three feet, is nearly 3/4th of the weight. Therefore \( \frac{M}{I} \) is the first constituent of m in the above investigation. If the observed velocity is greater or less than three feet per second, this value must be increased or diminished in the same proportion.

The second cause of resistance, viz. the immersion of the pump rods in water, is easily computed, being the weight of the water which they displace.

The third cause, the friction of the pistons, &c., is almost insignificant, and must be discovered by experiment.

The fourth cause depends on the structure of the pumps. These pumps, when made of a proper strength, can hardly have the perforation of the piston more than a fourth part of the area of the working-barrel; and the velocity with which the water passes through it is increased at least 4th by the contraction (see Pump). The velocity of the water is therefore five times greater than that of the piston. A piston 12 inches diameter, and moving one foot per second, meets with a resistance equal to 20 pounds; and this increases as the square of the diameter and as the square of the velocity. If the whole depth of the pit be divided into several lifts, this resistance must be multiplied by the number of lifts, because it obtains in each pump.

Thus we make up the value of m; and we must acknowledge that the method is still indirect, because it supposes the velocity to be known.

We may obtain it more easily in another way, but still with this circumstance of being indirect. We found that \( p = \sqrt{Lm} \), and consequently \( m = \frac{p^2}{L} \).

Now in any engine L and p can always be had; and unless p deviates greatly from the proportion which we determined to be the best, the value of m thus obtained will not be very erroneous.

It was farther presumed in this investigation, that the observations both up and down were uniformly accelerated; but this cannot be the case when the resistances increase something with the velocity. This circumstance makes very little difference in the working-stroke, and therefore the theorem which determines the best relation of P to L may be considered in. The resistances which vary with the velocity in this case are a mere trifle when compared with the moving power y. These resistances are, 1st, The strangling of the water at the entry and at the standing valve of each pump: This is about 37 pounds for a pump 12 inches diameter, and the velocity one foot per second, increasing in the duplicate ratio of the diameter and velocity. And, 2d, The friction of the water along the whole lift: This for a pump of the same size and with the same velocity, lifting 20 fathoms, is only about 25 pounds, and varies in the simple proportion of the diameter and the depth, and in the duplicate proportion of the velocity. The resistance arising from inertia is greater than in the returning stroke; because the M in this case must contain the momentum of the water both of the pit-pumps and the jackhead-pump: but this part of the resistance does not affect the uniform acceleration. We may therefore consider in the propriety of the formula \( y = \frac{L}{2} \). And we may obtain the velocity of this stroke at the end of a second with great accuracy as follows. Let \( 2g \) be the velocity communicated by gravity in a second, and the velocity at the end of the first second of the steam piston's descent will be somewhat less than \( \frac{y}{M} \cdot 2g \); where M expresses the inertia of all the parts which are in motion during the descent of the steam piston, and therefore includes L. Compute the two resistances just mentioned for this velocity. Call this r. Then \( \frac{y - r}{M} \cdot 2g \) will give another velocity infinitely near the truth.

But the case is very different in the returning stroke, and the proper ratio of p to L is not ascertained with the same certainty: for the moving force p is not so great in proportion to the resistance m; and therefore the acceleration of the motion is considerably affected by it, and the motion itself is considerably retarded, and in a very moderate time it becomes sensibly uniform: for it is precisely similar to the motion of a heavy body falling... falling through the air, and may be determined in the manner laid down in the article RESISTANCE of Fluids, viz. by an exponential calculus. We shall content ourselves here with saying, that the resistances in the present case are so great that the motion would be to all intents uniform before the pistons have descended one-third of their stroke, even although there were no other circumstance to affect it.

But this motion is affected by a circumstance quite unconnected with any thing yet considered, depending on conditions not mechanical, and so uncertain, that we are not yet able to ascertain them with any precision; yet they are of the utmost importance to the good performance and improvement of the engine, and therefore deserve a particular consideration.

The counter weight has not only to push down the pump rods, but also to drag up the great piston. This it cannot do unless the steam be admitted into the cylinder. If the steam be no stronger than common air, it cannot enter the cylinder except in consequence of the piston's being dragged up. If common air were admitted into the cylinder, some force would be required to drag up the piston, in the same manner as it is required to draw up the piston of a common syringe; for the air would rush through the small entry of the cylinder in the same manner as through the small nozzle of the syringe. Some part of the atmospheric pressure is employed in driving in the air with sufficient velocity to fill the syringe, and it is only with the remainder that the admitted air presses on the under surface of the syringe. Therefore some of the atmospheric pressure on its upper surface is not balanced. This is felt by the hand which draws it up. The same thing must happen in the steam-engine, and some part of the counter weight is expended in drawing up the steam-piston. We could tell how much is thus expended if we knew the density of the steam; for this would tell us the velocity with which its elasticity would cause it to fill the cylinder. If we suppose it 12 times rarer than air, which it certainly is, and the piston rises to the top of the cylinder in two seconds, we can demonstrate that it will enter with a velocity not less than 1450 feet per second, whereas 300 feet is enough to make it maintain a density $\frac{3}{4}$th of that of steam in equilibrium with the air. Hence it follows, that its elasticity will not be less than $\frac{3}{4}$th of the elasticity of the air, and therefore not more than $\frac{3}{4}$th of counter weight will be expended in drawing up the steam-piston.

But all this is on the supposition that there is an unbounded supply of steam of undiminished elasticity. This is by no means the case. Immediately before opening the steam-cock, the steam was filling through the safety-valve and all the crevices in the top of the boiler, and (in good engines) was about $\frac{3}{4}$th stronger or more elastic than air. This had been gathering during something more than the descent of the piston, viz. in about three seconds. The piston rises to the top in about two seconds; therefore about twice and a half as much steam as fills the dome of the boiler is now shared between the boiler and cylinder. The dome is commonly about five times more capacious than the cylinder. If therefore no steam is condensed in the cylinder, the density of the steam, when the piston has reached the top, must be about $\frac{3}{4}$th of its former density, and still more elastic than air. But as much steam is condensed by the cold cylinder, its elasticity must be less than this. We cannot tell how much less, both because we do not know how much is thus condensed, and because by this diminution of its pressure on the surface of the boiling water, it must be more copiously produced in the boiler; but an attentive observation of the engine will give us some information. The moment the steam-cock is opened we have a strong puff of steam through the stuffing valve. At this time, therefore, it is still more elastic than air; but after this, the stuffing valve remains shut during the whole rise of the piston, and no steam any longer issues through the safety-valve or crevices; nay, the whole dome of the boiler may be observed to sink.

These facts give abundant proof that the elasticity of the steam during the ascent of the piston is greatly diminished, and therefore much of the counter weight is expended in dragging up the steam piston in opposition to the unbalanced part of the atmospheric pressure. The piston motion of the returning stroke is therefore so much decreased by this foreign and unappreciated circumstance, that it would have been quite useless to engage in the intricate exponential investigation, and we must fit down contented with a less perfect adjustment of the counter weight and weight of water.—Any person who attends to the motion of a steam-engine will perceive that the descent of the pump-rods is so far from being accelerated, that it is nearly uniform, and frequently it is sensibly retarded towards the end. We learn by the way, that it is of the utmost importance not only to have a quick production of steam, but also a very capacious dome, or empty space above the water in the boiler. In engines where this space was but four or five times the capacity of the cylinder, we have always observed a very sensible check given to the descent of the pump-rods after having made half their stroke. This obliges us to employ a greater counter weight, which diminishes the column of water, or retards the working stroke; it also obliges us to employ a stronger steam, at the risk of bursting the boiler, and increases the expense of fuel.

It would be a most desirable thing to get an exact knowledge of the elasticity of the steam in the cylinder; and this is by no means difficult. Take a long glass tube exactly calibrated, and close at the farther end. Put in the cylinder a small drop of some coloured fluid into it, so as to stand inside at the middle nearly.—Let it be placed in a long box filled with water to keep it of a constant temperature. Let the open end communicate with the cylinder, with a cock between. The moment the steam-cock is opened, open the cock of this instrument. The drop will be pushed towards the closed end of the tube, while the steam in the cylinder is more elastic than the air, and it will be drawn the other way while it is less elastic, and, by a scale properly adapted to it, the elasticity of the steam corresponding to every position of the piston may be discovered. The same thing may be done more accurately by a barometer properly constructed, so as to prevent the oscillations of the mercury.

It is equally necessary to know the state of the cylinder during the descent of the steam-piston. We have hitherto supposed P to be the full pressure of the atmosphere on the area of the piston, supposing the vacuum below it to be complete. But the inspection of our table of elasticity shows that this can never be the case, because the cylinder is always of a temperature far above 32°. We have made many attempts to discover its temperature. We have employed a thermometer in close contact with the side of the cylinder, which soon acquired a steady temperature; this was never less than 145°. We have kept a thermometer in the water which lies on the piston; this never sunk below 135°. It is probable that the cylinder within may be cooled somewhat lower; but for this opinion we cannot give any very satisfactory reason. Suppose it cooled down to 120°; this will leave an elasticity which would support three inches of mercury. We cannot think therefore that the unbalanced pressure of the atmosphere exceeds that of 27 inches of mercury, which is about 13½ pounds on a square inch, or 10½ on a circular inch. And this is the value which we should employ in the equation \( P = L + y \). This question may be decided in the same way as the other, by a barometer connected with the inside of the cylinder.

And thus we shall learn the state of the moving forces in every moment of the performance, and the machine will then be as open to our examination as any water or horse mill; and till this be done, or something equivalent, we can only guess at what the machine is actually performing, and we cannot tell in what particulars we can lend it a helping hand. We are informed that Messrs Watt and Boulton have made this addition to some of their engines; and we are persuaded that, from the information which they have derived from it, they have been enabled to make the curious improvements from which they have acquired so much reputation and profit.

There is a circumstance of which we have as yet taken no notice, viz. the quantity of cold water injected. Here we confess ourselves unable to give any precise instructions. It is clear at first sight that no more than is absolutely necessary should be injected. It must generally be supplied by the engine, and this expands part of its power. An excess is much more hurtful by cooling the cylinder and piston too much, and therefore wasting steam during the next rise of the piston. But the determination of the proper quantity requires a knowledge, which we have not yet acquired, of the quantity of heat contained in the steam in a latent form. As much water must be injected as will absorb all this without rising near to the boiling temperature. But it is of much more importance to know how far we may cool the cylinder with advantage; that is, when will the loss of steam, during the next rise of the piston, compensate for the diminution of its elasticity during its present descent? Our table of elasticities shows us, that by cooling the cylinder to 120°, we still leave an elasticity equal to one-tenth of the whole power of the engine; if we cool it only to 140°, we leave an elasticity of one-fifth; if we cool it to a blood-heat, we leave an elasticity of one-twentieth. It is extremely difficult to choose among these varieties. Experience, however, informs us, that the best engines are those which use the smallest quantities of injection water. We know an exceedingly good engine having a cylinder of 30 inches and a six feet stroke, which works with something less than one-fifth of a cubic foot of water at each injection; and we imagine that the quantity should be nearly in the proportion of the capacity of the cylinder. Deaguliers observed, that a very good engine, with a cylinder of 32 inches, worked with 300 inches of water at each injection, which does not much exceed one-fifth of a cubic foot. Mr Watt's observations, by means of the barometer, must have given him much valuable information in this particular, and we hope that he will not always withhold them from the public.

We have gone thus far in the examination, in order to ascertain the motion of the engine when examined, loaded and balanced in any known manner, and in order to discover that proportion between the moving power and the load which will produce the greatest attention to the quantity of work. The result has been very unsatisfactory to the factory, because the computation of the returning stroke principal is acknowledged to be beyond our abilities. But it has circumstanced given us the opportunity of directing the reader's attention to the leading circumstances in this inquiry. By knowing the internal state of the cylinder in machines of very different goodness, we learn the connection between the state of the steam and the performance of the machine; and it is very possible that the result of a full examination may be, that in situations where fuel is expensive, it may be proper to employ a weak steam which will expend less fuel, although less work is performed by it. We shall see this confirmed in the clearest manner in some particular employments of the new engines invented by Watt and Boulton.

In the mean time, we see that the equation which we gave from the celebrated Abbé Boifut, is in every respect erroneous even for the purpose which he had in view. We also see that the equation which we substituted in its place, and which was intended for determining that proportion between the counter-weight and the moving force, and the load which would render the working stroke and returning stroke of equal duration, is also erroneous, because these two motions are extremely different in kind, the one being nearly uniform, and the other nearly uniformly accelerated. This being supposed true, it should follow that the counter-weight should be reduced to one half; and we have found this to be very nearly true in some good engines which we have examined.

We shall add but one observation more on this head. The practical engineers have almost made it a maxim, that the two motions are of equal duration. But the only reason which we have heard for the maxim, is that it is awkward to see an engine go otherwise. But equal duration, without being able to give any accurate determination, we think that the engine will do more work if the working stroke be made slower than the returning stroke. Suppose the engine so constructed that they are made in equal times; an addition to the counter-weight will accelerate the returning stroke and retard the working stroke. But as the counter-weight is but small in proportion to the unbalanced portion of the atmospheric pressure, which is the moving force of the machine, it is evident that this addition to the counter-weight must bear a much greater proportion to the counter-weight than it does to the moving force, and must therefore accelerate the returning stroke much more than it retards the working stroke, and the time of both strokes taken together must be diminished by this addition and the performance of the machine improved; and this must be the case as long as the machine is not extravagantly loaded. The best machines which we have seen, in respect of performance, raise a column of water whose weight is very nearly two-thirds of the pressure of the atmosphere. atmosphere on the piston, making 11 strokes of six feet each per minute, and the working stroke was almost twice as long as the other. This engine had worked pumps of 12 inches, which were changed for pumps of 14 inches, all other things remaining the same. In its former state it made from 12 and a half to 13 and a half strokes per minute, the working stroke being considerably slower than the returning stroke. The load was increased, by the change of the pumps, nearly in the proportion to three to four. This had retarded the working stroke; but the performance was evidently increased in the proportion of $3 \times 13$ to $4 \times 11$, or from 39 to 44. About 300 pounds were added to the counterweight, which increased the number of strokes to more than 12 per minute. No sensible change could be observed in the time of the working stroke. The performance was therefore increased in the proportion of 39 to 48. We have therefore no hesitation in saying, that the seeming equality of the two strokes is a sacrifice to fancy. The engineer who observes the working stroke to be slow, fears that his engine may be thought feeble and unequal to its work; a similar notion has long misled him in the construction of watermills, especially of overshot mills; and, even now, he is submitting with hesitation and fear to the daily correction of experience.

It is needless to engage more deeply in scientific calculations in a subject where so many of the data are so very imperfectly understood.

We venture to recommend as a maxim of construction (supposing always a large boiler and plentiful supply of pure steam unmixed with air), that the load of work be not less than 10 pounds for every square inch of the piston, and the counter-weight so proportioned that the time of the returning stroke may not exceed two-thirds of that of the working stroke. A serious objection may be made to this maxim, and it deserves mature consideration. Such a load requires the utmost care of the machine, that no admission be given to the common air; and it precludes the possibility of its working, in case the growth of water, or deepening of the pit, should make a greater load absolutely necessary. These considerations must be left to the prudence of the engineer. The maxim now recommended relates only to the best actual performance of the engine.

Before quitting this machine, it will not be amiss to give some easy rules, sanctioned by successful practice, for computing its performance. These will enable any artist, who can go through simple calculations, to suit the size of his engine to the task which it is to perform.

The circumstance on which the whole computation must be founded is the quantity of water which must be drawn in a minute, and the depth of the mine; and the performance which may be expected from a good engine is at least 12 strokes per minute of six feet each, working against a column of water whose weight is equal to half of the atmospheric pressure on the steam-piston, or rather to 7.64 pounds on every square inch of its surface.

It is most convenient to estimate the quantity of water in cubic feet, or its weight in pounds, recollecting that a cubic foot of water weighs $62\frac{1}{2}$ pounds. The depth of the pit is usually reckoned in fathoms of six feet, and the diameter of the cylinder and pump is usually reckoned in inches.

Let $Q$ be the quantity of water to be drawn per minute in cubic feet, and $f$ the depth of the mine in fathoms; let $c$ be the diameter of the cylinder, and $p$ that of the pump; and let us suppose the arms of the beam to be of equal length.

1. To find the diameter of the pump, the area of the piston in square feet is $\rho^2 \times \frac{0.7854}{144}$. The length of the column drawn in one minute is 12 times 6 or 72 feet, and therefore its solid contents is $\rho^2 \times \frac{72 \times 0.7854}{144}$ cubic feet, or $\rho^2 \times 0.3927$ cubic feet. This must be equal to $Q$; therefore $\rho^2$ must be $\frac{Q}{0.3927}$ or nearly $\frac{Q}{0.3927} \times 2\frac{1}{2}$. Hence this practical rule: Multiply the cubic feet of water which must be drawn in a minute by $2\frac{1}{2}$, and extract the square root of the product: this will be the diameter of the pump in inches.

Thus suppose that 58 cubic feet must be drawn every minute; 58 multiplied by $2\frac{1}{2}$ gives 145, of which the square root is 12, which is the required diameter of the pump.

2. To find the proper diameter of the cylinder.

The piston is to be loaded with 7.64 pounds on every square inch. This is equivalent to six pounds on a circular inch very nearly. The weight of a cylinder of water an inch in diameter and a fathom in height is $2\frac{1}{2}$ pounds, or nearly two pounds. Hence it follows that $6c^2$ must be made equal to $2f\rho^2$, and that $c^2$ is equal to $\frac{2f\rho^2}{6}$, or to $\frac{f\rho^2}{3}$.

Hence the following rule: Multiply the square of the diameter of the pump piston (found as above) by the fathoms of lift, and divide the product by 3; the square root of the quotient is the diameter of the cylinder.

Suppose the pit to which the foregoing pump is to be applied is 24 fathoms deep; then $\frac{24 \times 144}{3}$ gives 1152, of which the square root is 34 inches very nearly.

This engine constructed with care will certainly do the work.

Whatever is the load of water proposed for the engine, let 10 be the pounds on every circular inch of the steam piston, and make $c^2 = \rho^2 \times \frac{2f}{m}$, and the square root will be the diameter of the steam piston in inches.

To free the practical engineer as much as possible from all trouble of calculation, we subjoin the following Table of the Dimensions and Power of the Steam Engine, drawn up by Mr Beighton in 1717, and fully verified by practice since that time. The measure is in English ale gallons of 282 cubic inches. The first part of the table gives the size of the pump suited to the growth of water. The second gives the size of the cylinder suited to the load of water. If the depth is greater than any in this table, take its fourth part, and double the diameter of the cylinder. Thus if 150 hogheads are to be drawn in an hour from the depth of 100 fathoms, the last column of part first gives for 149.40 a pump of seven inches bore. In a line with this, under the depth of 50 yards, which is one fourth of 100 fathoms, we find 20½, the double of which is 41 inches for the diameter of the cylinder.

It is almost impossible to give a general rule for strokes of different lengths, &c., but any one who professes the ability to erect an engine, should surely know as much arithmetic as will accommodate the rule now given to any length of stroke.

We venture to say, that no ordinary engineer can tell *a priori* the number per minute which an engine will give. We took 12 strokes of six feet each for a standard, which a careful engineer may easily accomplish, and which an employer has a right to expect, the engine being loaded with water to half the pressure of the atmosphere: if the load be less, there is some fault—an improper counter weight, or too little boiler, or leaks, &c., &c.

Such is the state in which Newcomen's steam-engine had continued in use for 60 years, neglected by the philosopher, although it is the most curious object which human ingenuity has yet offered to his contemplation, and abandoned to the efforts of the unlettered artif. Its use has been entirely confined to the raising of water. Mr Keans Fitzgerald indeed published in the Philosophical Transactions a method of converting its reciprocating motion into a continued rotatory motion by employing the great beam to work a crank or a train of wheel-work.

As the real action of the machine is confined to its working stroke, to accomplish this, it became necessary to connect with the crank or wheeled work a very large and heavy fly, which should accumulate in itself the whole pressure of the machine during its time of action, and therefore continue in motion, and urge forward the working machinery, while the steam engine was going through its inactive returning stroke. This will be the case, provided that the resistance exerted by the working machine during the whole period of the working and returning stroke of the steam-engine, together with the friction of both, does not exceed the whole pressure exerted by the steam-engine during its working stroke; and provided that the momentum of the fly, arising from its great weight and velocity, be very great, so that the resistance of the work during one returning stroke of the steam-engine do not make any very sensible diminution of the velocity of the fly. This is evidently possible and easy. The fly may be made of any magnitude; and being exactly balanced round its axis, it will soon acquire any velocity consistent with the motion of the steam-engine. During the working stroke of the engine it is uniformly accelerated, and by its acquired momentum it produces in the beam the movement of the returning stroke; but in doing this, its momentum is shared with the inert matter of the steam-engine, and consequently its velocity diminished, but not entirely taken away. The next working stroke therefore, by prelading on it afresh, increases its remaining velocity by a quantity nearly equal to the whole that it acquired during the first stroke. We say nearly, but not quite equal, because the time of the second working stroke must be shorter than that of the first, on account of the velocity already in the machine. In this manner the fly will be more and more accelerated every succeeding stroke, because the pressure of the engine during the working stroke does more than restore to the fly the momentum which it lost in producing the returning movement of the steam-engine. Now suppose the working part of the machine to be added. The acceleration of the fly during each working stroke of the steam-engine will be less than it was before, because the impelling pressure is now partly employed in driving the working machine, and because the fly will lose more of its momentum during the returning stroke of the steam-engine, part of it being expended in driving the working machine. It is evident, therefore, that a time will come come when the successive augmentation of the fly's velocity will cease; for, on the one hand, the continual acceleration diminishes the time of the next working stroke, and therefore the time of action of the accelerating power. The acceleration must diminish in the same proportion; and on the other hand, the resistance of the working machine generally, though not always, increases with its velocity. The acceleration ceases whenever the addition made to the momentum of the fly during a working stroke of the steam-engine is just equal to what it loses by driving the machine, and by producing the returning movement of the steam-engine.

This must be acknowledged to be a very important addition to the engine, and though sufficiently obvious, it is ingenious, and requires considerable skill and address to make it effective (b).

The movement of the working machine, or mill of whatever kind, must be in some degree hobbling or unequal. But this may be made quite insensible, by making the fly exceedingly large, and disposing the greatest part of its weight in the rim. By these means its momentum may be made so great, that the whole force required for driving the mill and producing the returning movement of the engine may bear a very small proportion to it. The diminution of its velocity will then be very trifling.

No counter weight is necessary here, because the returning movement is produced by the inertia of the fly. A counter weight may, however, be employed, and should be employed, viz. as much as will produce the returning movement of the steam-engine. It will do this better than the same force accumulated in the fly; for this force must be accumulated in the fly by the intervention of rubbing parts, by which some of it is lost; and it must be afterwards returned to the engine with a similar loss. But, for the same reason, it would be improper to make the counter weight also able to drive the mill during the returning stroke.

By this contrivance Mr Fitzgerald hoped to render the steam-engine of more extensive use; and he, or others associated with him, obtained a patent excluding all others from employing the steam-engine for turning a crank. They also published proposals for erecting mills of all kinds driven by steam engines, and stated very fairly their powers and their advantages. But their proposals do not seem to have acquired the confidence of the public; for we do not know of any mill ever having been erected under this patent.

The great obstacle to this extensive use of the steam-engine is the prodigious expense of fuel. An engine having a cylinder of four feet diameter, working night and day, consumes about 3400 chaldron (London) of good coals in a year.

This circumstance limits the use of steam-engines exceedingly. To draw water from coal-pits, where they can be stocked with unsaleable small coal, they are of universal employment; also for valuable mines, for raising the supplying a great and wealthy city with water, and a few other purposes where a great expense can be borne, steam-engines are very proper engines; but in a thousand cases, where their unlimited powers might be vastly serviceable, the enormous expense of fuel completely excludes them. We cannot doubt but that the attention of engineers was much directed to every thing that could promise a diminution of this expense. Every one had his particular nostrum for the construction of his furnace, and some were undoubtedly more successful than others. But science was not yet sufficiently advanced: It was not till Dr Black had made his beautiful discovery of latent heat, that we could know the intimate relation between the heat expended in boiling off a quantity of water and the quantity of steam that is produced.

Much about the time of this discovery, viz. 1763, Mr James Watt, established in Glasgow in the commercial line, was amusing himself with repairing a working model of the steam engine which belonged to the philosophical apparatus of the university. Mr Watt was a person of a truly philosophical mind, eminently conversant in all branches of natural knowledge, and the pupil and intimate friend of Dr Black. In the course of the above-mentioned amusement many curious facts in the production and condensation of steam occurred to him; and among others, that remarkable fact discovers which is always appealed to by Dr Black as the proof of the immense quantity of heat which is contained in a very minute quantity of water in the form of elastic quantity steam. When a quantity of water is heated several degrees above the boiling point in a close digester, if a hole be opened, the steam rushes out with prodigious violence, and the heat of the remaining water is reduced, in the course of three or four seconds, to the boiling temperature. The water of the steam which has issued amounts only to a very few drops; and yet these have carried off with them the whole excess of heat from the water in the digester.

Since then a certain quantity of steam contains so in his at-great a quantity of heat, it must expend a great quantity of fuel; and no construction of furnace can prevent this. Mr Watt therefore set his invention to work to discover methods of husbanding this heat. The cylinder of his little model was heated almost in an instant, so that it could not be touched by the hand. It could not be otherwise, because it condensed the vapour by attracting its heat. But all the heat thus communicated to the cylinder, and wafted by it on surrounding bodies, contributed nothing to the performance of the engine,

(b) We do not recollect at present the date of this proposal of Mr Fitzgerald; but in 1781 the Abbé Arnal, canon of Alais in Languedoc, entertained a thought of the same kind, and proposed it for working lighters in the inland navigations; a scheme which has been successfully practised (we are told) in America. His brother, a major of engineers in the Austrian service, has carried the thing much farther, and applied it to manufactures; and the Aulic Chamber of Mines at Vienna has patronized the project: (See Journal Encyclopédique, 1781). But these schemes are long posterior to Mr Fitzgerald's patent, and are even later than the erection of several machines driven by steam-engines which have been erected by Messrs Watt and Boulton. We think it our duty to state these particulars, because it is very useful for our neighbours on the continent to assume the credit of British inventions. engine, and must be taken away at every injection, and again communicated and waited. Mr Watt quickly understood the whole process which was going on within the cylinder, and which we have considered so minutely, and saw that a very considerable portion of the steam must be wasted in warming the cylinder. His first attempts were made to ascertain how much was thus wasted, and he found that it was not less than three or four times as much as would fill the cylinder and work the engine. He attempted to diminish this waste by using wooden cylinders. But though this produced a sensible diminution of the waste, other reasons forced him to give them up. He then used his metal cylinders in a wooden case with light wood ashes between. By this, and using no more injection than was absolutely necessary for the condensation, he reduced the waste almost half. But by using too small a quantity of cold water, the inside of the cylinder was hardly brought below the boiling temperature; and there consequently remained in it a steam of very considerable elasticity, which robbed the engine of a proportional part of the atmospheric pressure. He saw that this was unavoidable as long as the condensation was performed in the cylinder. The thought struck him to attempt the condensation in another place. His first experiment was made in the simplest manner. A globular vessel communicated by means of a long pipe of one inch diameter with the bottom of his little cylinder of four inches diameter and 30 inches long. This pipe had a stop-cock, and the globe was immersed in a vessel of cold water. When the piston was at the top, and the cylinder filled with strong steam, he turned the cock. It was scarcely turned, nay he did not think it completely turned, when the sides of his cylinder (only strong tin-plate) were crushed together like an empty bladder. This surprised and delighted him. A new cylinder was immediately made of brass sufficiently thick, and nicely bored. When the experiment was repeated with this cylinder, the condensation was so rapid, that he could not say that any time was expended in it. But the most valuable discovery was, that the vacuum in the cylinder was, as he hoped, almost perfect. Mr Watt found, that when he used water in the boiler purged of air by long boiling, nothing that was very sensibly inferior to the pressure of the atmosphere on the piston could hinder it from coming quite down to the bottom of the cylinder. This alone was gaining a great deal, for in most engines the remaining elasticity of the steam was not less than one-eighth of the atmospheric pressure, and therefore took away one-eighth of the power of the engine.

Having gained this capital point, Mr Watt found many difficulties to struggle with before he could get the machine to continue its motion. The water produced from the condensed steam, and the air which was extricated from it, or which penetrated through unavoidable leaks, behaved to accumulate in the condensing vessel, and could not be voided in any way similar to that adopted in Newcomen's engine. He took another method: He applied pumps to extract both, which were worked by the great beam. The convenience is easy to any good mechanic; only we must observe, that the piston of the water-pump must be under the surface of the water in the condenser, that the water may enter the pump by its own weight, because there is no atmospheric pressure there to force it in. We must also observe, that a considerable force is necessarily expended here, because, as there is but one stroke for rarefying the air, and this rarefaction must be nearly complete, the air-pump must be of large dimensions, and its piston must act against the whole pressure of the atmosphere. Mr Watt, however, found that this force could be easily spared from his machine, already so much improved in respect of power.

Thus has the steam-engine received a very considerable improvement. The cylinder may be allowed to remain very hot; nay, boiling hot, and yet the condensation be completely performed. The only elastic steam that now remains is the small quantity in the pipe of communication. Even this small quantity Mr Watt at last got rid of, by admitting a small jet of cold water up this pipe to meet the steam in its passage to the condenser. This both cooled this part of the apparatus in a situation where it was not necessary to warm it again, and it quickened the condensation. He found at last that the small pipe of communication was of itself sufficiently large for the condensation, and that no separate vessel, under the name of condenser, was necessary. This circumstance shows the prodigious rapidity of the condensation. We may add, that unless this had been the case, his improvement would have been vastly diminished; for a large condenser would have required a much larger air-pump, which would have expended much of the power of the engine. By these means the vacuum below the piston is greatly improved: for it will appear, clear to anyone who understands the subject, that as long as any part of the condenser is kept of a low temperature, it will abstract and condense the vapour from the warmer parts, till the whole acquires the elasticity corresponding to the coldest part. By the same means much of the waste is prevented, because the cylinder is never cooled much below the boiling temperature. Many engines have been erected by Mr Watt in this form, and their performance gave universal satisfaction.

We have contented ourselves with giving a very slight description without a figure of this improved engine, because we imagine it to be of very easy comprehension, and because it is only a preparation for still greater improvements, which, when understood, will at the same time leave no part of this more simple form unexplained.

During the progress of these improvements Mr Watt made many experiments on the quantity and density of the steam of boiling water. These fully convinced him, that although he had greatly diminished the waste of the force steam, a great deal yet remained, and that the steam of steam expended during the rise of the piston was at least three times more than what would fill the cylinder. The cause of this was very apparent. In the subsequent descent of the piston, covered with water much below the boiling temperature, the whole cylinder was necessarily cooled and exposed to the air. Mr Watt's fertile genius immediately suggested to him the expedient of employing the elasticity of the steam from the boiler to impel the piston down the cylinder, in place of the pressure of the atmosphere; and thus he restored the engine to its first principles, making it an engine really moved by steam. As this is a new epoch in its history, we shall be more particular in the description; at the same fame time still restricting ourselves to the essential circumstances, and avoiding every peculiarity which is to be found in the prodigious varieties which Mr Watt has introduced into the machines which he has erected, every individual of which has been adapted to local circumstances, or diversified by the progress of Mr Watt's improvements.

Let A (fig. 9) represent the boiler. This has received great improvements from his complete acquaintance with the procedure of nature in the production of steam. In some of his engines the fuel has been placed in the midst of the water, surrounded by an iron or copper vessel, while the exterior boiler was made of wood, which transmits, and therefore wastes the heat very slowly. In others, the flame not only plays round the whole outside, as in common boilers, but also runs along several flues which are conducted through the midst of the water. By such contrivances the fire is applied to the water in a most extensive surface, and for a long time, so as to impart to it the greatest part of its heat. So skilfully was it applied in the Albion mills, that although it was perhaps the largest engine in the kingdom, its unconfined smoke was inferior to that of a very small brew-house. In this second engine of Mr Watt, the top of the cylinder is shut up by a strong metal plate g h, in the middle of which is a collar or box of leathers k l, formed in the usual manner of a jack-head pump, through which the piston rod PD, nicely turned and polished, can move up and down, without allowing any air to pass by its sides. From the dome of the boiler proceeds a large pipe BCIOQ, which, after reaching the cylinder with its horizontal part BC, descends parallel to its side, feeding off two branches, viz. IM to the top of the cylinder, and ON to its bottom. At I is a puppet valve opening from below upwards. At L, immediately below this branch, there is a similar valve, also opening from below upwards. The pipe descends to Q, near the bottom of a large cistern c d e f, filled with cold water constantly renewed. The pipe is then continued horizontally along the bottom of this cistern (but not in contact), and terminates at R in a large pump ST. The piston S has clack valves opening upwards, and its rod Sr, passing through a collar of leathers at T, is suspended by a chain to a small arch head on the outer arm of the beam. There is a valve R in the bottom of this pump, as usual, which opens when pressed in the direction QR, and shuts against a contrary pressure. This pump delivers its contents into another pump XY, by means of the small pipe t X, which proceeds from its top. This second pump has a valve at X, and a clack in its piston Z as usual, and the piston rod Z s is suspended from another arch head on the outer arm of the beam. The two valves I and L are opened and shut by means of spanners and handles, which are put in motion by a plug frame, in the same manner as in Newcomen's engine.

Lastly, there may be observed a crooked pipe a b o, which enters the upright pipe laterally a little above Q. This has a small jet hole at o; and the other end a, which is considerably under the surface of the water of the condensing cistern, is covered with a puppet valve v, whose long stalk v u rises above the water, and may be raised or lowered by hand or by the pump beam. The valves R and X, and the clacks in the pistons S and Z, are opened or shut by the pressures to which they are immediately exposed.

This figure is not an exact copy of any of Mr Watt's engines, but has its parts so disposed that all may come distinctly into view, and exactly perform their various functions. It is drawn in its quiescent position, the outer end of the beam preponderating by the counter weight, and the piston P at the top of the cylinder, and the pistons S and Z in their lowest situations.

In this situation let us suppose that a vacuum is (by any means) produced in all the space below the piston, the valve I being shut. It is evident that the valve R will also be shut, as also the valve v. Now let the valve I be opened. The steam from the boiler, as elastic as common air, will rush into the space above the piston, and will exert on it a pressure as great as that of the atmosphere. It will therefore press it down, raise the outer end of the beam, and cause it to perform the same work as an ordinary engine.

When the piston P has reached the bottom of the cylinder, the plug frame shuts the valve I, and opens L. By so doing the communication is open between the top and bottom of the cylinder, and nothing hinders the steam which is above the piston from going along the passage MLON. The piston is now equally affected on both sides by the steam, even though a part of it is continually condensed by the cylinder, and in the pipe IOQ. Nothing therefore hinders the piston from being dragged up by the counter weight, which acts with its whole force, undiminished by any remaining unbalanced elasticity of steam. Here therefore this form of the engine has an advantage (and by no means a small one) over the common engines, in which a great part of the counter weight is expended in overcoming unbalanced atmospheric pressure.

Whenever the piston P arrives at the top of the cylinder, the valve L is shut by the plug frame, and the valves I and v are opened. All the space below the piston is at this time occupied by the steam which came from the upper part of the cylinder. This being a little wasted by condensation, is not quite a balance for the pressure of the atmosphere. Therefore, during the ascent of the piston, the valve R was shut, and it remains so. When, therefore, the valve v is opened, the cold water of the cistern must spout up through the hole o, and condense the steam. To this must be added the coldness of the whole pipe OQS. As fast as it is condensed, its place is supplied by steam from the lower part of the cylinder. We have already remarked, that this successive condensation is accomplished with astonishing rapidity. In the mean time, steam from the boiler presses on the upper surface of the piston. It must therefore descend as before, and the engine must perform a second working stroke.

But in the mean time the injection water lies in the bottom of the pipe OQR, heated to a considerable degree by the condensation of the steam; also a quantity of air has been disengaged from it and from the water in the boiler. How is this to be discharged?—This is the office of the pumps ST and XY. The capacity of ST is very great in proportion to the space in which the air and water are lodged. When, therefore, the piston S has got to the top of its course, there must be a vacuum in the barrel of this pump, and the water and air must open the valve R and come into it. When the piston piston S comes down again in the next returning stroke, this water and air gets through the valve of the piston; and in the next working stroke they are discharged by the piston into the pump XY, and raised by its piston. The air escapes at Y, and as much of the water as is necessary is delivered into the boiler by a small pipe Yg to supply its wants. It is a matter of indifference whether the pistons S and Z rise with the outer or inner end of the beam, but it is rather better that they rise with the inner end. They are otherwise drawn here, in order to detach them from the rest and show them more distinctly.

Such is Mr Watt's second engine. Let us examine its principles, that we may see the causes of its avowed and great superiority over the common engines.

We have already seen one ground of superiority, the full operation of the counter weight. We are authorized by careful examination to say, that in the common engines at least one-half of the counter weight is expended in counteracting an unbalanced pressure of the air on the piston during its ascent. In many engines, which are not the worst, this extends to 1/3 of the whole pressure. This is evident from the examination of the engine at Montrelais by Boilat. This makes a very great counter weight necessary, which exhausts a proportional part of the moving force.

But the great advantage of Mr Watt's form is the almost total annihilation of the waste of steam by condensation in the cylinder. The cylinder is always boiling hot, and therefore perfectly dry. This must be evident to any person who understands the subject. By the time that Mr Watt had completed his improvements, his experiments on the production of steam had given him a pretty accurate knowledge of its density; and he found himself authorized to say, that the quantity of steam employed did not exceed twice as much as would fill the cylinder, so that not above one-half was unavoidably wasted. But before he could bring the engine to this degree of perfection, he had many difficulties to overcome: He inclosed the cylinder in an outer wooden case at a small distance from it. This diminished the expense of heat by communication to surrounding bodies. Sometimes he allowed the steam from the boiler to occupy this interval. This undoubtedly prevented all dissipation from the inner cylinder; but in its turn it dissipated much heat by the outer case, and a very sensible condensation was observed between them. This has occasioned him to omit this circumstance in some of his best engines. We believe it was omitted in the Albion mills.

The greatest difficulty was to make the great piston tight. The old and effectual method, by water lying on it, was inadmissible. He was therefore obliged to have his cylinders most nicely bored, perfectly cylindrical, and finely polished; and he made numberless trials of different soft substances for packing his piston, which should be tight without enormous friction, and which should long remain so, in a situation perfectly dry, and hot almost to burning.

After all that Mr Watt has done in this respect, he thinks that the greatest part of the waste of steam which he still perceives in his engines arises from the unavoidable escape by the sides of the piston during its descent.

But the fact is, that an engine of this construction, of the same dimensions with a common engine, making the same number of strokes of the same extent, does not consume above one-fourth part of the fuel that is consumed by the best engines of the common form. It is also a very fortunate circumstance, that the performance of the engine is not immediately destroyed, nor indeed sensibly diminished, by a small want of tightness in the piston. In the common engine, if air get in, in this way, it immediately puts a stop to the work; but although even a considerable quantity of steam get past the piston during its descent, the rapidity of condensation is such, that hardly any diminution of pressure can be observed.

Mr Watt's penetration soon discovered another most valuable property of this engine. When an engine of the common form is erected, the engineer must make an accurate estimate of the work to be performed, and must proportion his engine accordingly. He must be careful that it be fully able to execute its task; but its power must not exceed its load in any extravagant degree. This would produce a motion which is too rapid, and which, being alternately in opposite directions, would occasion jolts which no building or machinery could withstand. Many engines have been thwarted by the pumps drawing air, or a pump-rod breaking; by which accidents the steam-piston defends with such rapidity that every thing gives way. But in most operations of mining, the task of the engine increases, and it must be so constructed at first as to be able to bear this addition. It is very difficult to manage an engine that is much superior to its task; and the easiest way is, to have it almost full loaded, and to work it only during a few hours each day, and allow the pit water to accumulate during its repose. This increases the first cost, and wastes fuel during the inaction of the engine.

But this new engine can at all times be exactly fitted, so that it (at least during the working stroke) to the load of work can always be exactly minister steam of a proper elasticity. At the first erection the load the engine may be equal to twice its task, if the steam admitted above the cylinder be equal to that opens to be common boiling water; but when once the ebullition on it is fairly commenced, and the whole air expelled from all parts of the apparatus, it is evident, that by damping the fire, steam of half this elasticity may be continually supplied, and the water will continue boiling although its temperature does not exceed 184° of Fahrenheit's thermometer. This appears by inspecting our table of vaporous elasticity, and affords another argument for rendering that table more accurate by new experiments. We hope that Mr Watt will not withhold from the public the knowledge which he has acquired on this subject. It may very possibly result from an accurate investigation, that it would be advisable to work our steam-engines with weak steams, and that the diminution of work may be more than compensated by the diminution of fuel. It is more probable indeed, and it is Mr Watt's opinion, that the contrary is the case, and that it is much more economical to employ great heats. At any rate, the decision of this question is of great importance for improving the engine; and we see, in the mean time, that the engine can at all times be fitted so as to perform its task with a moderate and manageable motion, and that as the task increases we can increase the power of the engine. But the method now proposed has a great inconvenience. While the steam is weaker than the atmosphere, there is an external force tending to squeeze in the sides and bottom of the boiler. This could not be resisted when the difference is considerable, and common air would rush in through every crevice of the boiler and soon choke the engine: it must therefore be given up.

But the same effect will be produced by diminishing the passage for the steam into the cylinder. For this purpose, the puppet valve by which the steam enters the cylinder was made in the form of a long taper spigot, and it was lodged in a cone of the same shape; consequently the passage could be enlarged or contracted at pleasure by the distance to which the inner cone was drawn up.

In this way several engines were constructed, and the general purpose of fitting the power of the engine to its task was completely answered; but (as the mathematical reader will readily perceive) it was extremely difficult to make this adjustment precise and constant. In a great machine like this going by jerks, it was hardly possible that every successive motion of the valve should be precisely the same. This occasioned very sensible irregularities in the motion of the engine, which increased and became hazardous when the joints worked loose by long use.

Mr Watt's genius, always fertile in resources, found out a complete remedy for all these inconveniences. Making the valve of the ordinary form of a puppet clock, he adjusted the button of its stalk or tail so that it should always open full to the same height. He then regulated the pins of the plug-frame, in such a manner that the valve should shut the moment that the piston had descended a certain proportion (suppose one-fourth, one-third, one-half, &c.) of the cylinder. So far the cylinder was occupied by steam as elastic as common air. In pressing the piston farther down, it behoved the steam to expand, and its elasticity to diminish. It is plain that this could be done in any degree we please, and that the adjustment can be varied in a minute, according to the exigency of the case, by moving the plug pins.

In the mean time, it must be observed, that the pressure on the piston is continually changing, and consequently the accelerating force. The motion therefore will no longer be uniformly accelerated: it will approach much faster to uniformity; may, it may be retarded, because although the pressure on the piston at the beginning of the stroke may exceed the resistance of the load, yet when the piston is near the bottom the resistance may exceed the pressure. Whatever may be the law by which the pressure on the piston varies, an ingenious mechanic may contrive the connecting machinery in such a way that the chains or rods at the outer end of the beam shall continually exert the same pressure, or shall vary their pressure according to any law he finds most convenient. It is in this manner that the watchmaker, by the form of the fusee, produces an equal pressure on the wheel-work by means of a very unequal action of the main-spring. In like manner, by making the outer arch heads portions of a proper spiral instead of a circle, we can regulate the force of the beam at pleasure.

Thus we see how much more manageable an engine is in this form than Newcomen's was, and also more easily investigated in respect of its power in its various positions. The knowledge of this last circumstance was of mighty consequence, and without it no notion could be formed of what it could perform. This suggested to Mr Watt the use of the barometer communicating with the cylinder; and by the knowledge acquired by these means has the machine been so much improved by its ingenious inventor.

We must not omit in this place one deduction made by Mr Watt from his observations, which may be called a discovery of great importance in the theory of the engine.

Let ABCD (fig. 10.) represent a section of the cylinder of a steam-engine, and EF the surface of its pit of great size. Let us suppose that the steam was admitted important while EF was in contact with AB, and that as soon as in it had pressed it down to the situation EF the steam theory of the engine is shut. The steam will continue to press down, Fig. 10., and as the steam expands its pressure diminishes. We may express its pressure (exerted all the while the piston moves from the situation AB to the situation EF) by the line EF. If we suppose the elasticity of the steam proportional to its density, as is nearly the case with air, we may express the pressure on the piston in any other position, such as KL or DC, by K/L and D/C, the ordinates of a rectangular hyperbola F/c, of which AE, AB are the asymptotes, and A the centre. The accumulated pressure during the motion of the piston from EF to DC will be expressed by the area EFcDE, and the pressure during the whole motion by the area ABFcDA.

Now it is well known that the area EFcDE is equal to ABFE multiplied by the hyperbolic logarithm of AD/AE = L. AD/AE, and the whole area ABFcDA is = ABFE × (1 + L. AD/AE).

Thus let the diameter of the piston be 24 inches, and the pressure of the atmosphere on a square inch be 14 pounds; the pressure on the piston is 6333 pounds. Let the whole stroke be 6 feet, and let the steam be stopped when the piston has descended 18 inches, or 1.5 feet. The hyperbolic logarithm of 6/1.5 is 1.3862943.

Therefore the accumulated pressure ABFcDA is = 6333 × 2.3862943 = 15114 pounds.

As few professional engineers are possessed of a table of hyperbolic logarithms, while tables of common logarithms are, or should be in the hands of every person who is much engaged in mechanical calculations, let the following method be practised. Take the common logarithm of AD/AE, and multiply it by 2.3026; the product is the hyperbolic logarithm of AD/AE.

The accumulated pressure while the piston moves from AB to EF is 6333 × 1, or simply 6333 pounds. Therefore the steam while it expands into the whole cylinder adds a pressure of 8781 pounds.

Suppose that the steam had got free admission during the whole descent of the piston, the accumulated pressure would have been 6333 × 4, or 25332 pounds.

Here Mr Watt observed a remarkable result. The steam expended in this case would have been four times greater. greater than when it was stopped at one-fourth, and yet the accumulated pressure is not twice as great, being nearly five-thirds. One-fourth of the steam performs nearly three-fifths of the work, and an equal quantity performs more than twice as much work when thus admitted during one-fourth of the motion.

This is a curious and an important information, and the advantage of this method of working a steam-engine increases in proportion as the steam is sooner stopped; but the increase is not great after the steam is rarefied four times. The curve approaches near to the axis, and small additions are made to the area. The expense of such great cylinders is considerable, and may sometimes compensate this advantage.

Let the steam be stopped at Its performance is mult.

| Fraction | Performance | |---------|-------------| | 1/3 | 1.7 | | 1/4 | 2.1 | | 1/5 | 2.4 | | 1/6 | 2.6 | | 1/8 | 2.8 | | 1/9 | 3 | | 1/10 | 3.2 | | &c. | &c. |

It is very pleasing to observe so many unlooked-for advantages resulting from an improvement made with the sole view of lessening the waste of steam by condensation. While this purpose is gained, we learn how to husband the steam which is not thus wasted. The engine becomes more manageable, and is more easily adapted to every variation in its task, and all its powers are more easily computed.

The active mind of this ingenious inventor did not stop here: It had always been matter of regret that one-half of the motion was unaccompanied by any work. It was a very obvious thing to Mr Watt, that as the steam admitted above the piston pressed it down, so steam admitted below the piston pressed it up with the same force, provided that a vacuum were made on its upper side. This was easily done, by connecting the lower end of the cylinder with the boiler and the upper end with the condenser.

Fig. 11. is a representation of this construction exactly copied from Mr Watt's figure accompanying his specification. Here BB is a section of the cylinder, surrounded at a small distance by the case IIII. The section of the piston A, and the collar of leathers which embraces the piston rod, gives a distinct notion of its construction, of the manner in which it is connected with the piston-rod, and how the packing of the piston and collar contributes to make all tight.

From the top of the cylinder proceeds the horizontal pipe. Above the letter D is observed the seat of the steam valve, communicating with the box above it. In the middle of this may be observed a dark shaded circle. This is the mouth of the upper branch of the steam pipe coming from the boiler. Beyond D, below the letter N, is the seat of the upper condensing valve. The bottom of the cylinder is made spherical, fitting the piston, so that they may come into entire contact. Another horizontal pipe proceeds from this bottom. Above the letter E is the seat of the lower steam valve, opening into the valve box. This box is at the extremity of another steam pipe marked C, which branches off from the upper horizontal part, and descends obliquely, coming forward to the eye. The lower part is represented as cut open, to show its interior conformation. Beyond this steam valve, and below the letter F, may be observed the seat of the lower condensing valve. A pipe descends from hence, and at a small distance below unites with another pipe GG, which comes down from the upper condensing valve N. These two education-pipes thus united go downwards, and open at L into a rectangular box, of which the end is seen at L. This box goes backward from the eye, and at its farther extremity communicates with the air-pump K, whose piston is here represented in section with its butterfly valves. The piston delivers the water and air laterally into another rectangular box M, darkly shaded, which box communicates with the pump I. The piston-rods of this and of the air pump are suspended by chains from a small arch head on the inner arm of the great beam. The lower part of the education-pipe, the horizontal box L, the air-pump K, with the communicating box M between it and the pump I, are all immersed in the cold water of the condensing cistern. The box L is made flat, broad, and shallow, in order to increase its surface and accelerate the condensation. But that this may be performed with the greatest expedition, a small pipe H, open below (but occasionally stopped by a plug valve), is inserted laterally into the education-pipe G, and then divides into two branches; one of which reaches within a foot or two of the upper valve N, and the other approaches as near to the valve F.

As it is intended by this construction to give the piston a strong impulse in both directions, it will not be proper to suspend its rod by a chain from the great beam; for it must not only pull down that end of the beam, but also push it upwards. It may indeed be suspended by double chains like the pistons of the engines for extinguishing fires; and Mr Watt has accordingly done so in some of his engines. But in his drawing from which this figure is copied, he has communicated the force of the piston to the beam by means of a toothed rack OO, which engages or works in the toothed sector QQ on the end of the beam. The reader will understand, without any farther explanation, how the impulse given to the piston in either direction is thus transmitted to the beam without diminution. The fly XX, with its pinion Y, which also works in the toothed arch QQ, may be supposed to be removed for the present, and will be considered afterwards.

We shall take the present opportunity of describing Mr Watt's method of communicating the force of the steam-engine to any machine of the rotatory kind. VV represents the rim and arms of a very large and heavy metallic fly. On its axis is the concentric toothed wheel U. There is attached to the end of the great beam a strong and stiff rod TT, to the lower end of which a toothed wheel W is firmly fixed by two bolts, so that it cannot turn round. This wheel is of the same size and in the same vertical plane with the wheel U; and an iron link or strap (which cannot be seen here, because it is on the other side of the two wheels) connects the centres of the two wheels, so that the one cannot quit the other. The engine being in the position represented in the figure, suppose the fly to be turned once round by any external force in the direction of the darts. It is plain, that since the toothed wheels cannot quit each other, being kept together by the link, the inner half (that is, the half next the cylinder) of the wheel U will work on the inner half of the wheel W, so that at the end of the revolution of the fly wheel W must have got to the top of the wheel U, and the outer end of the beam must be raised to its highest position. The next revolution of the fly will bring the wheel W and the beam connected with it to their first positions; and thus every two revolutions of the fly will make a complete period of the beam's reciprocating movements. Now, instead of supporting the fly to drive the beam, let the beam drive the fly. The motions must be perfectly same, and the ascent or descent of the piston will produce one revolution of the fly.

A side view of this apparatus is given in fig. 12, marked by the same letters of reference. This shows the situation of parts which were fore-shortened in fig. 11, particularly the descending branch C of the steam-pipe, and the situation and communications of the two pumps K and L. S, S is the horizontal part of the steam-pipe. G is a part of it whose box is represented by the dark circle of fig. 11. D is the box of the steam-clack; and the little circle at its corner represents the end of the axis which turns it, as will be described afterwards. N is the place of the upper eduction valve. A part only of the upper eduction-pipe G is represented, the rest being cut off, because it would have covered the descending steam-pipe CC. When continued down, it comes between the eye and the box E of the lower steam-valve, and the box F of the lower eduction-valve.

Let us now trace the operation of this machine through all its steps. Recurring to fig. 11, let us suppose that the lower part of the cylinder BB is exhausted of all elastic fluids; that the upper steam-valve D and the lower eduction-valve F are open, and that the lower steam-valve E and upper eduction-valve N are shut. It is evident that the piston must be pressed toward the bottom of the cylinder, and must pull down the end of the working beam by means of the toothed rack OO and sector QQ, causing the other end of the beam to urge forward the machinery with which it is connected. When the piston arrives at the bottom of the cylinder, the valves D and F are shut by the plug frame, and E and N are opened. By this last passage the steam gets into the eduction-pipe, where it meets with the injection water, and is rapidly condensed. The steam from the boiler enters at the same time by E, and pressing on the lower side of the piston, forces it upwards, and by means of the toothed rack OO and toothed sector QQ forces up that end of the working beam, and causes the other end to urge forward the machinery with which it is connected: and in this manner the operation of the engine may be continued for ever.

The injection water is continually running into the eduction-pipe, because condensation is continually going on, and therefore there is a continual atmospheric pressure to produce a jet. The air which is disengaged from the water, or enters by leaks, is evacuated only during the rise of the piston of the air-pump K. When this is very copious, it renders a very large air-pump necessary; and in some situations Mr Watt has been obliged to employ two air-pumps, one worked by each arm of the beam. This in every case expends a very considerable portion of the power, for the air-pump is always working against the whole pressure of the atmosphere.

It is evident that this form of the engine, by maintaining an almost constant and uninterrupted impulsion, is much fitter for driving any machinery of continued motion than any of the former engines, which were inactive during half of their motion. It does not, however, seem to have this superiority when employed to draw water: But it is equally fitted for this task. Let the engine be loaded with twice as much as would be proper for it if a single-stroke engine, and let a fly be connected with it. Then it is plain that the power of the engine during the rise of the steam-piston will be accumulated in the fly; and this, in conjunction with the power of the engine during the descent of the steam-piston, will be equal to the whole load of water.

In speaking of the steam and eduction-valves, we said that they were all puppet-valves. Mr Watt employed cocks, and also sliding-valves, such as the regulator or steam-valves in the old engines. But he found them always lose their tightness after a short time. This is not surprising, when we consider that they are always perfectly dry, and almost burning hot. He was therefore obliged to change them all for puppet-clacks, which, when truly ground and nicely fitted in their motions at first, are not found to go out of order by any length of time. Other engineers now universally use them in the old form of the steam-engine, without the same reasons, and merely by fertile and ignorant imitation.

The way in which Mr Watt opens and shuts these valves is as follows. Fig. 13. represents a clack with its seat and box. Suppose it one of the eduction-valves. HH is part of the pipe which introduces the steam, and GG is the upper part of the pipe which communicates with the condenser. At EE may be observed a piece more faintly shaded than the surrounding parts. This is the seat of the valve, and is a brass or bell-metal ring turned conical on the outside, so as to fit exactly into a conical part of the pipe GG. These two pieces are fitted by grinding; and the cone being of a long taper, the ring sticks firmly in it, especially after having been there for some time and united by rust. The clack itself is a strong brass plate D, turned conical on the edge, so as to fit the conical or sloping inner edge of the seat. These are very nicely ground on each other with emery. This conical jointing is much more obtuse than the outer side of the ring; so that although the joint is air-tight, the two pieces do not stick strongly together. The clack has a round tail DG, which is freely moveable up and down in the hole of a cross piece FF. On the upper side of the valve is a strong piece of metal DC firmly joined to it, one side of which is formed into a toothed rack. A is the section of an iron axle which turns in holes in the opposite sides of the valve-box, where it is nicely fitted by grinding, so as to be air-tight. Collets of thick leather, well soaked in melted tallow and rosin, are screwed on the outside of these holes to prevent all ingress of air. One end of this axle projects a good way without the box, and carries a spanner or handle, which is moved by the plug-frame. To this axle is fixed a strong piece of metal B, the edge of which is formed into an arch of a circle, having the axis A in its centre, and is cut into teeth, which work in the teeth of the rack DC. K is a cover which is fixed by screws to the top of the box HJJH, and may be taken off in order to get at the valve when it needs repairs.

From this description it is easy to see that by turning the handle which is on the axis A, the sector B must lift up the valve by means of its toothed rack DC, till the upper end of the rack touch the knob or button K. Turning the handle in the opposite direction brings the valve down again to its seat.

This valve is extremely tight. But in order to open it for the passage of the steam, we must exert a force equal to the pressure of the atmosphere. This in a large engine is a very great weight. A valve of six inches diameter sustains a pressure not less than 400 pounds. But this force is quite momentary, and hardly impedes the motion of the engine; for the instant the valve is detached from its seat, although it has not moved the thousandth part of an inch, the pressure is over. Even this little inconvenience has been removed by a delicate thought of Mr Watt. He has put the spanner in such a position when it begins to raise the valve, that its mechanical energy is almost infinitely great. Let QR (fig. 14.) be part of the plug-frame descending, and P one of its pins just going to lay hold of the spanner NO moveable round the axis N. On the same axis is another arm NM connected by a joint with the leader ML, which is connected also by a joint with the spanner LA that is on the axis A of the sector within the valve-box. Therefore when the pin P pushes down the spanner NO, the arm NM moves sidewise and pulls down the spanner AL by means of the connecting rod. Things are so disposed, that when the cock is shut, LM and MN are in one straight line. The intelligent mechanic will perceive that, in this position, the force of the lever ONM is insuperable. It has this further advantage, that if anything should tend to force open the valve, it would be ineffectual; for no force exerted at A, and transmitted by the rod LM, can possibly push the joint M out of its position. Of such importance is it to practical mechanics, that its professors should be persons of penetration as well as knowledge. Yet this circumstance is unheeded by hundreds who have servilely copied from Mr Watt, as may be seen in every engine that is puffed on the public as a discovery and an improvement. When these puppet-valves have been introduced into the common engine, we have not seen one instance where this has been attended to; certainly because its utility has not been observed; and there is one situation where it is of more consequence than in Mr Watt's engine, viz. in the injection-cock. Here the valve is drawn back into a box, where the water is awkwardly disposed round it that it can hardly get out of its way, and where the pressure even exceeds that of the atmosphere. Indeed this particular substitution of the button-valve for the cock is most injudicious.

We postponed any account of the office of the fly XX (fig. 11.), as it is not of use in an engine regulated by the fly VV. The fly XX is only for regulating the reciprocating motion of the beam when the steam is not admitted during the whole descent of the piston. This it evidently must render more uniform, accumulating a momentum equal to the whole pressure of the full supply of steam, and then sharing it with the beam during the rest of the descent of the piston.

When a person properly skilled in mechanics and chemistry reviews these different forms of Mr Watt's steam-engine, he will easily perceive them susceptible of many intermediate forms, in which any one or more of the following improvements may be employed. The first great improvement was the condensation in a separate vessel. This increased the original powers of the engine, giving to the atmospheric pressure and to the counter-weight their full energy; at the same time the waste of steam is greatly diminished. The next improvement, by employing the pressure of the steam instead of that of the atmosphere, aimed only at a still farther diminution of the waste; but was fertile in advantages, rendering the machine more manageable, and particularly enabling us at all times, and without trouble, to suit the power of the engine to its load of work, however variable and increasing; and brought into view a very interesting proposition in the mechanical theory of the engine, viz. that the whole performance of a given quantity of steam may be augmented by admitting it into the cylinder only during a part of the piston's motion. Mr Watt has varied the application of this proposition in a thousand ways; and there is nothing about the machine which gives more employment to the sagacity and judgement of the engineer. The third improvement of the double impulse may be considered as the finishing touch given to the engine, and renders it as uniform in its action as any water-wheel. In the engine in its most perfect form there does not seem to be above one-fourth of the steam wasted by warming the apparatus; so that it is not possible to make it one-fourth part more powerful than it is at present. The only thing that seems susceptible of considerable improvement is the great beam. The enormous strains exerted now on its arms require a proportional strength. This requires a vast mass of matter, not less indeed in an engine with a cylinder of 54 inches than three tons and a half, moving with the velocity of three feet in a second, which must be communicated in about half a second. This mass must be brought into motion from a state of rest, must again be brought to rest, again into motion, and again to rest, to complete the period of a stroke. This consumes much power; and Mr Watt has not been able to load an engine with more than 10 or 11 pounds on the inch and preserve a sufficient quantity of motion, so as to make 12 or 15 five-feet strokes in a second. Many attempts have been made to lessen this mass by using a light framed wheel, or a light frame of carpentry, in place of a solid beam. These have generally been constructed by persons ignorant of the true scientific principles of carpentry, and have fared accordingly. Mr Watt has made similar attempts; but found, that although at first they were abundantly strong, yet after a short time's employment the straps and bolts with which the wooden parts were connected cut their way into the wood, and the framing grew loose in the joints, and, without giving any warning, went to pieces in an instant. A solid massive simple beam, of sufficient strength, bends, and sensibly complains (as the carpenters express it), before it breaks. In all great engines, therefore, such only are employed, and in smaller engines he sometimes uses cast-iron wheels or pulleys; nay, he frequently uses no beam or equivalent whatever, but employs the steam-piston rod to drive the machinery to which the engine is applied.

We presume that our thinking readers will not be disappointed. displeased with this rational history of the progress of this engine in the hands of its ingenious and worthy inventor. We owe it to the communications of a friend, well acquainted with him, and able to judge of his merits. The public see him always associated with the no less celebrated mechanic and philosopher Mr Boulton of Soho near Birmingham (see SOHO). They have shared the royal patent from the beginning; and the alliance is equally honourable to both.

The advantages derived from the patent-right show both the superiority of the engine and the liberal minds of the proprietors. They erect the engines at the expense of the employers, or give working drafts of all the parts, with instructions, by which any resident engineer may execute the work. The employers select the best engine of the ordinary kind in the kingdom, compare the quantities of fuel expended by each, and pay to Messrs Watt and Boulton one-third of the annual savings for a certain term of years. By this the patentees are excited to do their utmost to make the engine perfect; and the employer pays in proportion to the advantage he derives from it.

It may not be here improper to state the actual performance of some of these engines, as they have been ascertained by experiment.

An engine having a cylinder of 31 inches in diameter, and making 17 double strokes per minute, performs the work of forty horses working night and day (for which three relays or 120 horses must be kept), and burns 11,000 pounds of Staffordshire coal per day. A cylinder of 19 inches, making 25 strokes of 4 feet each per minute, performs the work of 12 horses working constantly, and burns 3700 pounds of coals per day. A cylinder of 24 inches, making 22 strokes of 5 feet, burns 5500 pounds of coals, and is equivalent to the constant work of 20 horses. And the patentees think themselves authorized by experience to say in general, that these engines will raise more than 20,000 cubic feet of water 24 feet high for every hundred weight of good pit-coal consumed by them.

In consequence of the great superiority of Mr Watt's engines, both with respect to economy and manageability, they have become of most extensive use; and in every demand of manufacture on a great scale they offer us an indefatigable servant, whose strength has no bounds. The greatest mechanical project that ever engaged the attention of man was on the point of being executed by this machine. The States of Holland were treating with Messrs Watt and Boulton for draining the Haarlem Meer, and even reducing the Zuider Zee; and we doubt not but that it will be accomplished whenever that unhappy nation has sufficiently felt the difference between liberty and foreign tyranny. Indeed such unlimited powers are afforded by this engine, that the engineer now thinks that no task can be proposed to him which he cannot execute with profit to his employer.

No wonder then that all classes of engineers have turned much of their attention to this engine; and seeing that it has done so much, that they try to make it do still more. Numberless attempts have been made to improve Mr Watt's engine; and it would occupy a volume to give an account of them, whilst that account would do no more than indulge curiosity. Our engineers by profession are in general miserably deficient in that accurate knowledge of mechanics and of chemistry which is necessary for understanding this machine; and we have not heard of one in this kingdom who can be put on a par with the present patentees in this respect. Most of the attempts of engineers have been made with the humbler view of availing themselves of Mr Watt's discoveries, so as to construct a steam-engine superior to Newcomen's, and yet of a form sufficiently different from Watt's to keep it without the reach of his patent. This they have in general accomplished by performing the condensation in a place which, with a little stretch of fancy, not unfrequent in a court of law, may be called part of the cylinder.

The success of most of these attempts has interfered so little with the interest of the patentees, that they succeed have not hindered the erection of many engines which they have deemed encroachments. We think not injured it our duty to give our opinion on this subject without reserve. These are most expensive undertakings, and few employers are able to judge accurately of the merits of a project presented to them by an ingenious artificer. They may see the practicability of the scheme, by having a general notion of the expansion and condensation of steam, and they may be misled by the ingenuity apparent in the construction. The engineer himself is frequently the dupe of his own ingenuity; and it is not always dishonesty, but frequently ignorance, which makes him prefer his own invention or (as he thinks it) improvement. It is a most delicate engine, and requires much knowledge to see what does and what does not improve its performance. We have gone into the preceding minute investigation of Mr Watt's progress with the express purpose of making our readers fully masters of its principles, and have more than once pointed out the real improvements, that they may be firmly fixed and always ready in the mind. By having recourse to them, the reader may pronounce with confidence on the merits of any new construction, and will not be deceived by the puffs of an ignorant or dishonest engineer.

We must except from this general criticism a construction by Mr Jonathan Hornblower near Bristol, on in favour account of its singularity, and the ingenuity and real skill which appears in some particulars of its construction. The following short description will sufficiently explain its principle, and enable our readers to appreciate its merit.

A and B (fig. 15.) represent two cylinders, of which A is the largest. A piston moves in each, having their rods C and D moving through collars at E and F. These cylinders may be supplied with steam from the boiler by means of the square pipe G, which has a flanch steam-plate to connect it with the rest of the steam-pipe. This square part is represented as branching off to both cylinders. c and d are two cocks, which have handles and tumblers as usual, worked by the plug-beam W. On the fore-side (that is, the side next the eye) of the cylinders is represented another communicating pipe, whose section is also square or rectangular, having also two cocks a, b. The pipe Y, immediately under the cock b, establishes a communication between the upper and lower parts of the small cylinder B, by opening the cock b. There is a similar pipe on the other side of the cylinder A, immediately under the cock d. When the cocks c and a are open, and the cocks b and d are shut, the steam from the boiler has free admission into the upper part of the cylinder B, and the steam... from the lower part of B has free admission into the upper part of A; but the upper part of each cylinder has no communication with its lower part.

From the bottom of the great cylinder proceeds the education-pipe K, having a valve at its opening into the cylinder, which bends downwards, and is connected with the conical condenser L (c). The condenser is fixed on a hollow box M, on which stand the pumps N and Q for extracting the air and water; which last runs along the trough T into a cistern U, from which it is raised by the pump V for recruiting the boiler, being already nearly boiling hot. Immediately under the condenser there is a spigot valve at S, over which is a small jet pipe, reaching to the bend of the education-pipe. The whole of the condensing apparatus is contained in a cistern R of cold water. A small pipe P comes from the side of the condenser, and terminates on the bottom of the trough T, and is there covered with a valve Q, which is kept tight by the water that is always running over it. Lastly, the pump-rods X cause the outer end of the beam to preponderate, so that the quiescent position of the beam is that represented in the figure, the pistons being at the top of the cylinders.

Suppose all the cocks open, and steam coming in copiously from the boiler, and no condensation going on in L; the steam must drive out all the air, and at last follow it through the valve Q. Now shut the valves b and d, and open the valve S of the condenser. The condensation will immediately commence. There is now no pressure on the under side of the piston of A, and it immediately descends. The communication between the lower part of B and the upper part of A being open, the steam will go from B into the space left by the piston of A. It must therefore expand, and its elasticity must diminish, and will no longer balance the pressure of the steam above the piston of B. This piston therefore, if not withheld by the beam, would descend till it is in equilibrium, having steam of equal density above and below it. But it cannot descend so far; for the cylinder A is wider than B, and the arm of the beam at which its piston hangs is longer than the arm which supports the piston of B; therefore when the piston of B has descended as far as the beam will permit it, the steam between the two pistons occupies a larger space than it did when both pistons were at the tops of their cylinders. Its density, therefore, and its elasticity, diminish as its bulk increases. It is therefore not a balance; for the steam on the upper side of B, and the piston B, pulls at the beam with all the difference of these pressures. The slightest view of the subject must show the reader, that as the pistons descend, the steam that is between them will grow continually rarer and less elastic, and that both pistons will pull the beam downwards.

Suppose now that each has reached the bottom of its cylinder. Shut the cock a and the education cock at the bottom of A, and open the cocks b and d'. The communication being now established between the upper and lower part of each cylinder, nothing hinders the counter weight from raising the pistons to the top. Let

(c) This, however, was stopped by Watt's patent; and the condensation must be performed as in Newcomen's engine, or at least in the cylinder A.

Shut the cocks b and d', and open the cock a, and the education cock at the bottom of A; the condensation will again operate, and the pistons descend. And thus the operation may be repeated as long as steam is supplied; and one full of the cylinder B of ordinary steam is expended during each working stroke.

Let us now examine the power of this engine. It is evident, that when both pistons are at the top of their respective cylinders, the active pressure (that is, the difference of the pressure on its two sides) on the piston of B is nothing, while that on the piston of A is equal to the full pressure of the atmosphere on its area. This, multiplied by the length of the arm by which it is supported, gives its mechanical energy. As the pistons descend, the pressure on the piston of B increases, while that on the piston of A diminishes. When both are at the bottom, the pressure on the piston of B is at its maximum, and that on the piston of A at its minimum.

Mr Hornblower saw that this must be a beneficial employment of steam, and preferable to the practice of condensing it while its full elasticity remained; but he has not considered it with the attention necessary for ascertaining the advantage with precision.

Let a and b represent the areas of the pistons of A and B, and let a and b be the lengths of the arms by which they are supported. It is evident, that when both pistons have arrived at the bottoms of their cylinders, the capacities of the cylinders are as a a and b b. Let this be the ratio of m to 1. Let g h i k (fig. 16.) and l m n o be two cylinders of equal length, communicating with each other, and fitted with a piston-rod p q, on which are fixed two pistons a a and b b, whose areas are as m and 1. Let the distance between the pistons be precisely equal to the height of each cylinder, which height we shall call h. Let x be the space g b or b a, through which the pistons have descended. Let the upper cylinder communicate with the boiler, and the lower cylinder with the condenser or vacuum V.

Any person in the least conversant in mechanics and pneumatics will clearly see that the strain or pressure on the piston-rod p q is precisely the same with the united energies of the two piston rods of Mr Hornblower's engine, by which they tend to turn the working beam round its axis.

The base of the upper cylinder being 1, and its height h, its capacity or bulk is 1 h or h; and this expresses the natural bulk of the steam which formerly filled it, and is now expanded into the space b h l a a m i b. The part b h i b is plainly = h - x, and the part l a a m is = m x. The whole space therefore is m x + h - x, = h + m x - x, or h + m - 1 x. Therefore the density of the steam between the pistons is

\[ \frac{h}{h + m - 1 x} \]

Let p be the downward pressure of the steam from the the boiler on the upper piston \( b h \). This piston is also pressed up with a force \( = p \frac{h}{h + m - 1 x} \) by the steam between the pistons. It is therefore, on the whole, pressed downward with a force \( = p \left( 1 - \frac{h}{h + m - 1 x} \right) \).

The lower piston \( a a \), having a vacuum below it, is pressed downwards with a force \( = p \frac{m h}{h + m - 1 x} \). Therefore the whole pressure on the piston rod downwards is \( = p \left( 1 + \frac{m h}{h + m - 1 x} - \frac{h}{h + m - 1 x} \right) = p \left( 1 + \frac{m h}{h + m - 1 x} \right) = p + \frac{p h}{h + m - 1 x} = p + \frac{p h}{m - 1 x} \).

This then is the momentary pressure on the piston rod corresponding to its descent \( x \) from its highest position. When the pistons are in their highest position, this pressure is equal to \( mp \). When they are in their lowest position, it is \( = p \frac{2 m - 1}{m} \). Here therefore is an accession of power. In the beginning the pressure is greater than on a single piston in the proportion of \( m \) to \( 1 \); and at the end of the stroke, where the pressure is weakest, it is still much greater than the pressure on a single piston. Thus, if \( m \) be 4, the pressure at the beginning of the stroke is \( 4p \), and at the end it is \( \frac{7}{4} p \), almost double, and in all intermediate positions it is greater. It is worth while to obtain the sum total of all the accumulated pressures, that we may compare it with the constant pressure on a single piston.

We may do this by considering the momentary pressure \( p + \frac{p h}{h + m - 1 x} \), as equal to the ordinate \( GF \), \( Hb \), or \( Mc \), of a curve \( Fbc \) (fig. 10.), which has for its axis the line \( GM \) equal to \( h \) the height of our cylinder. Call this ordinate \( y \). We have \( y = p + \frac{p h}{h + m - 1 x} \), and \( y = p \frac{p h}{h + m - 1 x} \). Now it is plain that \( \frac{p h}{h + m - 1 x} \) is the ordinate of an equilateral hyperbola, of which \( ph \) is the power or rectangle of the ordinate and absciss, and of which the absciss reckoned from the centre is \( \frac{h}{m - 1} + x \). Therefore make \( GE = p \), and draw \( DEA \) parallel to \( MG \), and make \( EA = \frac{GM}{m - 1} \), \( = \frac{h}{m - 1} \). The curve \( Fbc \) is an equilateral hyperbola, having \( A \) for its centre and \( AD \) for its asymptote. Draw the other asymptote \( AB \), and its ordinate \( FB \). Since the power of the hyperbola is \( = p h \), \( = GEDM \) (for \( GE = p \), and \( GM = h \)); and since all the inscribed rectangles, such as \( AEFB \), are equal to \( p h \), it follows that \( AEFB \) is equal to \( GEDM \), and that the area \( ABFD \) is equal to the area \( GFcMG \), which expresses the accumulated pressure in Hornblower's engine.

We can now compute the accumulated pressure very easily. It is evidently \( = p \times \left( 1 + \frac{AD}{AE} \right) \).

The intelligent reader cannot but observe that this is precisely the same with the accumulated pressure of the accumulated quantity of steam admitted in the beginning, and stopulated in Mr Watt's method, when the piston has descended through the \( m \)th part of the cylinder. In considering Mr Hornblower's engine, the thing was presented in so different a form that we did not perceive Watt's exact analogy at first, and we were surprised at the result.

We could not help even regretting it, because it had the appearance of a new principle and an improvement; and we doubt not but that it appeared so to the ingenious author; for we have had such proofs of his liberality of mind as permit us not to suppose that he saw it from the beginning, and availed himself of the difficulty of tracing the analogy. And as the thing may mislead others in the same way, we have done a service to the public by showing that this engine, so costly and so difficult in its construction, is no way superior in power to Mr Watt's simple method of stopping the steam. It is even inferior, because there must be a condensation in the communicating passages. We may add, that if the condensation is performed in the cylinder \( A \), which it must be unless with the permission of Watt and Boulton, the engine cannot be much superior to a common engine; for much of the steam from below \( B \) will be condensed between the pistons by the coldness of the cylinder \( A \); and this diminishes the downward pressure on \( A \) more than it increases the downward pressure on \( B \). We learn however that, by confining the condensation to a small part of the cylinder \( A \), Mr Hornblower has erected engines clear of Mr Watt's patent, which are considerably superior to Newcomen's: so has Mr Symington.

We said that there was much ingenuity and real skill still observable in many particulars of this engine. The engine differs in the disposition and connection of the cylinders, and the whole condensing apparatus, are contrived with peculiar neatness. The cocks are very ingenious; they are composed of two flat circular plates ground very true to each other, and one of them turns round on a pin through their centres; each is pierced with three sectional apertures, exactly corresponding with each other, and occupying a little less than one-half of their surfaces. By turning the moveable plate so that the apertures coincide, a large passage is opened for the steam; and by turning it so that the solid of the one covers the aperture of the other, the cock is shut. Such regulators are now very common in the cast iron stoves for warming rooms.

Mr Hornblower's contrivance for making the collars for the piston rods air-tight is also uncommonly ingenious. This collar is in fact two, at a small distance from each other. A small pipe, branching off from the main steam-pipe, communicates with the space between the collars. This steam, being a little stronger than the pressure of the atmosphere, effectually hinders the air from penetrating by the upper collar; and though a little steam should get through the lower collar into the cylinder \( A \), it can do no harm. We see many cases in which this pretty contrivance may be of signal service. But it is in the framing of the great working beam that Mr Hornblower's scientific knowledge is most conspicuous; and we have no hesitation in affirming that it is stronger than a beam of the common form, and containing twenty times its quantity of timber. There is hardly a part of it exposed to a transverse strain, if we except the strain of the pump V on the strutt by which it is worked. Every piece is either pushed or pulled in the direction of its length. We only fear that the bolts which connect the upper beam with the two iron bars under its ends will work loose in their holes, and tear out the wood which lies between them. We would propose to substitute an iron bar for the whole of this upper beam. This working beam highly deserves the attention of all carpenters and engineers. We have that opinion of Mr Hornblower's knowledge and talents, that we are confident that he will see the fairness of our examination of his engine, and we trust to his candour for an excuse for our criticism.

The reciprocating motion of the steam-engine has always been considered as a great defect; for though it be now obviated by connecting it with a fly, yet, unless it is an engine of double stroke, this fly must be an enormous mass of matter moving with great velocity. Any accident happening to it would produce dreadful effects: A part of the rim detaching itself would have the force of a bomb, and no building could withstand it. Many attempts have been made to produce a circular motion at once by the steam. It has been made to blow on the vanes of a wheel of various forms. But the rarity of steam is such, that even if none is condensed by the cold of the vanes, the impulse is exceedingly feeble, and the expense of steam, so as to produce any serviceable impulse, is enormous. Mr Watt, among his first speculations on the steam-engine, made some attempts of this kind. One in particular was uncommonly ingenious. It consisted of a drum turning air-tight within another, with cavities so disposed that there was a constant and great prelude urging it in one direction. But no packing of the common kind could preserve it air-tight with sufficient mobility. He succeeded by immersing it in mercury, or in an amalgam which remained fluid in the heat of boiling water; but the continual trituration soon calcined the fluid and rendered it useless. He then tried Parent's or Dr Barker's mill, including the arms in a metal drum, which was immersed in cold water. The steam rushed rapidly along the pipe which was the axis, and it was hoped that a great reaction would have been exerted at the ends of the arms; but it was almost nothing. The reason seems to be, that the greatest part of the steam was condensed in the cold arms. It was then tried in a drum kept boiling hot; but the impulse was now very small in comparison with the expense of steam. This must be the case.

Mr Watt has described in his specification to the patent office some contrivances for producing a circular motion by the immediate action of the steam. Some of these produce alternate motions, and are perfectly analogous to his double-stroke engine. Others produce a continued motion. But he has not given such a description of his valves for this purpose as can enable an engineer to construct one of them. From any guess that we can form, we think the machine very imperfect; and we do not find that Mr Watt has ever erected a continuous circular engine. He has doubtless found all his attempts inferior to the reciprocating engine with a fly. A very crude scheme of this kind may be seen in the Transactions of the Royal Society of Dublin, 1787. But although our attempts have hitherto failed, we hope that the case is not yet desperate: we see different principles which have not yet been employed.

We shall conclude our account of this noble engine for dinner with observing, that Mr Watt's form suggests the construction of an excellent air-pump. A large vessel may easily be made to communicate with a boiler at one side, and be employed with the pump-receiver on the other, and also with a condenser. Suppose this vessel of ten times the capacity of the receiver; fill it with steam from the boiler, engine tug, and drive out the air from it; then open its communication with the receiver and the condenser. This will rarify the air of the receiver ten times. Repeating the operation will rarify it 100 times; the third operation will air pump, rarify it 1000 times; the fourth 10,000 times, &c. All this may be done in half a minute.

STEAM-Kitchen. Ever since Dr Papin contrived his digester (about the year 1692), schemes have been proposed for dressing viands by the steam of boiling water. A philosophical club used to dine at Saltero's coffee-house, Chelsea, about 40 years ago, and had their viands dressed by hanging them in the boiler of the steam-engine which raised water for the supply of Piccadilly and its neighbourhood. They were completely drest, and both expeditiously and with high flavour.

A patent was obtained for an apparatus for this purpose by a tin-man in London; we think of the name of Tate. They were afterwards made on a much more effective plan by Mr Gregory, an ingenious tradesman in Edinburgh, and are coming into very general use.

It is well known to the philosopher that the steam of boiling water contains a prodigious quantity of heat, which it retains in a latent state ready to be faithfully accounted for, and communicated to any colder body. Every cook knows the great scalding power of steam, and is disposed to think that it is much hotter than boiling water. This, however, is a mistake; for it will raise the thermometer no higher than the water from which it comes. But we can assure the cook, that if he make the steam from the spout of a tea-kettle pass through a great body of cold water, it will be condensed or changed into water; and when one pound of water has in this manner been boiled off, it will have heated the mass of cold water as much as if we had thrown into it seven or eight hundred pounds of boiling hot water.

If, therefore, a boiler be properly fitted up in a furnace, and if the steam of the water boiling in it be conveyed by a pipe into a pan containing viands to be dressed, every thing can be cooked that requires no higher degree of heat than that of boiling water: And, this will be done without any risk of (cooking, or any kind of overheating, which frequently spoils our dishes, and proceeds from the burning heat of air coming to those parts of the pot or pan which is not filled with liquor, and is covered only with a film, which quickly burns and taints the whole dish. Nor will the cook be scorched by the great heat of the open fire that is necessary for dressing, at once a number of dishes, nor have his person and clothes soiled by the smoke and foot unavoidable in the cooking on an open fire. Indeed the whole whole process is so neat, so manageable, so open to inspection, and so cleanly, that it need neither fatigue nor offend the delicacy of the nicest lady.

We had great doubts, when we first heard of this as a general mode of cookery, as to its economy; we had none as to its efficacy. We thought that the steam, and consequently the fuel expended, must be vastly greater than by the immediate use of an open fire; but we have seen a large tavern dinner expeditiously dressed in this manner, seemingly with much less fuel than in the common method. The following simple narration of facts will show the superiority. In a paper manufactory in this neighbourhood, the vats containing the pulp into which the frames are dipped are about six feet diameter, and contain above 200 gallons. This is brought to a proper heat by means of a small coddle or furnace in the middle of the liquor. This is heated by putting in about one hundred weight of coals about eight o'clock in the evening, and continuing this till four next morning, renewing the fuel as it burns away. This method was lately changed for a steam heater. A furnace, having a boiler of five or six feet diameter and three feet deep, is heated about one o'clock in the morning with two hundred weight of coals, and the water kept in brisk ebullition. Pipes go off from this boiler to five vats, some of which are at 90 feet distance. It is conveyed into a flat box or vessel in the midst of the pulp, where it condenses, imparting its heat to the sides of the box, and thus heats the surrounding pulp. These five vats are as completely heated in three hours, expending about three hundred weight of coals, as they were formerly in eight hours, expending near eighteen hundred weight of coals. Mr Gregory, the inventor of this steam-heater, has obtained (in company with Mr Scott, plumber, Edinburgh) a patent for the invention; and we are persuaded that it will come into very general use for many similar purposes. The dyers, hatmakers, and many other manufacturers, have occasion for large vats kept in a continual heat; and there seems no way to effectual.

Indeed when we reflect seriously on the subject, we see that this method has immense advantages considered merely as a mode of applying heat. The steam may be applied to the vessel containing the viands in every part of its surface; it may even be made to enter the vessel, and apply itself immediately to the piece of meat that is to be dressed, and this without any risk of scorching or overdoing.—And it will give out about 75% of the heat which it contains, and will do this only if it be wanted; so that no heat whatever is wasted except what is required for heating the apparatus. Experience shows that this is a mere trifle in comparison of what was supposed necessary. But with an open fire we only apply the flame and hot air to the bottom and part of the sides of our boiling vessels; and this application is hurried in the extreme; for to make a great heat, we must have a great fire, which requires a prodigious and most rapid current of air. This air touches our pans but for a moment, imparts to them but a small portion of its heat; and we are persuaded that three-fourths of the heat is carried up the chimney, and escapes in pure waste, while another great portion beams out into the kitchen to the great annoyance of the scorched cook. We think, therefore, that a page or two of this work will not be thrown away in the description of a contrivance by which a saving may be made to the entertainer, and the providing the pleasures of his table prove a less fatiguing task to this valuable corps of practical chemists.

Let A (fig. 1.) represent a kitchen-boiler, either properly fitted up in a furnace, with its proper fire-place, ash-pit, and flue, or set on a tripod on the open fire, or built up in the general fire-place. The steam-pipe AC rises from the cover of this boiler, and then is led away with a gentle ascent in any convenient direction. C represents the section of this conducting steam-pipe. Branches are taken off from the side at proper distances. One of these is represented at CDE, furnished with a cock D, and having a taper nozzle E, fitted by grinding into a conical piece F, which communicates with an upright pipe GH, which is soldered to the side of the stewing vessel PQRS, communicating with it by the short pipe I. The vessel is fitted with a cover OT, having a flapple handle V. The piece of meat M is laid on a tin-plate grate KL, pierced with holes like a cullender, and standing on three short feet n.n.n.

The steam from the boiler comes in by the pipe I, and is condensed by the meat and by the sides of the vessel, communicating to them all its heat. What is not so condensed escapes between the vessel and its cover. The condensed water lies on the bottom of the vessel, mixed with a very small quantity of gravy and fatty matter from the viands. Frequently, instead of a cover, another stew-vessel with a cullender bottom is set on this one, the bottom of the one fitting the mouth of the other; and it is observed, that when this is done, the dish in the under vessel is more expeditiously and better dressed, and the upper dish is more slowly, but as completely stewed.

This description of one stewing vessel may serve to give a notion of the whole; only we must observe, that when broths, soups, and dishes with made sauces or containing liquids, are to be dressed, they must be put into a smaller vessel, which is let into the vessel PQRS, and is supported on three short feet, so that there may be a space all round it of about an inch or three quarters of an inch. It is observed, that dishes of this kind are not to be expeditiously cooked as on an open fire, but as completely in the end, only requiring to be turned up now and then to mix the ingredients; because as the liquids in the inner vessel can never come into ebullition, unless the steam from the boiler be made of a dangerous heat, and every thing be close confined, there cannot be any of that tumbling motion that we observe in a boiling pot.

The performance of this apparatus is far beyond any expectation we had formed of it. In one which we examined, six pans were stewing together by means of a boiler 10½ inches in diameter, standing on a brisk open fire. It boiled very briskly, and the steam puffed frequently through the chinks between the stew-pans and their covers. In one of them was a piece of meat considerably above 30 pounds weight. This required above four hours stewing, and was then very thoroughly and equally cooked; the outside being no more done than the heart, and it was near two pounds heavier than when put in, and greatly swelled. In the mean time, several dishes had been dressed in the other pans. As far, Far as we could judge, this cooking did not consume one-third part of the fuel which an open fire would have required for the same effect.

When we consider this apparatus with a little more knowledge of the mode of operation of fire than falls to the share of the cooks (we speak with deference), and consider the very injudicious manner in which the steam is applied, we think that it may be improved so as to surpass anything that the cook can have a notion of.

When the steam enters the stew-pan, it is condensed on the meat and on the vessel; but we do not want it to be condensed on the vessel. And the surface of the vessel is much greater than that of the meat, and continues much colder; for the meat grows hot, and continues so, while the vessel, made of metal, which is a very perfect conductor of heat, is continually robbed of its heat by the air of the kitchen, and carried off by it. If the meat touch the side of the pan in any part, no steam can be applied to that part of the meat, while it is continually imparting heat to the air by the intermediate medium of the vessel. Nay, the meat can hardly be drenched unless there be a current of steam through it; and we think this confirmed by what is observed above, that when another stew-pan is let over the first, and thus gives occasion to a current of steam through its cullender bottom to be condensed by its sides and contents, the lower dish is more expeditiously drest. We imagine, therefore, that not less than half of the steam is wasted on the sides of the different stew-pans. Our first attention is therefore called to this circumstance, and we wish to apply the steam more economically and effectually.

We would therefore construct the steam-kitchen in the following manner:

We would make a wooden chest (which we shall call the STEW-CHEST) ABCD (fig. 2.). This should be made of deal, in very narrow slips, not exceeding an inch, that it may not shrink. This should be lined with very thin copper, lead, or even strong tinfoil. This will prevent it from becoming a conductor of heat by soaking with steam. For further security it might be set in another chest, with a space of an inch or two all round, and this space filled with a composition of powdered charcoal and clay. This should be made by first making a mixture of fine potter's clay and water about as thick as poor cream: then as much powdered charcoal must be beat up with this as can be made to stick together. When this is rammed in and dry, it may be hot enough on one side to melt glass, and will not discolour white paper on the other.

This chest must have a cover LMNO, also of wood, having holes in it to receive the stew-pans P, Q, R. Between each pan is a wooden partition, covered on both sides with milled lead or tinfoil. The whole top must be covered with very spongy leather or felt, and made very flat. Each stew-pan must have a bearing or shoulder all round it, by which it is supported, resting on the felt, and lying so true and close that no steam can escape. Some of the pans should be simple, like the pan F, for dressing broths and other liquid dishes. Others should be like E and G, having in the bottom a pretty wide hole H, K, which has a pipe in its upper side, rising about an inch or an inch and half into the stew-pan. The meat is laid on a cullender plate, as in the common way; only there must be no holes in the cullender immediately above the pipe.—These stew-pans must be fitted with covers, or they may have others fitted to their mouths, for warming sauces or other dishes, or stewing greens, and many other subordinate purposes for which they may be fitted.

The main-pipe from the boiler must have branches, (each furnished with a cock), which admit the steam into these divisions. At its first entry some will be condensed on the bottom and sides; but we imagine that these will in two minutes be heated so as to condense no more, or almost nothing. The steam will also quickly condense on the stew-pan, and in half a minute make it boiling hot, so that it will condense no more; all the rest will now apply itself to the meat and to the cover. It may perhaps be advisable to allow the cover to condense steam, and even to waste it. This may be promoted by laying on it flannel soaked in water. Our view in this is to create a demand for steam, and thus produce a current through the stew-pan, which will be applied in its passage to the viands. But we are not certain of the necessity of this. Steam is not like common air of the same temperature, which would glide along the surfaces of bodies, and impart to them a small portion of its heat, and escape with the rest. To produce this effect there must be a current; for air hot enough to melt lead, will not boil water, if it be kept stagnant round the vessel. But steam imparts the whole of its latent heat to any body colder than boiling water, and goes no farther till this body be made boiling hot. It is a most faithful carrier of heat, and will deliver its whole charge to any body that can take it. Therefore, although there were no partitions in the stew-chest, and the steam were admitted at the end next the boiler, if the pan at the farther end be colder than the rest, it will all go thither; and will, in short, communicate to everything impartially according to the demand. If any person has not the confidence in the steam which we express, he may still be certain that there must be a prodigious saving of heat by confining the whole in the stew-chest; and he may make the pans with entire bottoms, and admit the steam into them in the common way, by pipes which come through the sides of the chest and then go into the pan. There will be none lost by condensation on the sides of the chest; and the pans will soon be heated up to the boiling temperature; and hardly any of their heat will be wasted, because the air in the chest will be stagnant. The chief reason for recommending our method is the much greater ease with which the stew-pans can be shifted and cleaned. There will be little difference in the performance.

Nay, even the common steam-kitchen may be prodigiously improved by merely wrapping each pan in three or four folds of coarse dry flannel, or making flannel bags of three or four folds fitted to their shape, which can be put on or removed in a minute. It will also greatly conduce to the good performance to wrap the main steam pipe in the same manner in flannel.

We said that this main-pipe is conducted from the boiler with a gentle ascent. The intention of this is, that the water produced by the unavoidable condensation of the steam may run back into the boiler. But the rapid motion of the steam generally sweeps it up hill, and it runs into the branch-pipes and descends into the stew-pans. Perhaps it would be as well to give the main- STEAM kitchen.

ROOMS heated by STEAM. main pipe a declivity the other way, and allow all the water to collect in a hot well at the farther end, by means of a descending pipe, having a loaded valve at the end. This may be so contrived as to be close by the fire, where it would be so warm that it would not check the boiling if again poured into the boiler. But the utmost attention must be paid to cleanliness in the whole of this passage, because this water is boiled again, and its steam passes through the heart of every dish. This circumstance forbids us to return into the boiler what is condensed in the stew-pans. This would mix the tastes and flavours of every dish, and be very disagreeable. All this must remain in the bottom of each stew-pan; for which reason we put in the pipe rising up in the middle of the bottom. It might indeed be allowed to fall down into the stew-chest, and to be collected in a common receptacle, while the fat would float at top, and the clear gravy be obtained below, perhaps fit for many sauces.

The completest method for getting rid of this condensed steam would be to have a small pipe running along the under side of the main conductor, and communicating with it at different places, in a manner similar to the air discharger on the mains of water-pipes. In the paper manufacture mentioned above, each steam-box has a pipe in its bottom, with a float-cock, by which the water is discharged; and the main pipe being of great diameter, and laid with a proper acclivity, the water runs back into the boiler.

But these precautions are of little moment in a steam-kitchen even for a great table; and for the general use of private families, would hurt the apparatus, by making it complex and of nice management. For a small family, the whole apparatus may be set on a table four feet long and two broad, which may be placed on casters, so as to be wheeled out of the way when not in use. If the main conductor be made of wood, or properly cased in flannel, it will condense so little steam that the cooking table may stand in the remotest corner of the kitchen without sensibly impairing its performance; and if the boiler be properly set up in a small furnace, and the fire made so that the flame may be applied to a great part of its surface, we are persuaded that three-fourths of the fuel used in common cookery will be saved. Its only inconvenience seems to be the indispensible necessity of the most anxious cleanliness in the whole apparatus. The most trifling neglect in this will destroy a whole dinner.

We had almost forgotten to observe, that the boiler must be furnished with a funnel for supplying it with water. This should pass through the top, and its pipe reach near to the bottom. It will be proper to have a cock on this funnel. There should also be another pipe in the top of the boiler, having a valve on the top. If this be loaded with a pound on every square inch, and the fire so regulated that steam may be observed to puff sometimes from this valve, we may be certain that it is passing through our dishes with sufficient rapidity; and if we shut the cock on the funnel, and load the valve a little more, we shall cause the steam to blow at the covers of the stew-pans. If one of these be made very tight, and have a hole also furnished with a loaded valve, this pan becomes a digester, and will dissolve bones, and do many things which are impracticable in the ordinary cookery.

Vol. XIX. Part II.

STEAM applied to Heating Rooms. Steam has been successfully applied as a substitute for open fires in heating manufactories, and promises to be highly beneficial, not only in point of economy in saving fuel, but also in lessening the danger of accidental fire. The following mode of heating a cotton mill by steam was proposed and practised in 1799 by Mr Niel Snodgraf of Paisley. We shall give an account of it in his own words.

"Fig. I. presents a view of an inner gable, which is at one extremity of the preparation and spinning rooms of the mill. On the other side of this gable there is a space of 17 feet, enclosed by an outer gable, and containing the water-wheel, the staircase, and small rooms for the accommodation of the work. In this space the furnace and boiler are placed on the ground. The boiler cannot be shown here, as it lies behind the gable exhibited; nor is it of any consequence, as there is nothing peculiar in it. It may be of any convenient form. The feeding apparatus, &c., are in every respect the same as in the boiler of a common steam-engine. A circular copper boiler, two feet diameter by two feet deep, containing 30 gallons of water, with a large copper head as a reservoir for the steam, was found to answer in the present instance. The steam is conveyed from the boiler through the gable, by the copper pipe B, into the tin pipe, C, C. From C it passes into the centres of the perpendicular pipes E, E, E, by the small bent copper tubes D, D, D. The pipes E, E, E, are connected under the garret floor by the tubes F, F, for the more easy circulation of the steam. The middle pipe, E, is carried through the garret floor, and communicates with a lying pipe, 36 feet in length (the end of which is seen at G), for heating the garret. At the further extremity of the pipe G, there is a valve falling inwards to prevent a vacuum being formed on the cooling of the apparatus; the consequence of which would be the crusting of the pipes by the pressure of the atmosphere. Similar valves K, K, are placed near the top of the perpendicular pipes, E, E; and from the middle one E, the small pipe passes through the roof, and is furnished with a valve at I, opening outwards, to suffer the air to escape while the pipes are filling with steam, or the steam itself to escape when the charge is too high.

"The water condensed in the perpendicular pipes E, E, E, trickles down their sides into the three funnels L, L, L, the necks of which may either pass through or round the pipe C, into the copper tube M, M, which also receives the water condensed in C, C, by means of the short tubes N, N. The pipe C, C, is itself so much inclined as to cause the water to run along it to the tubes N, N, and the pipe G in the garret has an inclination of 18 inches in its length, to bring the water condensed in it back to the middle pipe E. The tube M, M, carries back the water through the gable to the boiler, which stands five feet lower than this tube. It is material to return the water to the boiler, as, being nearly at a boiling heat, a considerable expense of fuel is thereby saved.

"The large pipes are ten inches in diameter, and are made of the second kind of tinned iron plates. The dimensions of the smaller tubes may be seen by their comparative size in the engraving, and perhaps they might be varied without inconvenience.

"The apparatus erected as here described, has been found sufficiently strong, and has required no material repairs." Steam repairs since the first alterations were made. The leading object in the instance under consideration being to save fuel, in order to derive as much heat as possible from a given quantity of fuel, the flue from the furnace, which heats the boiler, is conveyed into common stone pipes placed in the gable. These are erected so as to prevent any danger of fire, in the manner shown in the engraving, fig. 2. The steam with this auxiliary communicates a heat of about 76° to the mill, the rooms of which are 50 feet long, 32½ feet wide, and 8½ feet high, except the lower story and garret; the former of which is 11, and the latter seven feet high. The rooms warmed in this manner are much more wholesome and agreeable than those heated by the best constructed stoves, being perfectly free from vapour or contaminated air.

"The application of the principle to buildings already constructed, it is presumed, will be sufficiently obvious from the foregoing details. In new manufactories, where the mode of heating may be made a part of the original plan, a more convenient apparatus may be introduced. This will be best explained by a description of fig. 2, which gives a section of a cotton-mill constructed so as to apply the steam apparatus to a new building.

"The furnace for the boiler is shown at a (fig. 2). The flue of the furnace conveys the smoke into the cast iron flue pipes, 1, 2, 3, 4. These pipes are placed in a space in the gable, entirely inclosed with brick, except at the small apertures, 5, 6, 7, 8. A current of air is admitted below at 9, and thrown into the rooms by those openings, after being heated by contact with the pipes. This part of the plan is adopted with a view to prevent, as much as possible, any of the heat, produced by the fuel used, from being thrown away. It may be omitted where any danger of fire is apprehended from it, and the smoke may be carried off in any way that is considered absolutely secure. So far, however, as appears from experience, there seems to be little or no danger of fire from a stove of this construction. The greatest inconvenience of a common stove is, that the coke or metal furnace is liable to crack from the intensity of the heat. By the continuity of the metal from the fireplace, an intense heat is also conducted along the pipes, which exposes them to the same accident. Here the smoke being previously conveyed through a brick flue, can never communicate to the pipes a degree of heat sufficient to crack them. In like manner the pipes, having no communication with the rooms but by the small apertures, cannot come in contact with any combustible substance; and from being surrounded with air, which is constantly changing, can impart only a very moderate degree of heat to the walls. The iron supports of the pipes may be imbedded in some substance which is a bad conductor of heat, as furnace ashes and lime, &c. The emission of heated air into the rooms may be regulated by valves. As the pipes are not exposed to cracking, there is no risk of their throwing smoke or vapour into the rooms.

"The boiler b, b, is six feet long, three and a half broad, and three feet deep. As there is nothing peculiar in the feeding apparatus, it is omitted. The boiler may be placed in any convenient situation. Where a steam engine is used for other purposes, the steam may be taken from its boiler. The pipe c, c, conveys the steam from the boiler to the first perpendicular pipe d, d, d, d. There is an expanding joint at e, fluted, to make it steam-tight. The steam ascending in the first pipe d, d, d, enters the horizontal pipe f, f, f, f, (which is slightly inclined) expelling the air, which partly escapes by the valve g, and is partly forced into the other pipes. The valve g being considerably loaded, forces the accumulating steam down into the rest of the pipes d, d, d. The air in these pipes recedes before the steam, and is forced through the tubes h, h, h, into the pipe m, m, m, whence it escapes at the valve i, and the syphon k. The water, condensed in the whole of the pipes, passes also through the tubes h, h, h, h, into the pipe m, m, m, which has such a declivity as to discharge the water at the syphon k, into the hot well n, whence it is pumped back into the boiler.

"The whole of the pipes are of cast iron, except m, m, m, which is of copper. The perpendicular pipes serve as pillars for supporting the beams of the house, by means of the projecting pieces o, o, o, which may be raised or lowered at pleasure by the wedges p, p, p. The pipes are sunk in the beams about an inch, and are made fast to them by the iron straps g, g. Those in the lower story rest on the stones s, s, s, s, and are made tight at the junction with stuffing. The pipe in each story supports the one in the story above by a stuffed joint as shown at r. The pipes in the lower story are seven inches in diameter; those in the higher fix inches; those in the other two are of intermediate diameters. The thickness of the metal is three-eighths of an inch. The lower pipes are made larger than the upper, in order to expose a greater heated surface in the lower rooms, because the steam being thrown from above into all the pipes, except the first, would otherwise become incapable of imparting an equal heat as it descends. There is no necessity for valves opening inwards in this apparatus, the pipes being strong enough to resist the pressure of the atmosphere.

"The cotton mill is 60 feet long, 33 wide, and four stories high, the upper being a garret story. In the engraving, five parts out of nine in the length of the building are only shown. The apparatus will heat the rooms to 81° in the coldest season. It is evident that, by increasing the size, or the number of the pipes, and the supply of steam, any degree of heat up to 212° may be easily produced. It may even be carried beyond that point by an apparatus strong enough to compress the steam; this, however, can seldom be wanted. At first it was objected to this construction, that the expansion of the pipes, when heated, might damage the building; but experience has proved, that the expansion occasioned by the heat of steam is quite infensible."

Steam has also been advantageously employed in drying muslin goods, when the state of the weather interrupts this process out of doors. This application of steam, we understand, was the invention of an ingenious mechanic in Paisley, who never derived the smallest benefit from the discovery. It was adopted immediately by some bleachers in the neighbourhood, and has now come into very general use. The steam is introduced into cylinders of tin plate, and the goods to be dried are wrapped round the cylinders which communicate to them a heat equal at least to the temperature of boiling water, and in this way the process of drying is expeditiously accomplished.