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 weakness 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 the marquis of Worcester during the reign of Cha. II. This nobleman published in 1663 a small book entitled A Century of Inventions; giving some obscure and 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 their inventions seem not so much intended to instruct the public, as to raise wonder; and his encomiums on their utility and importance are to a great degree extravagant, resembling more the puff of an advertising tradesman than the patriotic communications of a gentleman. The marquis of Wor-

Captain Savary obtained his patent after having actually Papin has erected several machines, of which he gave a description to claim in a book entitled The Miner's Friend, published in the invention as he 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. them in close 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 Ada Eruditorum, Lipsia, 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 Bofus, 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 a 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 DeLandes) also presented to the academy a project of a steam-wheel, where the impulsive force of the vapour 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 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 b 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 H G, 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 is pressed the water out of the receiver into the rising pipe. 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 steelyard L M. This weight is so adjusted that its pressure on the safety-valve is somewhat greater than the pressure of a column of water V k 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 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 K E D f, 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 elasticity of the steam. This must in the boiler, and 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 of this thickness to be just in equilibrium with this pressure; and the soldered 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 solder is just ready to melt, and has no tenacity; even soldered joints are considerably weakened by it. Accordingly, in a machine erected by Captain Savary at York Buildings in London, 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 Marquis'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 advantage this case the greatest part of it was performed by the only in connection-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 so particular a 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 clips 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. 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 some 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 contrivance 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 is 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 subside 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 \(\frac{3}{4}\)ths of the whole steam is uselessly 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 to condense a prodigious quantity of steam in the succeeding stroke. Mr Blackey has attempted to lessen this 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°, we should lose one half of the pressure of the atmosphere; if cooled to 120°, we should still lose \(\frac{1}{4}\)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; which 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 \(\frac{1}{4}\)th 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 very 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 as 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 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 stood most in need of hydraulic assistance; 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 a Mr Newcomen, an iron-monger or blacksmith, and Mr Cawley a glazier at Dartmouth in Devonshire, who had dabbled much with this machine. Newcomen was a person of some rearing, 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 dissuaded 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 desideratum 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 allocated. Posterity has done justice to the modest inventor, and the machine is universally called Newcomen's Engine. Its principle and mode of operation may be clearly conceived as follows.

Let A (fig. 7.) represent a great boiler properly built in a furnace. At a small height above it is a cylinder CBBC of metal, bored very truly and smoothly. The boiler communicates with this cylinder by means of the throat or steam-pipe NQ. The lower aperture of this pipe is shut by the plate N, which is ground very flat, so as to apply very accurately to the whole circumference of the orifice. This plate is called the regulator or steam-cock, and it turns horizontally round an axis b which passes through the top of the boiler, and is nicely fitted to the socket, like the key of a cock, by grinding. The upper end of this axis is furnished with a handle T.

A piston P is suspended in this cylinder, and made air-tight by a packing of leather or soft rope, well filled with tallow; and, for greater security, a small quantity of water is kept above the piston. The piston-rod PD is suspended by a chain which is fixed to the upper extremity F of the arched head FD of the great lever or Working Beam HK, which turns on the gudgeon O. There is a similar arched head EG at the other end of the beam. To its upper extremity E is fixed a chain carrying the pump-rod XL, which raises the water from the mine. The load on this end of the beam is made to exceed considerably the weight of the piston P at the other extremity.

At some small height above the top of the cylinder is a cistern W called the Injection Cistern. From this descends the Injection Pipe ZSR, which enters the cylinder through its bottom, and terminates in a small hole R, or sometimes in a nozzle pierced with many smaller holes diverging from a centre in all directions. This pipe has at S a cock called the Injection Cock, fitted with a handle V.

At the opposite side of the cylinder, a little above its bottom, there is a lateral pipe, turning upwards at the extremity, and there covered by a clack-valve f, called the Shifting Valve, which has a little dish round it to hold water for keeping it air-tight.

There proceeds also from the bottom of the cylinder a pipe d e g h (passing behind the boiler), of which the lower end is turned upwards, and is covered with a valve h. This part is immersed in a cistern of water Y, called the Hot Well, and the pipe itself is called the Eduction Pipe. Lastly, the boiler is furnished with a safety-valve called the Puppet Clock (which is not represented in this sketch for want of room), in the same manner as Savary's engine. This valve is generally loaded with one or two pounds on the square inch, so that it allows the steam to escape when its elasticity is 3/4th greater than that of common air. Thus all risk of bursting the boiler is avoided, and the pressure outwards is very moderate; so also is the heat. For, by inspecting the table of vaporous elasticity, we see that the heat corresponding to 32 inches of elasticity is only about 216° of Fahrenheit's thermometer.

These are all the essential parts of the engine, and are here drawn in the most simple form, till our knowledge of their particular offices shall show the propriety of the peculiar forms which are given to them. Let us now see how the machine is put in motion, and what is the nature of its work.

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 motion, resting both the steam-cock and injection cock shut, and the retort position or attitude of the machine must be such as appears in this 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 boiler 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 education-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 inflame 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 inflamed 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 shoot 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 a 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 distant 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 spouts 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 education-valve \( b \) 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 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 rarified 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 piston is 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 made exceeds this preponderancy. 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 joists loaded by heavy walls. It is usual to frame these joists 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 sees the piston as low as he thinks the 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 pitman's clack. The moment therefore that the steam-cock is opened, it rushes violently into the cylinder, having an elasticity greater than that of the air. It therefore immediately blows open the snifting valve, and allows (at least) the water which had come in by the former injection, and what arose from the condensed fed steam, to descend by its own weight through the eduction pipe \(d e g h\) to open the valve \(h\), and to run out into the hot 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 waste 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 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 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 boiler 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 less air. Such engines as are so unfortunately situated that they are obliged to employ the very water which they have brought up from great depths, are found greatly inferior in their performance to others. The air collected below the piston greatly diminishes the accelerating force, and the expulsion of such a quantity requires a long continued blast of the bell 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 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 dropped 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 \(\frac{1}{2}\)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 \(30 - \frac{1}{2}\), 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 rising 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 284 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 \(\frac{1}{10000}\)th of the density of water. Defaguliers says that it is above 140,000 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 \(\frac{1}{4}\)th were employed in allowing the piston to rise, and the remaining \(\frac{3}{4}\)ths were employed to warm the cylinder.

The moving force during the ascent of the piston must be considered as resulting chiefly, if not solely, from the preponderating weight of the pit piston-rods. The office of this is to return the steam-piston to the pit piston-rods. 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 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 brilliancy 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 column 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 Difference 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 forever 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; and 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 Boffin in his Hydrodynamique; these being by far the most accurate and perspicuous of any that have been published.

Fig. 8, n° 1., is a perspective view of the boiler cylinder, and all the parts necessary for turning the cocks. Fig. 8, n° 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 e. This 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, n° 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 this 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 jackhead or jacquette, is worked by a rod r, suspended from a concentric arch v near the outer end of the working beam. This forces a small portion of the pit water along the rising pipe i L M into the injection cistern.

In figure 8, n° 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 spouts 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 A, that the pieces connected 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 b f g, whose long tail b 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 b f g backwards and forwards horizontally, and by 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 o 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 caught 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 spread 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 b 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 on the horizontal bolt e, and then it pushes the stirrup and the fork towards the cylinder, and opens the regulator. It lets 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 w 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 & 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. stalk of the Y till it passes the perpendicular, and it then falls down, outwards from the cylinder, and the claw again drives the fork in the direction b 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 and at a proper height in 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 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 underside of a wooden lever DE, moveable round the centre D, and supported at a proper height by a string r 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 g 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 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 g 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 is a considerable propriety in the gradual shutting 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 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 large quantity of injected water lying at the bottom of the viscous 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 slender spring), and it will fill the cup to the level of the entry 2 of the education-pipe, and will then 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 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 quite insignificant, when compared with the quantity of heat which must then be combined with it in order to convert the water into steam. 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 judgment of the internal state of the engine during the performance. Mr Beighton paid a minute attention to the situation of the water in the feeders and education-pipe of an engine, which seems to have been one of the best which has yet been erected. It was lifting a column of water whose weight was 7ths of the pressure of the air on its piston, and made 16 strokes, of 6 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 4½ feet 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 1/8th weaker than air. By grasping this pipe in his hand during the rise of the piston, he could gauge 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 6ths of the elasticity of the air.

Mr Buat, in his examination of an engine which is erected at Montrelaix, 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 1/7th stronger and 1/7th weaker than common air. But this engine has not been nearly so perfect. Its lift was not more than 1/2 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 last 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. This valve is therefore called the shifting 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 S near the bottom of the boiler, by which it may be emptied when it needs repairs or cleansing.

There are two small pipes t, t, t, and t, t, t, with cocks called gage-pipes. The first extends 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 set 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 posts, which are framed into the joists 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 slight 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... 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 1/3ths 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. Boffin's solution has solved it very shortly and simply, upon this 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 \(b\) 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 \(b c^2 a - W b = (b - H) c^2 a + W b\). 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

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

If we suppose the arms of the lever equal and \(H = b\), we have \(W = \frac{c^2b}{2}\); 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 1/8th 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 and fundamentally another respect. The \(W\) in the first member is not the same with the \(W\) in the second. In the first respect, 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 \(P_0\); this we may express by \(ab\), \(a\) being \(P_0\).

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 \(P_1\); we may express this by \(bb\). 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 is \(ab - Wc\), and during the returning stroke it is \(c - bb\). Therefore \(c - bb = ab - Wc\); and \(c = \frac{b(a + b) - W}{2}\); which, on the above supposition of the values of \(a\) and \(b\), gives us \(c = \frac{b - W}{2}\). We shall make some use of this equation afterwards; but it affords us no information concerning the most advantageous proportion of \(b\) and \(W\), which is the material point.

We must consider this matter in another way: And 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 piston to rise again, but without giving it any impetus. The pump-rods must therefore be returned to be used at the bottom of the working barrels by means of a 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 cient 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 \( p = L \), no water will be raised; if \( p = 0 \), 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 \( p : p - m = g : \frac{g(p-m)}{p} \).

The fourth term of this analogy is the space required.

Let \( t \) be the whole time of the descent in seconds. Then \( t^2 : t = \frac{g(p-m)}{p} : \frac{g(p-m)}{p} \). This last term 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 \( \frac{g(p-m)}{p} \). Therefore the performance of the engine must be \( \frac{g(p-m)}{p} (L - p) \).

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

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 \( \sqrt{Lm} \).

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 \( m = \frac{p^2}{L} \).

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

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

\[ m = \frac{L}{1}, \frac{L}{4}, \frac{L}{9}, \frac{L}{16}, \frac{L}{25}, \ldots \]

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

The circumstance which is commonly obtruded on 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 every thing in conformity to this.

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

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 seen that the greater 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. variable quantity, and make the fluxion of this formula

\[ y = \frac{L}{2} \]

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 \( \frac{3}{2} \)ds 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 judgment 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 considered in as pretty exact.

It is a satisfactory thing to observe these results agree very well with the most successful practice. By many 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 water should be \( \frac{1}{4} \) of the atmospheric pressure. They call this loading the engine with \( \frac{7}{8} \) pounds on the inch, and they say that too 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 \( \frac{1}{4} \) or \( \frac{1}{2} \)ths of \( P \), and \( L \) should be \( \frac{1}{4} \)ths of \( P \) and \( L \) \( = w + p \); it follows, that the counter weight should be \( \frac{1}{4} \)th of \( P \); 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 thro' 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 this, 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 \( \frac{1}{4} \)ths 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 \( \frac{1}{2} \) feet, and would have acquired the velocity of three feet per second. But in one second gravity would have produced a velocity of 32 feet in the same mass. Therefore the accelerating force which has produced the velocity of three feet is nearly \( \frac{1}{16} \)th of the weight. Therefore \( M \) 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 \( \frac{1}{4} \)th 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 fill with this circumstance of being indirect. We found that \( p \) was equal to \( \sqrt{Lm} \), 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 motions both up and down were uniformly accelerated; but this cannot be the case when the resistances increase with the velocity. This circumstance makes very little change in the working-stroke, and therefore the theorem which determines the best relation of P to L may be confided 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 straining 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 2½ 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 confide in the propriety of the formula \( y = \frac{I}{z} \). And we may obtain the velocity of this stroke at the end of a second with great accuracy as follows. Let \( z g \) 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 \( \frac{y}{M} z g \); 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 \( \frac{y - \frac{1}{2} r}{M} z g \) 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 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 senses uniform before the pistons have descended ¼ 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 1400 feet per second, whereas 500 feet is enough to make it maintain a density \( \frac{1}{\sqrt{5}} \)ths of that of steam in equilibrium with the air. Hence it follows, that its elasticity will not be less than \( \frac{2}{3} \)ths of the elasticity of the air, and therefore not more than \( \frac{1}{3} \)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 issuing through the safety-valve and all the crevices in the top of the boiler, and (in good engines) was about \( \frac{1}{6} \)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 \( \frac{1}{6} \)ths 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 safety-valve. At this time, therefore, it is still more elastic than air; but after this, the safety-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 greatly diminished 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. motion of the returning stroke is therefore so much deranged by this foreign and inappreciated circumstance, that it would have been quite useless to engage in the intricate exponential investigation, and we must sit 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 feebly 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 a small drop of some coloured fluid into it, so as to stand 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 close 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 elasticities 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 expends 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 \( \frac{1}{8} \)th of the whole power of the engine; if we cool it only to 145°, we leave an elasticity of \( \frac{1}{4} \)th; if we cool it to a blood-heat, we leave an elasticity of \( \frac{1}{2} \)th. 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-foot stroke, which works with something less than \( \frac{1}{4} \)th 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 \( \frac{1}{8} \)th 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 seemingly to ascertain the motion of the engine when 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 quantity of work. The result has been very unsatisfactory, because the computation of the returning stroke is acknowledged to be beyond our abilities. But it has 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. formed by it. We shall see this confirmed in the clear- est 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 Bofllet is in every re- spect erroneous even for the purpose which he had in view. We also see that the equation which we substi- tuted in its place, and which was intended for determi- ning 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 ex- tremely 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 we doubt exceedingly the truth of this maxim, and, 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 ac- celerate the returning stroke and retard the working stroke. But as the counter-weight is but small in pro- portion 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 ac- celerate 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 machine which we have seen, in re- spect of performance, raises a column of water whose weight is very nearly \( \frac{3}{4} \) of the pressure of the atmosphere on the piston, making 12 strokes of six feet each per mi- nute, and the working stroke was almost twice as slow 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\(\frac{1}{2}\) to 13\(\frac{1}{2}\) 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 of 3 to 4. This had retarded the working stroke; but the performance was evidently increased in the proportion of 3 x 13 to 4 x 11, or of 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 feebly 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 mislaid him in the construction of water-mills, especially of overshot mills; and, even now, he is submitting with hesitation and fear to the daily correction of ex- perience.

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

We venture to recommend as a maxim of construction The load (supposing always a large boiler and plentiful supply of pure steam unmixed with air), that the load of work be kept not less than 10 pounds for every square inch of the piston, and the counterweight so proportioned that the for every time of the returning stroke may not exceed \( \frac{3}{4} \) of that of the working stroke. A serious objection may be raised made to this maxim, and it deserves mature considera- tion. Such a load requires the utmost care of the ma- chine, 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 the pit, should make a greater load absolutely necessary. These considera- tions 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 Rules for give some easy rules, sanctioned by successful practice, com- puting the perform- ance artificer, who can go through simple calculations, to suit of the the size of his engine to the task which it is to per- form.

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 wa- ter 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 usu- ally 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.

To find the diameter of the pump, the area of the piston in square feet is \( \frac{0.7854}{144} \). The length of the column drawn in one minute is 12 times 6 or 72 feet, and therefore its solid contents is \( \frac{72 \times 0.7854}{144} \) cubic feet, or \( \frac{0.3927}{144} \) cubic feet. This must be equal to \( Q \); therefore \( p^2 \) must be \( \frac{Q}{0.3927} \) or nearly \( Q \times 2\(\frac{1}{2}\) \). Hence this practical rule: Multiply the cu- bic 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.

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 fix pounds on a circular inch very nearly. The weight of a cylinder of water an inch in diameter and a fathom in height is \( \frac{2}{3} \) pounds, or nearly 2 pounds. Hence it follows that \( e^2 \) must be made equal to \( \frac{2fp^2}{6} \), and that \( e^2 \) is equal to \( \frac{fp^2}{3} \).

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 \( e^2 = p^2 \times \frac{2f}{m} \), 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.

| Diam. of pump | Holds in one yard | Draws by a 6 feet stroke | Weighs in one yard | At 16 strokes per min. | Ditto in hogsheads | Ditto per hour | |---------------|------------------|-------------------------|-------------------|----------------------|------------------|---------------| | Inch. Gall. | Gall. | Lb. avoirdupois | Gall. | Hdgall. | Hdgall. | | 12 | 14.4 | 28.8 | 146 | 462 | 7.21 | 440 | | 11 | 12.13 | 24.26 | 123.5 | 338 | 6.20 | 309.33 | | 10 | 10.02 | 20.04 | 102 | 320 | 5.5 | 304.48 | | 9 | 8.12 | 16.24 | 82.7 | 259.8 | 4.7 | 247.7 | | 8½ | 7.26 | 14.52 | 73.9 | 234.8 | 3.43 | 221.15 | | 8 | 6.41 | 12.82 | 65.3 | 205.2 | 3.16 | 195.22 | | 7½ | 6.01 | 12.02 | 61.2 | 192.3 | 3.2 | 182.13 | | 7 | 5.66 | 11.32 | 57.6 | 181.1 | 2.55 | 172.30 | | 6½ | 4.91 | 9.82 | 50.9 | 157.1 | 2.31 | 149.40 | | 6 | 4.23 | 8.46 | 43 | 135.3 | 2.9 | 128.54 | | 5½ | 3.61 | 7.2 | 36.7 | 115.5 | 1.52 | 110.1 | | 5 | 3.13 | 6.2 | 31.8 | 99.2 | 1.36 | 94.30 | | 4½ | 2.51 | 5.0 | 25.5 | 80.3 | 1.7 | 66.61 | | 4 | 2.02 | 4.04 | 20.5 | 64.6 | 1.1 | 60.60 |

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 hogsheads 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 7 inches bore. In a line with this, under the depth of 50 yards, which is \( \frac{1}{4} \)th of 100 fathoms, we find 20¼, 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 the mind of human ingenuity has yet offered to his contemplation, converting and abandoning to the efforts of the unlettered artist. Its use has been entirely confined to the raising of water. Mr Keane 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 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 working 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 pressing 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 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 most 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 of having a cylinder of four feet diameter, working night and day, consumes about 3400 chaldron (Loudon) of good coals in a year.

This circumstance limits the use of steam-engines extensively. To draw water from coal-pits, where they can be stocked with unseizable small coal, they are of universal employment; also for valuable mines, for supplying a great and wealthy city with water, and a few other purposes where a great expense can be borne, they are very proper engines; but in a thousand cases where their unlimited powers might be vastly serviceable, the enormous expenses 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 notion 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.

(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 Public 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 usual for our neighbours on the continent to assume the credit of British inventions. 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 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 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 great a quantity of heat, it must expend a great quantity of fuel; and no construction of furnace can prevent this. Mr Watt therefore let 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 abstracting its heat. But all the heat thus communicated to the cylinder, and wasted by it on surrounding bodies, contributed nothing to the performance of the engine, and must be taken away at every injection, and again communicated and wasted. 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 cast 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 method of 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 feebly 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 1/4th of the atmospheric pressure, and therefore took away 1/4th 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 contrivance was 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 any person who understands the subject, that as long as any part of the condenser is kept 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 steam, a great deal yet remained, and that the 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 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 P D, 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 B C I O Q, which, after reaching the cylinder with its horizontal part BC, descends parallel to its side, sending 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 S T. The piston S has slack valves opening upwards, and its rod S r, 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 Q R, and shuts against a contrary pressure. This pump delivers its contents into another pump X Y, by means of the small pipe i X, which proceeds from its top. This second pump has a valve at X, and a slack in its piston Z as usual, and the piston rod Z z 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 w rises above the water, and may be raised or lowered by hand or by the plug beam. The valves R and X and the slacks 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 quietest 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 M L O N. 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 I O Q. 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 O Q S. 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 O Q R, 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 S T and X Y. The capacity of S T 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 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 X Y, 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 Y g 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 1/3th of the whole pressure. This is evident from the examination of the engine at Montrelais by Bofin. 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, and the waste of steam is the only inconvenience.

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 of it 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 shattered by the pumps drawing air, or a pump-rod breaking; by which accidents the steam-piston descends 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 safest 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 (at least during the working stroke) to the load of work that then happens to be on it. We have only to administer steam of a proper elasticity. At the first erection the engine may be equal to twice its task, if the steam admitted above the cylinder be equal to that of common boiling water; but when once the ebullition 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 185° 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 flail 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; nay, 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. 1c.) represent a section of the cylinder of a steam-engine, and EF the surface of its piston. Let us suppose that the steam was admitted while EF was in contact with AB, and that as soon as it had pressed it down to the situation EF the steam cock is shut. The steam will continue to press it down, 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 KI and DC, the ordinates of a rectangular hyperbola F/2, 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 EF & DE, and the pressure during the whole motion by the area ABF & DA. Now it is well known that the area \( \text{EF} \cdot \text{DE} \) is equal to \( \text{ABFE} \times (1 + L \cdot \frac{\text{AD}}{\text{AE}}) \).

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 8 inches, or 1.5 feet. The hyperbolic logarithm of \( \frac{6}{1.5} \) is 1.3862943.

Therefore the accumulated pressure \( \text{ABFE} \cdot \text{DA} \) is \( 6333 \times 2.3862943 = 15114 \) 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 \( \frac{\text{AD}}{\text{AE}} \), and multiply it by 2.3026; the product is the hyperbolic logarithm of \( \frac{\text{AD}}{\text{AE}} \).

The accumulated pressure while the piston moves from AB to EF is 6333 \times 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 \times 4, or 25332 pounds.

Here Mr Watt observed a remarkable result. The steam expended in this case would have been four times greater than when it was stopped at \( \frac{1}{4} \)th, and yet the accumulated pressure is not twice as great, being nearly 4ds. One-fourth of the steam performs nearly \( \frac{3}{4} \)ths of the work, and an equal quantity performs more than twice as much work when thus admitted during \( \frac{1}{4} \)th 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 retarded 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 \( \frac{1}{4} \)th. Its performance is multiplied:

\[ \begin{align*} \frac{1}{2} & : 1.7 \\ \frac{1}{4} & : 2.1 \\ \frac{1}{8} & : 2.4 \\ \frac{1}{16} & : 2.6 \\ \frac{1}{32} & : 2.8 \\ \frac{1}{64} & : 3.1 \\ \frac{1}{128} & : 3.2 \\ & \text{&c.} \end{align*} \]

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 its 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 cage 1111. The section of the piston A, and the collar of leathers which Watt's embraces the piston rod, gives a distinct notion of its steam-enclosure, 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 end 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 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 the 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 supposing the fly to drive the beam, let the beam drive the fly. The motions must be perfectly the 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 I. 8, 8 is the horizontal part of the steam pipe. 9 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 education valve. A part only of the upper education 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 education 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 education valve F are open, and that the lower steam valve E and upper education 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 education-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 education-pipe, because condensation is continually going on, and therefore there is a continual atmospheric pressure to produce a jet. The air which is discharged 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 education valves, we said that they were all puppet valves. Mr Watt employed cocks, and also sliding valves, such as the regulator or steam-valve of 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 servile 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 joining 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 any thing 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 fervently 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-pan. Here the valve is drawn back into a box, where the water is so 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 review of 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 distinguishing 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 judgment 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 more powerful than it is at present. The only thing that seems susceptible of considerable improvement Steam Engine.

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T it s equ eng dial Bu the tac to fitt the a f ch: (h: on mo the M ne wa th do T The enormous strains exerted 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 massy 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 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 Haerlem Meer by treating with Messrs Watt and Boulton for draining the Haerlem Meer, and even reducing the Zuyderzee engine. We doubt not but that it will be accomplished whenever that unhappy nation has sufficiently felt the difference between liberty and democratic 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 feeling 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 of some account little advantage; 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 have not hindered the erection of many engines which not injured the law would have deemed encroachments. We think the other 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 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 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 O 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 with-held by the beam, would descend till it is in equilibrio, 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 them arrive there. The cylinder B is at this time filled with steam of the ordinary density, and the cylinder A with an equal absolute quantity of steam, but expanded into a larger space.

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.

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

Let \(m\) 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 \(pq\), on which are fixed two pistons \(aa\) and \(bb\), 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 \(gb\) or \(ba\), 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 \(pq\) 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 \(hb\) or \(bh\); and this expresses the natural bulk of the steam which formerly filled it, and is now expanded into the space \(babami\). The part \(bbib\) is plainly \(=b-x\), and the part \(laamib\) is \(=mx\). The whole space therefore is \(mx+b-x=b+mx-x\), or \(b+mx-1x\). Therefore the density of the steam between the pistons is \(\frac{b}{b+mx-1x}\).

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

The lower piston \(aa\), having a vacuum below it, is pressed downwards with a force \(=\frac{mb}{b+mx-1x}\). Therefore the whole pressure on the piston rod downwards is \(=p(1+\frac{mb}{b+mx-1x})\), \(=p(1+\frac{mb}{b+mx-1x})\), \(=p+\frac{pb}{b+mx-1x}=\frac{pb}{b+mx-1x}\).

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 \(=\frac{2m-1}{m}\). 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 \(=\frac{pb}{b+mx-1x}\), as equal to the ordinate \(GF\),

\[Hb\text{, or }Mc\text{, of a curve }Fbc\text{ (fig. 10.), which has for its axis the line }GM\text{ equal to }h\text{ the height of our cylinder. Call this ordinate }y.\text{ We have }y=\frac{pb}{b+mx-1x},\text{ and }y-p=\frac{pb}{b+mx-1x}.\text{ Now it is plain that }\frac{pb}{b+mx-1x}\text{ is the ordinate of an equilateral hyperbola, of which }pb\text{ is the power or rectangle of the ordinate and abscissa, and of which the abscissa reckoned from the centre is }\frac{b}{b+mx-1x}.\text{ Therefore make }GE=p,\text{ and draw }DEA\text{ parallel to }MG,\text{ and make }EA=\frac{GM}{b+mx-1x}.\]

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 \(pb\), \(GEDM\) (for \(GE=p\), and \(GM=b\)); and since all the inscribed rectangles, such as \(AEFB\), are equal to \(pb\), it follows that \(AEFB\) is equal to \(GEDM\), and that the area \(ABFD\) is equal to the area \(GFE MG\), which expresses the accumulated pressure in Hornblower's engine.

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

The intelligent reader cannot but observe that this is precisely the same with the accumulated pressure of a mated quantity of steam admitted in the beginning, and stop-pressed in Mr Watt's method, when the piston has descended through the \(m\)th part of the cylinder. In consideration of Mr Hornblower's engine, the thing was presented in so different a form that we did not perceive the 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 observable in many particulars of this engine. 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 strut 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 servicable 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 pressure 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 titration soon calcined the fluid and rendered it useless. He then tried Parent's or Dr Barker's mill, inclosing 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 still all his attempts inferior to the reciprocating engine with a fly. A very crude scheme of this kind may be seen for difference in the Transactions of the Royal Society of Dublin rent in 1787. But although our attempts have hitherto failed, we hope that the case is not yet desperate; we employ different principles which have not yet been employed.

We shall conclude our account of this noble engine Mr Watt with observing, that Mr Watt's form suggests the construction of an excellent air-pump. A large vessel may be made to communicate with a boiler at one side, and 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, and drive out the air from it; then open its communication with the receiver and the condenser. This will rarefy the air of the receiver 10 times. Repeating the operation will rarefy it 100 times; the third operation will rarefy 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 1690), schemes have been proposed for dressing victuals by the steam of boiling water. A philosophical club used to dine at Saltero's coffee-house, Chelsea, about 30 years ago, and had their victuals 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 dressed, and both expeditiously and with high flavour. A patent was lately obtained for an apparatus for this purpose by a tin-man in London; we think of the name of Tate. They are made on a much more effective plan by 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 scorching, 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 soot unavoidable in the cooking on an open fire. Indeed the 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 manufacture 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 cockle 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 six 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 six vats are as completely heated in three hours, expending about three hundred weight of coals, as they were formerly in eight hours, expending near 18 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 so 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 $\frac{3}{4}$ 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 talk to this valuable corps of practical chemists.

Let A 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 BC 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 CD, 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 staple 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 STE [774] STE

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 set 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 brick 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 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 of the vessel. Nay, the meat can hardly be dressed 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 set 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 dressed. 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. 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 flouncy 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 every thing 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 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 complete 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 activity, 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 too 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 flue 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 indispensable 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.

Si quid novijs rectius illis, Condider imperitii—f non, his utere nostris.

STEATITES or Soap-earth, a genus of the magnesian order of earths. Of this genus there are several species, for which see MINERALOGY. According to the analysis of Bergman, 100 parts of steatites contain 80 of silic, 17 of mild magnesia, 2 of argillaceous earth, and nearly 1 of iron in a semioxidated state.

This substance may be formed into a paste with water, sufficiently ductile to be worked on the potter's wheel; and by exposure to a great heat it is hardened so as to strike fire with steel. It has also the property of FULZER'S Earth in cleansing clothes from grease; but it does not dissolve in water so well as clays do; and when digested with nitric acid, it does not form alum, as clays do, but a salt similar to Epsom salt. From its softness and ductility it may be easily formed into pots for the kitchen; and hence it has got the name of lapis ollaris.