(πλυντήρια, from πλύνω, to wash), a Grecian festival in honour of Athena, who received from the daughter of Cecrops the name of Aglauros. During the solemnity they undressed the statue of the goddess and washed it. The day on which it was observed the people regarded as unfortunate and inauspicious; and therefore no person was permitted to appear in the temples, which were purposely surrounded with ropes. It was customary at this festival to bear in procession a cluster of figs, thus intimating the progress of civilization amongst the primitive inhabitants of the earth. According to the present usage of our language, this term is restricted to that part of natural philosophy which treats of mechanical properties of elastic fluids. The word, in its original meaning, expresses a quality of air, or more properly, of breath.

We have extended the term Pneumatics to the study of the mechanical properties of all elastic or sensibly compressible fluids,—that is, of fluids whose elasticity and compressibility become an interesting object of our attention; as the term Hydrostatics is applied to the study of the mechanical properties of such bodies as interest us by their fluidity or liquidity only, or whose elasticity and compressibility are not familiar or interesting, though not less real or general than in the case of air and all vapours.

There is no precise limit to the different classes of bodies with respect to their mechanical properties. There is no such thing as a body perfectly hard, perfectly soft, perfectly elastic, or perfectly incompressible. All bodies have some degree of elasticity intermixed with some degree of ductility. Water, mercury, oil, are compressible; but their compressibility need not be attended to in order perfectly to understand the phenomena consequent on their materiality, fluidity, and gravity. But if we neglect the compressibility of air, we remain ignorant of the cause and nature of its most interesting phenomena, and are but imperfectly informed with respect to those in which its elasticity has no share; and it is convenient to attend to this distinction in our researches, in order to understand those phenomena which depend solely or chiefly on compressibility and elasticity. This observation is important; for here elasticity appears in its most simple form, unaccompanied with any other mechanical affection of matter (if we except gravity), and lies most open to our observation, whether employed for investigating the nature of this very property of bodies, or for explaining its mode of action. We shall even find that the constitution of an avowedly elastic fluid, whose compressibility is so very sensible, will give us the distinctest notions of fluidity in general, and enable us to understand its characteristic appearances, by which it is distinguished from solidity,—namely, the equal distribution of pressure through all its parts in every direction, and the horizontality which its surface assumes by the action of gravity; phenomena which have been assumed as equivalent to the definition of a perfect fluid, and from which all the laws of hydrostatics and hydraulics have been derived. And these laws have been applied to the explanation of the phenomena around us; and water, mercury, oil, &c., have been denominated fluid only because their appearances have been found to tally exactly with these consequences of this definition, while the definition itself remains in the form of an assumption, unsupported by any other proof of its obtaining in nature.

Of all the sensible compressible fluids, air is the most familiar, was the first studied, and has been the most minutely examined. It has therefore been generally taken as the example of their mechanical properties, whilst those mechanical properties which are peculiar to any of them, and therefore characteristic, have usually been treated as an appendix to the general science of pneumatics.

But although the mechanical properties are the proper subjects of our consideration, it will be impossible to avoid considering occasionally properties which are more of a chemical nature; because they occasion such modifications of the mechanical properties as would frequently be unintelligible without considering them in conjunction with the other; and, on the other hand, the mechanical properties produce such modifications of the properties merely chemical, and of very interesting phenomena consequent on them, that these would often pass unexplained unless we give an account of them in this place.

By mechanical properties we mean such as produce, or Mechanical are connected with, sensible changes of motion, and which properties indicate the presence and agency of moving or mechanical powers. They are therefore the subject of mathematical discussion; admitting of measure, number, and direction. We shall therefore begin with the consideration of air.

It is by no means an idle question to put, What is this air? What is of which so much is said and written? We see nothing, we hear nothing of it. We find ourselves at liberty to move about in any direction without any obstacle or hindrance. Whence, then, the assertion, that we are surrounded with a matter called air? A few very simple observations and experiments will show us that this assertion is well founded.

We are accustomed to say, that a vessel is empty when proofs that we have poured out of it the water which it contained, it is matter Take a cylindrical glass jar, having a small hole in its bottom; and having stopped this hole, fill the jar with water, and then pour out the water, leaving the glass empty, in the common acceptation of the word. Now, throw a bit of cork, or any light body, on the surface of water in a cistern; cover this with the glass jar A held in the hand with its bottom upwards, and move it downwards, as at B, keeping it all the while in an upright position. The cork will continue to float on the surface of the water in the inside of the glass, and will most distinctly show whereabout that surface is. It will thus be seen, that the water within the glass has its surface considerably lower at C than that of the surrounding water; and however deep we immerge the glass, we shall find that the water will never rise in the inside of it so as to fill it. If plunged to the depth of 32 feet, the water will only half fill it; and yet the acknowledged laws of hydrostatics tell us, that the water would fill the glass if there were nothing to hinder it. Therefore something already within the glass which prevents the water from getting into it; manifesting in this manner the most distinctive property of matter, viz. the hindering other matter from occupying the same place at the same time.

While things are in this condition, pull the stopper D out of the hole in the bottom of the jar, and the water will instantly rise in the inside of the jar, and stand at an equal height within and without. This is justly ascribed to the escape through the hole of the matter which formerly obstructed the entry of the water; for if the hand be held before the hole, a puff will be distinctly felt, or a feather held there will be blown aside; indicating in this manner that what prevented the entry of the water, and now escapes, possesses another characteristic property of matter, impulsive force. The materiality is concluded from this appearance, in the same manner that the materiality of water is concluded from the impulse of a jet from a pipe. We also see the mobility of the formerly pent up, and now liberated substance, in consequence of external pressure, viz. the pressure of the surrounding water. Also, if we take a smooth cylindrical tube, shut at one end, and fit a plug or cork to its open end, so as to slide along it, but so tightly as to prevent all passage by its sides; and if the plug be well soaked in grease, we shall find that no force whatever can push it to the bottom of the tube. There is therefore something within the tube preventing by its impenetrability the entry of the plug, and therefore possessing this characteristic of matter.

In like manner, if, after having opened a pair of common bellows, we shut up the nozzle and valve hole, and try to bring the boards together, we find it impossible. There is something included which prevents this, in the same manner as if the bellows were filled with wood; but on opening the nozzle we can easily shut them, viz. by expelling this something; and if the compression be forcible, the something will issue with considerable force, and very sensibly impel any thing in its way.

It is not accurate to say, that we move about without obstruction: for we find, that if we endeavour to move a large fan with rapidity, a very sensible hindrance is perceived, and a sensible wind is produced, which will agitate the neighbouring bodies. It is therefore justly concluded that the motion is possible only in consequence of having driven this obstructing substance out of the way; and that this impenetrable, resisting, moveable, impelling substance, is matter. We perceive the perseverance of this matter in its state of rest when we wave a fan, in the same manner that we perceive the inertia of water when we move a paddle through it. The effects of wind in impelling our ships and mills, in tearing up trees, and overturning buildings, are equal indications of its perseverance in a state of motion. To this matter, when at rest, we give the name air; and when it is in motion we call it wind.

Air, therefore, is a material fluid; a fluid, because its parts are easily moved, and yield to the smallest inequality of pressure. Air possesses some others of the very general, though not essential, properties of matter. It is heavy. This appears from the following facts.

1. It always accompanies this globe in its orbit round the sun, surrounding it to a certain distance, under the name of atmosphere, which indicates the being connected with the earth by its general force of gravity. It is chiefly in consequence of this that it is continually moving round the earth from east to west; forming what is called the trade-wind, to be more particularly considered afterwards. All that is to be observed on this subject at present is, that, in consequence of the disturbing forces of the sun and moon, there is an accumulation of the air of the atmosphere, in the same manner as of the waters of the ocean, in those parts of the globe which have the moon near their zenith or nadir; and as this happens successively, going from the east to the west, by the rotation of the earth round its axis in the opposite direction, the accumulated air must gradually flow along to form the elevation. This is chiefly to be observed in the torrid zone; and the generality and regularity of this motion are greatly disturbed by the changes which are continually taking place in different parts of the atmosphere from causes which are not mechanical.

2. It is in like manner owing to the gravity of the air that it supports the clouds and vapours which we see constantly floating in it. We have even seen bodies of no considerable weight float, and even rise, in the air. Soap bubbles, and balloons filled with inflammable gas, (hydrogen or gas obtained from oil or coal,) rise and float in the same manner as a cork rises in water. This phenomenon proves the weight of the air, in the same manner that the swimming of a piece of wood indicates the weight of the water which supports it.

3. But we are not left to these refined observations for the proof of the air's gravity. We observe many familiar phenomena, which must be immediate consequences of the supposition that air is a heavy fluid, and, like other heavy fluids, presses on the outsides of all bodies immersed in or surrounded by it. Thus, for instance, if we shut the nozzle and valve hole of a pair of bellows after having squeezed the air out of them, we shall find that even some hundred pounds, are necessary for separating the boards. They are kept together by the pressure of the heavy air which surrounds them in the same manner as if they were immersed in water. In like manner, if we stop the end of a syringe after its piston has been pressed down to the bottom, and then attempt to draw up the piston, we shall find a considerable force necessary, viz. about fifteen or sixteen pounds for every square inch of the section of the syringe. Exerting this force, we can draw up the piston to the top, and we can hold it there; but the moment we cease acting, the piston rushes down and strikes the bottom. It is called a suction, as we feel something as it were drawing in the piston; but it is really the weight of the incumbent air pressing it in. And this obtains in every position of the syringe; because the air is a fluid, and presses in every direction. Nay, it presses on the syringe as well as on the piston; and if the piston be hung by its ring on a nail, the syringe requires force to draw it down, just as much as to draw the piston up; and if it be let go, it will spring up unless loaded with at least fifteen pounds for every square inch of its transverse section. See fig. 2.

4. But the most direct proof of the weight of the air is It may had by weighing a vessel empty of air, and then weighing even be it again when the air has been admitted; and this, as it is weighed, the most obvious consequence of its weight, has been asserted as long ago as the days of Aristotle. If we take a very large and limber bladder, and squeeze out the air very carefully, and weigh it, and then fill it till the wrinkles just begin to disappear, and weigh it again, we shall find no difference in the weight. But this is not Aristotle's meaning; because the bladder, considered as a vessel, is equally full in both cases, its dimensions being changed. We cannot take the air out of a bladder without its immediately collapsing. But what would be true of a bladder would be equally true of any vessel. Therefore, take a round vessel A, (fig. 3.) fitted with a stopcock B, and syringe C. Fill the whole with water, and press the piston to the bottom of the syringe. Then keeping the cock open, and holding the vessel upright, with the syringe undermost, draw down the piston. The water will follow it by its weight, and leave part of the vessel empty. Now shut the cock, and again push up the piston to the bottom of the syringe; the water escapes through the piston valve, as will be explained afterwards; then opening the cock, and again drawing down the piston, more water will come out of the vessel. Repeat this operation till all the water have come out. Shut the cock, unscrew the syringe, and weigh the vessel very accurately. Now open the cock, and admit the air, and weigh the vessel again; it will be found heavier than before, and this additional weight is the weight of the air which fills it; and it will be found to be 523 grains, about an ounce and a fifth avoirdupois, for every cubic foot that the vessel contains. Now since a cubic foot of water would weigh 1000 ounces, this experiment would show that water is about 840 times heavier than The most accurate judgment of this kind of which we have met with an account, is that recorded by Sir George Shuckburgh, in the sixty-seventh volume of the Philosophical Transactions, (p.560.) From this it follows, that when the air is of the temperature 53, and the barometer stands at 29½ inches, the air is 836 times lighter than water. But the experiment is not susceptible of sufficient accuracy for determining the exact weight of a cubic foot of air. Its weight is very small; and the vessel must be strong and heavy, so as to overload any balance that is sufficiently nice for the experiment.

To avoid this inconvenience, the whole may be weighed in water, first loading the vessel so as to make it preponderate an ounce or two in the water. By this means the balance will be loaded only with this small preponderance. But even in this case there are considerable sources of error arising from changes in the specific gravity of the water and other causes. The experiment has often been repeated with this view, and the air has been found at a medium to be about 840 times as light as water, but with great variations, as may be expected from its very heterogeneous nature, in consequence of its being the menstruum of almost every fluid, or all vapours, and even of most solid bodies; all which it holds in solution, forming a fluid perfectly transparent, and of very different density according to its composition. It is found, for instance, that perfectly pure air of the temperature of our ordinary summer is considerably denser than when it has dissolved about half as much water as it can hold in that temperature; and that with this quantity of water the difference of density increases in proportion as the mass grows warmer, for damp air is more expansible by heat than dry air. We have had occasion to consider this subject when treating of the connection of the mechanical properties of air with the state of the weather.

Such is the result of the experiment suggested by Aristotle, evidently proving the weight of the air; and yet the Peripatetics uniformly refused it this property. It was a matter long debated among the philosophers of the last century. The reason was, that Aristotle assigns a different cause to many phenomena which any man led by common observation would ascribe to the weight of the air. Of this kind is the rise of water in pumps and syphons. Aristotle had asserted that all nature was full of being, and that nature abhorred a void. He adduces many facts, in which it appears, that if not absolutely impossible, it is very difficult, and requires great forces, to produce a space void of matter. When the operation of pumps and syphons came to be known, the philosophers of Europe, found in this fancied horror a ready solution of the phenomena. We shall state the facts that every reader may see what kind of reasoning was received not two centuries ago.

Pumps were then constructed in the following manner: A long pipe GB was set in the water of the well A. This was fitted with a sucker or piston C, having a long rod CF, and was furnished with a valve B at the bottom, and a lateral pipe DE at the place of delivery, also furnished with a valve. The fact is, that if the piston be thrust down to the bottom, and then drawn up, the water will follow it; and upon the piston being again pushed down, the water shuts the valve B by its weight, and escapes or is expelled at the valve E; and on drawing up the piston again the valve E is shut, the water again rises after the piston, and is again expelled at its next descent.

The Peripatetics explain all this by saying, that if the water did not follow the piston there would be a void between them. But nature abhors a void; therefore the water follows the piston; this reasoning is overturned by one observation. Suppose the pipe shut at the bottom, the piston can be drawn up, and thus a void produced.

Galileo seems to have been the first who seriously ascribed this to the weight of the air. Many had supposed first pre-air heavy; and thus explained the difficulty of raising the dipter the board of bellows, or the piston of a syringe, &c. But he distinctly applies to this allowed weight of the air all the consequences of hydrostistical laws; and he reasons as follows. The heavy air rests on the water in the cistern, and presses it with its weight. It does the same with the water in the pipe, and therefore both are on a level; but if the piston, after being in contact with the surface of the water, be drawn up, there is no longer any pressure on the surface of the water within the pipe; for the air now rests on the piston only, and thus occasions a difficulty in drawing it up. The water in the pipe, therefore, is in the same situation as if more water were poured into the cistern, that is, as much as would exert the same pressure on its surface as the air does. In this case we are certain that the water will be pressed into the pipe, and will raise up the water in it, and follow it till it is equally high within and without. The same pressure of the air shuts the valve E during the descent of the piston. He did not wait for the very obvious objection, that if the rise of the water was the effect of the air's pressure, it would also be its measure, and would be raised and supported only to a certain height. He directly said so, and adduced this as a decisive experiment. If the horror of a void be the cause, says he, the water must rise to any extent however great; but if it be owing to the pressure of the air, it will only rise till the weight of the water in the pipe is in equilibrium with the pressure of the air, according to the common laws of hydrosatics. And he adds, that this is well known; for it is a fact, that pumps will not draw water much above forty palms, although they may be made to propel it, or to lift it to any height. He then makes an assertion, which, he says, if true, will be decisive. Let a very long pipe, shut at one end, be filled with water, and let it be erected perpendicularly with the close end uppermost, and a stopper in the other end, and then its lower orifice immersed into a vessel of water; the water will subside in the pipe upon removing the stopper, till the remaining column is in equilibrium with the pressure of the external air. This experiment he proposes to the curious; saying, however, that he thought it unnecessary, there being already such abundant proofs of the air's pressure.

It is probable that the clumsiness of the necessary apparatus protracted the making of this experiment. Another equally conclusive, and much easier, was made in 1642 after Galileo's death, by his zealous and learned disciple Torricelli. He filled a glass tube, close at one end, with mercury; judging, that if the support of the water was owing to the pressure of the air, and was the measure of this pressure, mercury would in like manner be supported by it, and this at a height which was also the measure of the air's pressure, and therefore thirteen times less than water. He had the pleasure of seeing his expectation verified in the completest manner; the mercury descending in the tube AB, and finally settling at the height... Air. fB of 293 Roman inches: and he found, that when the tube was inclined, the point f was in the same horizontal plane with f in the upright tube, according to the received laws of hydrostatical pressure. The experiment was often repeated, and soon became famous. About three years afterwards the same experiment was published, at Warsaw in Poland, by Valerianus Magnus, as his own discovery; but it appears from the letters of Roeraval, not only that Torricelli was prior, and that his experiment was the general topic of discussion among the curious; but also highly probable that Valerianus Magnus was informed of it when at Rome, and daily conversant with those who had seen it. He denies, however, even having heard of the name of Torricelli.

This was the era of philosophical ardour; and we think that it was Galileo's invention and immediate application of the telescope which gave it vigour. Discoveries of the most wonderful kind in the heavens, and which required no extent of previous knowledge to understand them, were thus put into the hands of every person who could purchase a spy-glass; whilst the high degree of credibility which some of the discoveries, such as the phases of Venus and the rotation and satellites of Jupiter, gave to the Copernican system, immediately set the whole body of the learned in motion. About the years 1642 and 1644 we find clubs of gentlemen associated in Oxford and London for the cultivation of knowledge by experiment; and before 1655 all the doctrines of hydromatics and pneumatics were familiar there, and established by experiment. Mr. Boyle procured a coalition of these clubs under the name of the Invisible and Philosophical Society. In May 1658 Mr. Hooke finished for Mr. Boyle an air-pump, which had employed him a long time, and occasioned him several journeys to London for things which the workmen of Oxford could not execute. He speaks of this as a great improvement on Mr. Boyle's own pump, which he had been using some time before. Boyle therefore must have invented his air-pump, and was not indebted for it to Schottus's account of Otto Guericke's, published in the Mechanica Hydroaero-pneumatica of Schottus in 1657, as he asserts Technica Curiosa. The Royal Society of London arose in 1666 from the coalition of these clubs, after fifteen years' co-operation and correspondence. The Montmorin Society at Paris had subsisted nearly about the same time; for we find Pascal in 1648 speaking of the meetings in the Sorbonne College, from which we know that society originated. Nuremberg, in Germany, was also a distinguished seminary of experimental philosophy. The magistrates, sensible of its valuable influence in many manufactures, the source of the opulence and prosperity of their city, and many of them philosophers, gave philosophy a professed and magnificent patronage, furnishing the philosophers with a copious apparatus, a place of assembly, and a fund for the expense of their experiments; so that this was the first academy of sciences out of Italy under the patronage of government. In Italy, indeed, there had long existed institutions of this kind. Rome was the centre of church-government, and the resort of all expectants for preferment. The clergy was the majority of the learned in all Christian nations, and particularly of the systematic philosophers. Each, eager to recommend himself to notice, brought forward everything that was curious; and they were the willing vehicles of philosophical communication. Thus the experiments of Galileo and Torricelli were rapidly diffused by persons of rank, the dignitaries of the church, or by the monks their obsequious servants.

All now agree in giving Torricelli the honour of the first invention; and it universally passes by the name of the Torricellian Experiment. The tube is called the Torricellian tube; and the space left by the mercury is called the Torricellian Vacuum, to distinguish it from the Boylean Vacuum, which is only an extreme rarefaction. The experiment was repeated in various forms, and with apparatus which enabled philosophers to examine several effects which the vacuum produced on bodies exposed to it. This was done by making the upper part of the tube terminate in a vessel of some capacity, or communicate with such a vessel, in which were included along with the mercury bodies on which the experiments were to be made. When the mercury had run out, the phenomena of these bodies were carefully observed.

An objection was made to the conclusion drawn from Torricelli's experiment, which appears formidable. If the Torricellian tube be suspended on the arm of a balance, it is concluded that the counterpoise must be equal to the weight both of the tube and of the mercury it contains. This could not be, say the objectors, if the mercury were supported by the air. It is evidently supported by the balance; and this gave rise to another notion of the cause different from the Peripatetic fugae vacui. A suspensive force, or rather attraction, was assigned to the upper part of the tube.

But the true explanation of the phenomena is most easy and satisfactory. Suppose the mercury in the cistern and tube to freeze, but without adhering to the tube, so that the tube could be freely drawn up and down. In this case the mercury is supported by the base, without any dependence on the pressure of the air; and the tube is in the same condition as before, and the solid mercury performs the office of a piston to this kind of syringe. Suppose the tube thrust down till the top of it touches the top of the mercury. It is evident that it must be drawn up in opposition to the pressure of the external air, and it is precisely similar to the syringe in fig. 2. The weight sustained thereby by this arm of the balance is the weight of the tube and the downward pressure of the atmosphere on its top.

The curiosity of philosophers being thus excited by this Galileo's very manageable experiment, it was natural now to try the original experiment proposed by Galileo. Accordingly Ber-perimenti in Italy, Pascal in France, and many others in different places, made the experiment with a tube filled with water, wine, oil, &c., and all with the success which might be expected in so simple a matter; and hence the doctrine of the weight and pressure of the air was established beyond contradiction or doubt. All this was done before the year 1648. A very beautiful experiment was exhibited by Auxout, which completely satisfied all who had any remaining doubts.

A small box or vial EFGH, had two glass tubes, AB, CD, three feet long, inserted into it in such a manner as to be firmly fixed in one end, and to reach nearly to the other end. AB was open at both ends, and CD was close at D. This apparatus was completely filled with mercury, by unscrewing the tube AB, filling the box and the tube CD; then screwing in the tube AB, and filling it; then holding a finger on the orifice A, the whole was inverted and set upright in the position represented in the figure B, immersing the orifice A (now a of fig. B) in a small vessel of quicksilver. The result was, that the mercury ran out at the orifice a, till its surface m n within the flask descended to the top of the tube b a. The mercury also began to descend in the tube d e (formerly DC,) and run over into the tube b a, and run out at a, till the mercury in d e was very near equal in a level with m n. The mercury descending in b a till it stood at k, 293 inches above the surface o p of the mercury in the cistern, just as in the Torricellian tube.

The rationale of this experiment is very easy. The whole apparatus may first be considered as a Torricellian tube of an uncommon shape, and the mercury would flow out at a. But as soon as a drop of mercury comes out, leaving a space above m n, there is nothing to keep up the mercury in the tube d e. Its mercury, therefore, descends also; and running over into b a, continues to supply its expence till the tube d e is almost empty, or can no longer supply the waste of b a. The inner surface therefore falls as low as it can, till it is level with b. No more mercury can enter b a, yet its column is too heavy to be supported by the pressure of the air on the cistern below; it therefore descends in b a, and finally settles at the height h o, equal to that of the mercury in the Torricellian tube.

The prettiest circumstance of the experiment remains. Make a small hole g in the upper cap of the box. The external air immediately rushes in by its weight, and now presses on the mercury in the box. This immediately raises the mercury in the tube d e to l, 29½ inches above m n. It presses on the mercury at k in the tube b a, balancing the pressure of the air in the cistern. The mercury in this tube therefore is left to the influence of its own weight, and it descends to the bottom. Nothing can be more opposite or decisive.

And thus the doctrine of the gravity and pressure of the air is established by the most unexceptionable evidence; and we are entitled to assume it as a statical principle, and to affirm a priori all its legitimate consequences.

And in the first place, we obtain an exact measure of the pressure of the atmosphere. It is precisely equal to the weight of the column of mercury, of water, of oil, &c., which it can support; and the Torricellian tube, or others fitted up upon the same principle, are justly termed baroscopes and barometers with respect to the air. Now it is observed that water is supported at the height of 32 feet nearly; the weight of the column is exactly 2000 avoirdupois pounds on every square foot of base, or 13½ on every square inch. The same conclusion very nearly may be drawn from the column of mercury, which is nearly 29½ inches high when in equilibrium with the pressure of the air. We may here observe, that the measure taken from the height of a column of water, wine, spirits, and the other fluids of considerable volatility, as chemists term it, is not so exact as that taken from mercury, oil, and the like. For it is observed, that the volatile fluids are converted by the ordinary heat of our climates into vapour when the confining pressure of the air is removed; and this vapour, by its elasticity, exerts a small pressure on the surface of the water, &c., in the pipe, and thus counteracts a small part of the external pressure; and therefore the column supported by the remaining pressure must be lighter, that is, shorter. Thus it is found, that rectified spirits will not stand much higher than is competent to a weight of 13 pounds on an inch, the elasticity of its vapour balancing about ¼ of the pressure of the air. We shall afterwards have occasion to consider this matter more particularly.

As the medium height of the mercury in the barometer is 29½ inches, we see that the whole globe sustains a pressure equal to the whole weight of a body of mercury of this height; and that all bodies on its surface sustain a part of this in proportion to their surfaces. An ordinary sized man sustains a pressure of several thousand pounds. How comes it then that we are not sensible of a pressure which one should think enough to crush us together? This has been considered as a strong objection to the pressure of the air; for when a man is plunged a few feet under water, he is very sensible of the pressure. The answer is by no means so easy as is commonly imagined. We feel very distinctly the effects of removing this pressure from any part of the body. If any one will apply the open end of a syringe to his hand, and then draw up the piston, he will find his hand sucked into the syringe with great force, and it will give pain; and the soft part of the hand will swell into it, being pressed in by the neighbouring parts, which are subject to the action of the external air. If one lays his hand on the top of a long perpendicular pipe, such as a pump filled to the brim with water, which is at first prevented from running out by the valve below; and if the valve be then opened, so that the water descends, he will then find his hand so hard pressed to the top of the pipe, that he cannot draw it away. But why do we only feel the inequality of pressure? There is a similar instance wherein we do not feel it, although we cannot doubt of its existence. When a man goes slowly to a great depth under water in a diving-bell, we know unquestionably that he is exposed to a new and very great pressure, yet he does not feel it. But those facts are not sufficiently familiar for general argument. The human body is a bundle of solids, hard or soft, filled or mixed with fluids, and there are few or no parts of it which are empty. All communicate either by vessels or pores; and the whole surface is a sieve through which the insensible perspiration is performed. The whole extended surface of the lungs is open to the pressure of the atmosphere; everything is therefore in equilibrium; and if free or speedy access be given to every part, the body will not be damaged by the pressure, however great, any more than a wet sponge would be deranged by plunging it any depth in water. The pressure is instantaneously diffused by means of the incompressible fluids with which the parts are filled; and if any parts are filled with air or other compressible fluids, these are compressed till their elasticity again balances the pressure. Besides, all our fluids are acquired slowly, and gradually mixed with that proportion of air which they can dissolve or contain. The whole animal has grown up in this manner from the first vital atom of the embryo. For such reasons the pressure can occasion no change of shape by squeezing together the flexible parts; nor any obstruction by compressing the vessels or pores. We cannot say what would be felt by a man, were it possible that he could have been produced and grown up in vacuo, and then subjected to the compression. We even know that any sudden and considerable change of general pressure is very severely felt. Persons in a diving-bell have been almost killed by letting them down or drawing them up too suddenly. In drawing up, the elastic matters within have suddenly swelled, and not finding an immediate escape, have burst the vessels. Dr. Halley experienced this, the blood gushing out from his ears by the expansion of air contained in the internal cavities of this organ, from which there are but very slender passages.

A very important observation recurs here. The pressure of the atmosphere is variable. This was observed almost as soon as philosophers began to attend to the barometer. Pascal observed it in France, and Descartes observed it in Sweden in 1650. Mr. Boyle and others observed it in England in 1656. And before this, observers, who took notice of the concomitancy of these changes of aerial pressure with the state of the atmosphere, remarked, that it was generally greatest in winter and in the night; and certainly most variable during winter and in the northern regions. Familiar now with the weight of the air, and considering it as the vehicle of the clouds and vapours, they noted with care the connection between the weather and the pressure of the air, and found that a great pressure of the air was generally accompanied with fair weather, and a diminution of it with rain and mista. Hence the barometer came to be considered as an index not only of the present state of the air's weight, but also as indicating by its variations changes of weather.

In the next place, we may conclude that the pressure of the air will be different in different places, according to sure of the their elevation above the surface of the ocean; for if air be portion to a heavy fluid, it must press in some proportion according to the elevation perpendicular height. If it be a homogeneous fluid of uniform density and weight in all its parts, the mercury in the cistern of a barometer must be pressed precisely in proportion to the depth to which that cistern is immersed in it; and as this pressure is exactly measured by the height of The mercury in the tube, the height of the mercury in the Torricellian tube must be exactly proportional to the depth of the place of observation under the surface of the atmosphere.

This celebrated Descartes first entertained this thought (Epist. 67. of Pr. III.) and soon after him Pascal. His occupation in Paris not permitting him to try the justness of his conjecture, he requested Mr. Perrier, a gentleman of Clermont in Auvergne, to make the experiment, by observing the height of the mercury at one and the same time at Clermont and on the top of a very high mountain in the neighbourhood. His letters to Mr. Perrier in 1647 are still extant. Accordingly, Mr. Perrier, in September 1648, filled two equal tubes with mercury, and observed the heights of both to be the same, viz. 26½ inches, in the garden of the convent of the Friars Minims, situated in the lowest part of Clermont. Leaving one of them there, and one of the fathers to observe it, he took the other to the top of Puy de Dome, which was elevated nearly 500 French fathoms above the garden. He found its height to be 23½ inches. On his return to the town, in a place called Font de l'Arbre, 150 fathoms above the garden, he found it 25 inches; when he returned to the garden it was again 26½, and the person set to watch the tube which had been left, said that it had not varied the whole day. Thus a difference of elevation of 3000 French feet had occasioned a depression of 3½ inches; from which it may be concluded, that 3½ inches of mercury weighs as much as 3000 feet of air, and one-tenth of an inch of mercury as much as 96 feet of air. The next day he found, that taking the tube to the top of a steeple 120 feet high, made a fall of one-sixth of an inch. This gives 72 feet of air for one-tenth of an inch of mercury; but ill agreeing with the former experiment. But it is to be observed, that a very small error of observation of the barometer would correspond to a great difference of elevation, and also that the height of the mountain had not been measured with any precision. This has been since done (Mem. Acad. par. 1703.) and found to be 529 French toises.

Pascal published an account of this great experiment, and it was quickly repeated in many places of the world. In 1653, it was repeated in England by Dr. Power (Power's Exper. Phil.) and in Scotland in 1661, by Mr. Sinclair, professor of philosophy in the university of Glasgow, who observed the barometer at Lanark, on the top of mount Tinto in Clydesdale, and on the top of Arthur's seat at Edinburgh. He found a depression of two inches between Glasgow and the top of Tinto, three quarters of an inch between the bottom and top of Arthur's seat, and ¾ of an inch at the cathedral of Glasgow on a height of 126 feet.

Hence we may derive a method of measuring the heights of mountains. Having ascertained with great precision the elevation corresponding to a fall of one-tenth of an inch of mercury, which is nearly 90 feet, we have only to observe the length of the mercurial column at the top and bottom of the mountain, and to allow 90 feet for every tenth of an inch. Accordingly this method has been practised with great success; but it requires an attention to many things not yet considered; such as the change of density of the mercury by heat and cold; the changes of density of air from its compressibility; a change immediately connected with or dependent on the very elevation we wish to measure.

These observations give us the most accurate measure of the density of air and its specific gravity. This is but vaguely though directly measured by weighing air in a bladder or vessel. The weight of a manageable quantity is so small, that a balance sufficiently ticklish to indicate even very sensible fractions of it is overloaded by the weight of the vessel which contains it, and ceases to be exact; and when we take Bernouilli's ingenious method of suspending it in water, we expose ourselves to great risk of error by the variation of the water's density. Also it must necessarily be humid air which we can examine in this way; but the proportion of an elevation in the atmosphere to the depression of the column of mercury or other fluid, by which we measure its pressure, gives us at once the proportion of this weight or their specific gravity. Thus since it is found that in such a state of pressure the barometer stands at 30 inches, and the thermometer at 32°, 87 feet of rise produces one-tenth of an inch of fall in the barometer, the air and the mercury being both of the freezing temperature, we must conclude that mercury is 10,440 times heavier or denser than air. Then, by comparing mercury and water, we get nearly for the density of air relative to water; but this varies so much by heat and moisture, that it is useless to retain anything more than a general notion of it; nor is it easy to determine whether this method or that by actual weighing be preferable. It is extremely difficult to observe the height of the mercury in the barometer nearer than ½ of an inch; and this will produce a difference of even five feet, or ¼ of the whole. Perhaps this is a greater proportion than the error in weighing.

From the same experiments we also derive some knowledge of the height of the aerial covering which surrounds our globe. When we raise our barometer 87 feet above the surface of the sea, the mercury falls about one-tenth of an inch in the barometer; therefore if the barometer shows 30 inches at the sea-shore, we may expect that, by raising it 300 times 87 feet or five miles, the mercury in the tube will descend to the level of the cistern, and that this is the height of our atmosphere. But other appearances lead us to suppose a much greater height. Meteors are seen with us much higher than this, and which yet give undoubted indication of being supported by our air. There can be little doubt, too, that the visibility of the expanse above us is owing to the reflection of the sun's light by our air. Were the heavenly spaces perfectly transparent, we should no more see them than the purest water through which we see other objects; and we see them as we see water tinged with milk or other fleecy. Now it is easy to show, that the light which gives us what is called twilight must be reflected from the height of at least 50 miles; for we have it when the sun is depressed 18 degrees below our horizon.

A little attention to the constitution of our air will convince us, that the atmosphere must extend to a much greater height than 300 times 87 feet. We see from the most familiar facts that it is compressible; we can squeeze it in an ox-bladder. It is also heavy; pressing on the air in this bladder with a very great force, not less than 1500 pounds. We must therefore consider it as in a state of compression, existing in smaller room than it would assume if it were not compressed by the incumbent air. It must therefore be in a condition something resembling that of a quantity of fine carded wool thrown loosely into a deep pit; the lower strata carrying the weight of the upper strata, and being compressed by them; and so much the more compressed as they are further down, and only the upper stratum in its unconstrained and most expanded state. If we shall suppose this wool thrown in by a hundred weight at a time, it will be divided into strata of equal weights, but of unequal thickness; the lowest being the thinnest, and the superior strata gradually increasing in thickness. Now, suppose the pit filled with air, and reaching to the top of the atmosphere, the weights of all the strata above any horizontal plane in Air. It is measured by the height of the mercury in the Toricelli tube placed in that plane; and one-tenth of an inch of mercury is just equal to the weight of the lowest stratum 87 feet thick; for on raising the tube 87 feet from the sea, the surface of the mercury will descend one-tenth of an inch. Raise the tube till the mercury fall another tenth. This stratum must be more than 87 feet thick; how much more we cannot tell, being ignorant of the law of the air's expansion. In order to make it fall a third tenth, we must raise it through a stratum still thicker; and so on continually.

All this is abundantly confirmed by the very first experiment made by the order and directions of Pascal. For by carrying the tube from the garden of the convent to a place 150 fathoms higher, the mercury fell \( \frac{1}{10} \) inches, or 1.2916 inches, which gives about 69 feet eight inches of aerial stratum for \( \frac{1}{10} \) of an inch of mercury; and by carrying it from thence to a place 350 fathoms higher, the mercury fell \( \frac{1}{10} \), or 1.9167 inches, which gives 109 feet seven inches for \( \frac{1}{10} \) of an inch of mercury. These experiments were not accurately made. It is evident, however, from the whole tenor of them, that the strata of air decrease in density as we ascend through the atmosphere; but it remained to be discovered what is the force of this decrease, that is, the law of the air's expansion. Till this be done we can say nothing about the constitution of our atmosphere; we cannot tell in what manner it is fittest for raising and supporting the exhalations and vapours which are continually arising from the inhabited regions; not as an excrementitious waste, but to be supported, perhaps manufactured, in that vast laboratory of nature, and to be returned to us in beneficent showers. We cannot use our knowledge for the curious, and frequently useful, purpose of measuring the heights of mountains and taking the levels of extensive regions; in short, without an accurate knowledge of this, we can hardly acquire any acquaintance with those mechanical properties which distinguish air from those liquids which circulate here below.

Having therefore considered at some length the leading consequences of the air's fluidity and gravity, let us consider its compressibility with the same care; and then, combining the agency of both, we shall answer all the purposes of philosophy, discover the laws, explain the phenomena of nature, and improve art. We proceed therefore to consider a little the phenomena which indicate and characterise this other property of the air. All fluids are elastic and compressible as well as air; but in them the compressibility makes no figure, or does not interest us while we are considering their pressures, motions, and impulsions. But in air the compressibility and expansion draw our chief attention, and make it a proper representative of this class of fluids.

Nothing is more familiar than the compressibility of air. It is seen in a bladder filled with it, which we can forcibly squeeze into less room; it is seen in a syringe, of which we can push the plug farther and farther as we increase the pressure. But these appearances bring into view another, and the most interesting, property of air, viz., its elasticity. When we have squeezed the air in the bladder or syringe into less room, we find that the force with which we compressed it is necessary to keep it in this bulk; and that if we cease to press it together, it will swell out and regain its natural dimensions. This distinguishes it essentially from such a body as a mass of flour, salt, or such like, which remain in the compressed state to which we reduce them. There is something therefore which opposes the compression different from the simple impenetrability of the air; there is something that opposes mechanical force; there is something too which produces motion, not only resisting compression, but pushing back the compressing body, and communicating motion to it. As an arrow is gradually accelerated by the bow-string pressing it forward, and at the moment of its discharge is brought to a state of rapid motion; so the ball from a pop-gun or wind-gun is gradually accelerated along the barrel by the pressure of the air during its expansion from its compressed state, and finally quits it with an accumulated velocity. These two motions are indications perfectly similar of the elasticity of the bow and of the air.

Thus it appears that air is heavy and elastic. It needs Fluidity of little consideration to convince us in a vague manner that the air is fluid. The case with which it is penetrated, and driven almost in every direction, and the motion of its pipes and channels, however crooked and intricate, entitle it to this character. But before we can proceed to deduce consequences from its fluidity, and to offer them as a true account of what will happen in these circumstances, it is necessary to exhibit some distinct and simple case, in which the characteristic mechanical property of a fluid is clearly and unequivocally observed in it. That property of fluids is, that any pressure applied to any part of them is propagated through the whole mass in every direction; and that in consequence of this diffusion of pressure, any two external forces can be put in equilibrium by the interposition of a fluid, in the same way as they can be put in equilibrium by the intervention of any mechanical engine.

Let a close vessel ABC (fig. 7.), of any form, have two upright pipes EDC, GFB, inserted into any parts of its top, sides, or bottom, and let water be poured into them, so as to stand in equilibrium with the horizontal surfaces at E, D, G, F; and let D d, Ff, be horizontal lines, it will be found that the height of the column E d is sensibly equal to that of the column G f. This is a fact universally observed in whatever way the pipes are inserted. Now the surface of the water at D is undoubtedly pressed upwards with a force equal to a column of water, having its surface for its base, and E d for its height; it is therefore prevented from rising by some opposite force. This can be nothing but the elasticity of the confined air pressing it down. The very same thing must be said of the surface at F; and thus there are two external pressures at D and F set in equilibrium by the interposition of air. The force exerted on the surface D, by the pressure of the column E d, is therefore propagated to the surface at F; and thus air has this characteristic mark of fluidity.

In this experiment the weight of the air is insensible when the vessel is of small size, and has no sensible share in the pressure reaching at D and F. But if the elevation of the point F above D is very great, the column E d will be observed sensibly to exceed the column G f. Thus if F be 70 feet higher than D, E d will be an inch longer than the column G f; for in this case there is reacting at D, not only the pressure propagated from F, but also the weight of a column of air, having the surface at D for its base and 70 feet high. This is equal to the weight of a column of water one inch high.

It is by this propagation of pressure, this fluidity, that the pellet is discharged from a child's pop-gun. It sticks fast in the muzzle; and he forces in another pellet at the other end, which he presses forward with the rammer, condensing the air between them, and thus propagating to the other pellet the pressure which he exerts, till the friction is overcome, and the pellet is discharged by the air expanding and following it.

There is a philosophical toy which illustrates this property Hero's air, and which we shall have occasion to consider as converted into a useful hydraulic machine. This is what is usually called Hero's fountain. It consists of two vessels KLMN (fig. 8.), OPQR, which are close on all sides. The tube AB, having a funnel a-top, passes through the upper- most vessel without communicating with it, being soldered into its top and bottom. It also passes through the top of the under-vessel, where it is also soldered, and reaches almost to its bottom. This tube is open at both ends. There is another open tube ST, which is soldered into the top of the under vessel and the bottom of the upper vessel, and reaches almost to its top. These two tubes serve also to support the upper vessel.

A third tube GF is soldered into the top of the upper vessel, and reaches almost to its bottom. This tube is open at both ends, but the orifice G is very small. Now suppose the uppermost vessel filled with water to the height EN, E e being its surface a little below T. Stop the orifice G with the finger, and pour in water at A. This will descend through AB, and compress the air in OPQR into less room. Suppose the water in the under vessel to have acquired the surface C c, the air which formerly occupied the whole of the spaces OPQR, and KLE will now be contained in the spaces o P c C and KL e E; and its elasticity will be in equilibrium with the weight of the column of water, whose base is the surface E e, and whose height is A c. As this pressure is exerted in every part of the air, it will be exerted on the surface E e of the water of the upper vessel; and if the pipe FG were continued upwards, the water would be supported in it to an height e H above E e, equal to A c. Therefore if the finger be now taken off the orifice G, the water will spout up to the same height as if it had been immediately forced out by a column of water A c without the intervention of the air, that is, nearly to H. If instead of the funnel at A, the vessel have a brim VW which will cause the water discharged at G to run down the pipe AB, this fountain will play till all the water in the upper vessel is expended. The operation of this second fountain will be better understood from fig. 9, which an intelligent reader will see is perfectly equivalent to fig. 8. A very powerful engine for raising water upon this principle has long been employed in the Hungarian mines; where the pipe AB is about 200 feet high, and the pipe FG about 120; and the condensation is made in the upper vessel, and communicated to the lower at the bottom of the mine, by a long pipe.

We may now then apply to air the laws of hydrostatics and hydraulics, in perfect confidence that their legitimate consequences will be observed in all its situations. We shall in future substitute, in place of any force acting on a surface of air, a column of water, mercury, or any other fluid whose weight is equal to this force; and as we know distinctly from theory what will be the consequences of this hydrostatic pressure, we shall determine a priori the phenomena in air; and in cases where theory does not enable us to say with precision what is the effect of this pressure, experience informs us in the case of water, and analogy enables us to transfer this to air.

From such familiar and simple observations and experiments, the fluidity, the heaviness, and elasticity, are discovered of the substance with which we are surrounded, and which we call air. But to understand these properties, and completely to explain their numerous and important consequences, we must call in the aid of more refined observations and experiments, which even this scanty knowledge of them enables us to make; we must contrive some methods of producing with precision, any degree of condensation or rarefaction, of employing or excluding the gravitating pressure of air, and of modifying at pleasure the action of all its mechanical properties.

Nothing can be more obvious than a method of compressing a quantity of air to any degree. Take a cylinder or prismatic tube AB (fig. 10.) shut at one end, and fit it with a piston or plug C, so nicely that no air can pass by its sides. This will be best done in a cylindric tube by a turned stopper, covered with oiled leather, and fitted with a long handle CD. When this is thrust down, the air which formerly occupied the whole capacity of the tube is condensed into less room. The force necessary to produce any degree of compression may be concluded from the weight necessary for pushing down the plug to any depth. But this instrument leaves us little opportunity of making interesting experiments on or in this condensed air; and the force required to make any degree of compression cannot be measured with much accuracy; because the piston must be very close, and have great friction, in order to be sufficiently tight.

And as the compression is increased, the leather is more squeezed to the side of the tube; and the proportion of the external force, which is employed merely to overcome this variable and uncertain friction, cannot be ascertained with any tolerable precision. To get rid of these imperfections, the following addition may be made to the instrument, which then becomes what is called the condensing syringe.

The end of the syringe is perforated with a very small hole e f, and being externally turned to a small cylinder, a narrow slit of bladder, or of waxed silk, must be tied over the hole at f. Now let us suppose the piston pushed down to the bottom of the barrel, to which it applies close; when it is drawn up to the top, it leaves a void behind, and the weight of the external air presses on the slip of bladder, which therefore clasps close to the brass, and thus performs the part of a valve, and keeps it close, so that no air can enter. But the piston having reached the top of the barrel, a hole F in the side of it is just below the piston, and the air rushes through this hole, and fills the barrel. Now push the piston down again, it immediately passes the hole F, and no air escapes through it; it therefore forces open the valve at f, and escapes while the piston moves to the bottom.

Now let E be any vessel, such as a glass bottle, having its mouth furnished with a brass cap firmly cemented to it, having a hollow screw which fits a solid screw p p, turned to the cylindric nozzle of the syringe. Screw the syringe into this cap, and it is evident that the air forced out of the syringe will be accumulated in this vessel; for upon drawing up the piston, the valve f always shuts by the elasticity or expanding force of the air in E; and on pushing it down again, the valve will open as soon as the piston has got so far down that the air in the lower part of the barrel is more powerful than the air already in the vessel. Thus at every stroke an additional barrelful of air will be forced into the vessel E; and it will be found, that after every stroke the piston must be farther pushed down before the valve will open. It cannot open till the pressure arising from the elasticity of the air condensed in the barrel, is su-