a Grecian festival in honour of Aglauros, or rather of Minerva, who received from the daughter of Cecrops the name of Aglauros. The word is derived from ἀγλαος, λαυρα, because during the solemnity they undressed the statue of the goddess and washed it. The day on which it was observed the people looked upon as unfortunate and inauspicious; and therefore no person was permitted to appear in the temples, which were purposely surrounded with ropes. The arrival of Alcibiades in Athens on that day was thought very unfortunate, but the success which ever afterwards attended him proved it to be otherwise. 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, as figs served them for food after they had contracted a dislike for acorns. 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. Under the article Physics we observed, that in a great number of languages the term employed to express breath was also one of the terms used to express the animating principle, nay, the intellectual substance, the soul.

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 natural 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 equable 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 feel 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. 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 whereabouts 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 immerse 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. There is 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 sive force, 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 wool; 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 anything in its way.

It is not accurate to say, that we move about without any 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 force 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 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 preponderancy. 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 menstrum of almost every fluid, of 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 expensible 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 force, 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, air heavy; and thus explained the difficulty of raising the dioted 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 hydrostatal 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 hydrostatics. 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...