CHAPTER I.

matics.

(1.) MATERIAL SUBSTANCES, in refer ence to their mechanical properties, are divided into solids and fluids.

Wood, stone, metal in its ordinary state, are instances of solids. One of the most striking mechanical peculiarities of this class is that, if to such a body any force be applied, the whole mass will be moved without suffering any change in its figure or shape. Water, quicksilver, melted metal, air, steam, are instances of fluids. This class of bodies differ essentially in their mechanical properties from the former. That cohesion of parts which is the cause of the preservation of the figure of a solid, notwithstanding the application of a force tending to change the figure, has here no existence whatever. The parts of a fluid are perfectly free to move among each other, and immediately yield upon the application of the smallest force. Fluids easily allow solid bodies to pass through them, and their surfaces always compose themselves into a perfect level. On the other hand, the cohesion of the parts of solids do not permit the passage of another body through them unless extraordinary force be used, and their surfaces maintain any position with respect to a level or horizontal plane in which they may happen to be placed.

(2.) Fluids are divided into two very distinct classes, denominated, from their characteristic mechanical properties, elastic and inelastic.

If a strong cylindrical vessel, of which AB (fig. 1.) is a section, be filled to the height C with water, and a piston or plug D, accurately fitting the vessel, and capable of moving water-tight in it, be introduced over the surface of the water, and a pressure be exerted on

will force the surface of the water lower in the vessel than its original height C.

Now let us suppose the water discharged from the vessel A B, and its place occupied by common air. Let the piston, as before, be introduced into the cylinder, fitting it so that no air can escape between the piston and the cylinder. A pressure being now exerted on the piston it will immediately descend; but the moment the pressure is removed, it will again ascend, and resume its first position.

(3.) The property, in virtue of which the water resisted the descent of the piston and would not admit of a diminished bulk, is called incompressibility; and, on the other hand, the property by which the air admitted the descent of the piston and was forced into a less bulk, is called compressibility. Again, the property manifested by the air in forcing back the piston when the pressure was removed, and resuming its original bulk, is called elasticity.

It appears, therefore, that elasticity supposes compressibility, and a body which is incompressible must necessarily be also inelastic. Hence water is an inelastic, air an elastic fluid.

Strictly speaking, neither water, nor any other liquid which we know of, is perfectly incompressible or inelastic. It was long supposed that water possessed neither of these properties; but accurate experiments have shown, that it and other liquids are both compressible and elastic. A pressure of 15lbs. on each square inch of surface, reduces the bulk of water by one part in 21740. The degrees of compressibility, however, which

B

are found in liquids, are so inconsider able, that the quality of compressibility in these substances is rather to be looked upon as a philosophical fact, than as a property to be taken into account in mechanical investigations. Accordingly, in all mechanical treatises, liquids and aëriform bodies are considered to be distinguished as we have already explained; and when their properties are expressed mathematically, the formulæ for the one are founded on the supposition of their being inelastic, and for the other of their being elastic.

The same property of incompressibility which we have just explained in water, is common to all that class of fluids which are called liquids, such as mercury, alcohol, &c.; and the property of elasticity explained in the instance of air is common to all fluids of the gaseous or vaporous form, such as all the gases, steam raised by heat from all species of liquids, &c.

This manifest and important mechanical distinction between the two classes of fluids gives rise to a corresponding division in that part of mechanical philosophy which treats of their properties. That which treats of the mechanical properties of elastic fluids, and which forms the subject of the present treatise, is called Pneumatics, from the Greek word μ, (pneuma,) which signifies breath or air.

(4.) The various elastic fluids differ one from another in many respects. One of the most striking distinctions is, that some are permanently elastic, and others not. Those which are incapable by any known means of being converted into a liquid are called permanently elastic fluids. Such, for example, is air. On the other hand, those which, by being submitted to pressure or exposed to cold, are reduced to liquids, are not permanently elastic, and are generally called vapours. Such, for example, is steam. Besides this, there are many other distinctions between elastic fluids, arising out of their chemical properties. It will not, however, be necessary here to inquire into these, since the mechanical properties which we shall have to consider in the present treatise are common to all elastic fluids, whatever be their differences as to permanent elasticity or any other properties. The elastic fluid with which we are most familiar is atmospheric air, and it possesses all the mechanical properties which we shall have to notice in any elastic fluid. For

this and other reasons it will be convenient to adopt it as the representative of elastic fluids in general, and there will be no difficulty in applying to them the conclusions to which we may arrive.

CHAPTER II.

Air possesses the universa! Properties of Matter-Impenetrability, Inertia, Mobility, and Weight.

(5.) Air being apparently an invisible, intangible substance in which we freely move, it may at first be doubted whether it be matter or not. It should be observed, that the properties of substances, even those with which we are most habitually conversant, do not always offer themselves immediately to the observation of the senses, and that in noticing them our senses must often be guided by philosophical considerations. Not that these philosophical considerations add any thing to the certitude derived from the senses, but rather that they direct the senses to the proper objects of attention. Let us then consider how we should use the senses in deciding the question, whether air be material or not? We know that there are certain properties which any thing must have in order to be material, and that having these properties it is necessarily one of that class of beings which we denote by the term matter. Air, then, will be material or not, according as it is found to possess or not these requisite qualifications. The principal of these properties are, impenetrability, inertia, mobility, and weight.

(6.) Impenetrability is that property by which a body occupies any space to the exclusion of every other body, or so that no other body can fill that space until the other deserts it.

(7.) Air is impenetrable.

There are various experimental proofs of this proposition. Let A B (fig 2.) be a glass receiver, or cylindrical vessel, containing water to the level B, and let CD (fig. 3.) be a smaller vessel of the same kind empty, and having an aperture in the bottom furnished with a stopcock at F. Let a cork or other light body be placed floating on the surface of the water at G, and the stop-cock F being closed, let the vessel CD be inverted over the cork G, (as at fig 4.) and let its mouth D be pressed into the water to any convenient depth. It will

be found that the water will not enter the inverted receiver, except to a very limited height as E, as will be made visible by the cork floating on the surface and seen through the glass. Thus the air in the receiver which occupies the space CE excludes the water. That this is the cause of its exclusion will be rendered apparent by opening the stopcock F, by which the air which hitherto prevented the ascent of the water be yond the level E will escape, and the water will immediately rise until it assumes the same level in both receivers. Again, if the piston described in (2.) be forced against the air confined in the cylinder, it will be found that no force whatever will compel it to reach the bottom; however strong the apparatus may be, it will burst or break before this can happen.

(8.) In both of these experiments it might seem that the air was partially penetrable, since in the former the water entered the cylinder to the height E, and in the latter, although the piston could not be forced to the bottom, yet it was found to approach it. These effects, however, were not produced by the penetration of the air contained in the respective vessels, but by its compression. The air in both cases yielded to the body, which entered the cylinder and contracted itself into a smaller space.

It would be very easy to multiply proofs of the existence of this property in air, but it is the less necessary, as additional proofs of it will be perceived in many of the experiments which we shall have to describe for other purposes. The same, indeed, may be said of all the other properties of air which we are about to establish, and such is ever the nature of principles established by philosophical investigation. They are continually reappearing and soliciting our notice when we are not seeking them, and we recognise fresh proofs of them an investigations apparently the most

remote from those in which they first originated.

The quality in air which we have called impenetrability, is sometimes called solidity, and air is said to be solid. There is, however, an objection to the use of this term in this sense. Solid has already been used in opposition to fluid, and air is of the latter class. The word solid would thus be used in two different senses, which should be avoided. We have, therefore, in the present instance, thought it better to express that universal property of all species of matter by which it refuses admission of other matter to the space it fills until it has deserted it, by the negative term impenetrability.

(9.) Air is inert and moveable.

The quality of inertia which is known as an universal property of matter, is that in virtue of which it requires a certain effort or force to produce motion in matter if it be at rest, and to destroy or modify any motion which it has if not at rest.

That air possesses inertia we have numerous and familiar proofs. Wind is nothing but air in motion. Any obstacle which opposes this motion sustains a considerable pressure, and must exert a proportionate resistance, otherwise it will be carried forward with the body of moving air. Such are the effects of wind on balloons and other bodies floating in the atmosphere. Also on ships and all bodies floating on water which present sufficient surfaces in opposition to the wind, such as sails. Nay, we find, notwithstanding the extreme thinness of the atmosphere which surrounds us, that it is capable of exerting very powerful degrees of force when it acquires sufficient velocity: witness the effects of hurricanes in agitating the waters of the ocean, in tearing up by the roots the largest trees, in overthrowing buildings, &c. These effects establish beyond question the