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Compressibility and Dilatability of Air.

Book II,

fluid ocean in which they only can move, and within which they are al-ways immersed. It is to them, what the sea is to fish : remove them fromit, and they necessarily die. In some respects nature has been more fa-vorable to fishes than to us : most of them can ascend to the surface ofthe fluid in which they live, but we can only exist in the lowest depths ofthe atmospheric ocean that confines us : if we ascend but a little, ourenergies begin to fail, and we are compelled to descend to the bottom, theplace she designed us to occupy.

Possibly, some people may suppose that the velocity with which theearth shoots forward in her orbit, might sometimes cause this atmosphere(which hangs as a mantle so loosely about her) to be left floating, like thetail of a comet, behind; or be entirely separated from her, like the cloud ofvapor which the impetuous ball leaves at the cannon’s mouth. Such how-ever is not the fact; on the contrary, it revolves uniformly with the earthon the axis of the latter, and accompanies her, as a part of herseif, roundthe sun. Were it indeed separated from her, but for a moment, either byan increase or diminution of her velocity, the present Organization of na-ture would be destroyed ; every mountain would behurled from its base;every house on the globe would be leveled ; and no human being couldsurvive. Had the atmosphere not a rotatory motion also, in commonwith that of the earth, i. e. of the same velocity and in the same direction,a very different state of things, as regards the arts, would have subsistedthan those which we behold. For example, aerial navigation would cer-tainly have superseded nearly all traveling by land and water; and rail-roads, and locomotive carriages, and steamboats, would hardly have beenknown ; for the project of that individual who proposed to visit distantcountries, by merely ascending in a balloon, tili the rotation of the earthon its axis brought them under him, when he intended to descend, wouldhave been no visionary scheme.

The air is tangible. —Although the substance of the atmosphere is notvisible, it is tangible ; w efeel it when in motion as wind, whether it begently disturbed as in the evening breeze, or by the slight waving of alady’s fan; or when greatly excited, as in the hurricane, or the violent blastfrom a bellows’ mouth. We also see its effects when thus in motion, inthe direction of smoke, extinction of our tapers, slamming of doors, in thebeautiful waving of grass, and of the full eared grain of the fields; treesyielding to its impulse, buildings unroofed, and sometimes in the prostra-tion of large tracts of forests; in windmills, sailing of ships, and the con-vulsions into which it throws the otherwise placid ocean.

Air is compressible. —Indeed compressibility and expansibility are pro-perties of all bodies; by the abstraction of heat, airs are compressed mtoliquid s, and liquids into solids, while an increase of temperature expandssolids into liquids, and these into airs. In the common air gun, four or livegallons of the dense air around us are compressed into a pint, and byfurther pressure they may be squeezed into a few drops of liquid, whicha tea spoon might contain.

Its expansibility or dilatability is, so far as known, illimitable ; the spaceit occupies being always in proportion to the pressure that confines it. Ifa collapsed and apparently empty bladder be placed under a receiver,and the air around its exterior be remored, the small portion within willexpand and swell it out to its natural shape. If it were possible to with-draw the whole of the air from this room, and a globule no larger. than a peawere then admitted, it would instantly dilate and fill the room. The upperstrata of the atmosphere decrease in density as they recede from theearth’s surface, on account of the diminution of the pressure from super-