of the Atmosphere.

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Chap. 1.]

pressure which his body sustains is not less than 14 or 15 tons. “Not lessthan what ?” once exclaimed an elderly and corpulent lady. “ Why howcan that be ? We could neither talk, nor walk, nor even move ; and be-sides, sir, if that is the ease, why don’t we feel itl” For a very simplereason, though at the first view not a very obvious one. Air, as a fluid,presses equally in every direction —upwards as well as downwards—side-ways and every way. Its oomponent particles are so inconceivablyminute, that they enter all substances, even liquids. Air is mixed up andcirculates with the blood of all animals; it penetrates all the ramificationsand innermost recesses of our porous bodies, and by the pressure of itssuperincumbent strata is urged through them, almost as freely as throughthe fleece of wool on a sheep’s back, or between the fibres and threadsof a ball of silk. Now, it is this circulation through the interior of ourbodies that balances its pressure without. If its weight upon us were notthus neutralized, we certainly could neither talk nor walk : the lips of theloudest Speaker, when once closed, could never be opened. We should beas mute and immoveable as if enclosed in statues of lead. And we shouldfeel it, too—that is, for a moment; for it would as effectually crush ois todeath, as if We Were placed in mortars, and pestles , each weighing 14 or15 tons, were suddenly dropped upon us.

It is the air within the breast of the mother that forces milk into aninfant’s mouth, when the latter, by instinct, removes the external pressurefrom the nipple by sucking. It is the same with all mammiferous animals.The Operation of cupping is another illustration of the same thing: therarefaction of the air under the cup produces a partial vacuum within it;and as the external pressure of the atmosphere is removed from that partwhich is under it, the internal pressure urges the blood through thewounds. Were cupping instruments applied over the eyes, those Organs would be protruded from their sockets.

As it is the pressure of the atmosphere upon which the action of themachines about to be described principally depends, we shall extend ourremarks upon it.

Suppose a specimen of delicate fillagrane work, formed of the finestthreads and plates, and of the human form and size, were sunk in water tothe depth of 34 or 35 feet; it would then be exposed to the same degreeof pressure to which our bodies are subject from the atmosphere; andwhen drawn up, it would be found uninjured, because the water enteringmto all its cavities, pressed just as much against its- interior surfaces as theliquid around it against the exterior. But if it were enclosed in a skin orflexible covering, impervious to water, and then sunk as before, the pres-sure of the liquid around its exterior (not being balanced by any within)would crush it into a shapeless mass. Just so would it be with ourbodies, and those of all terrestrial animals, if the air within them did notcounteräct the pressure without. And as long as this interior circulationremams, we can no more feel the pressure of the atmosphere than a fishfeels that of water ; nor can we be deranged or compressed by it, anymore than a bündle of wool is, or a mass of entangled wire. It wasignorance of this simple fact—air in the interior of bodies exactly balancingthe exterior pressure—that led the ancients astray, and induced one of themost sagacious intellects that was ever clothed in humanity (Aristotle) toascnbe this pressure to “ nature’s abhorrence of a vacuum.”

Since the invention of the air-pump in 1654, numerous experimentsare made, which d'emonstrate the pressure of the atmosphere. By it, thispressure may be removed from one part of the body, while it is left freeto act with undiminished energy on the opposite part; as when the palm