o2 PHYSICS. 
pounds upon the square inch, which, upon a surface equal to that of the 
human body, amounts to from 30,000 to 40,000 pounds. ‘This at first appears 
incredible, as it seems impossible to resist so enormous a pressure; the 
matter becomes more intelligible, however, when it is considered that the 
pressure acts on all parts, both inside and out, at the same time, so that the 
pressure from one direction is exactly neutralized by that from the other. 
This weight then is only sensible when the equilibrium is disturbed, as in a 
violent wind, &c. The compression or crushing of the body is resisted 
by the penetration of the external air into all the cavities of the body by 
means of innumerable fine pores as well as of larger passages, so that both 
inside and out, air is present in the same state of tension. This atmospheric 
pressure is of the greatest importance to the animal organism, as will be 
made evident by a single example. It is known that the head of the thigh 
bone consists of a ball playing in a socket of the pelvis inclosed in a cap- 
sular ligament, and possessing motion in almost every direction. If the leg 
be unsupported, and even if all the muscles and tendons be severed, the 
head of the thigh bone does not fall out of its place. If, however, the cap- 
sular ligament be pierced, or communication be made in any other way 
with the external air, the thigh immediately descends out of its place. It 
is thus evident that the pressure of the air upon this air-tight joint must play 
a great part in keeping it in position. In this manner may be explained the 
peculiar sensation of weakness and relaxation experienced at great eleva- 
tions on mountains ; the diminished pressure of the air takes from the whole 
frame its compact and well knit character. 
One of the most important propositions in the theory of equilibrium of 
gaseous bodies, is the law discovered by Mariotte, and called after him 
Mariotte’s law: that the volume of a gas is inversely as the pressure to 
which it is subjected. Thus twice the pressure is required to reduce a gas 
to half the volume, &c. Arago and Dulong have shown the accuracy of 
this law up to a pressure of 27 atmospheres, or a pressure 27 times that of 
one atmosphere. For this purpose they employed the apparatus represented 
in pl. 18, fig. 39. In the middle of an old tower, a mast, a, of about 100 
feet in height, was erected, to which a long glass tube, ¢, was attached, com- 
posed of 13 single tubes of six feet in length. At the foot of the mast was 
a cast iron vessel, v, filled with mercury, with a forcing pump, p, attached 
at b,and provided with a manometer tube, mn, closed above, graduated, and 
filled with dry air. When the mercury stood at an equal height in the 
tubes, ¢ and mn, the air in the latter, of known volume, experienced the 
ordinary pressure or that of one atmosphere. Forcing water, however, by 
means of the forcing pump, into the upper part of the vessel, v, the air in the 
tube, mn, would become compressed, and the mercury rise in the tube, ¢. 
The scale on the first tube gave the volume of the included air; the differ- 
ence of height of mercury in the two tubes gave the corresponding pressure. 
Fig. 40 represents the manner in which the single parts of the vertical glass 
tube were united by strong rings, aa’; c is an upward projecting rim, filled 
with melted cement, to rerder any escape of mercury impossible. Jig. 41 
shows how the manometes tube, mn, was fastened to the plate, c, of the cast 
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