292 THE BAROMETER. 



feet. Now the specific gravity of a stratum of atmosphere contiguous to the 

 surface is about 840 times less than the specific gravity of water ; that is, a 

 cubic inch of water weighs 840 times more than a cubic inch of air. If as 

 we ascend in the atmosphere it continued to have the same density, then its 

 height would be evidently 840 times the height of 34 feet, which would amount 

 to 28,560 feet, or 5 miles and a quarter. It is obvious, therefore, that since 

 v even at a small elevation the density of the atmosphere is reduced to half its 

 density at the surface, the whole height must be many times greater than this. 

 The barometer in the balloon in which De Luc ascended, fell to the height of 

 12 inches. Supposing the barometer at the surface to have stood at that time 

 at 30 inches, it follows that he must have left three fifths of the whole atmo- 

 sphere below him. His elevation was upward of 20,000 feet. 



A column of pure mercury, whose base is a square inch, and whose height 

 is 30 inches, weighs about 15 Ibs. avoirdupois. It follows, therefore, that 

 when the barometer stands at 30 inches the atmosphere exerts a pressure on 

 each square inch of the surface of the mercury on the cistern, amounting to 

 15 Ibs. Now it is the nature of a fluid to transmit pressure equally in every 

 direction, and if the surface on which the atmosphere acts were presented to 

 it laterally, obliquely, or downward, still the pressure will be the same. Ta- 

 king, therefore, the medium height of the barometric column at 30 inches, it 

 follows that the pressure sustained by all bodies which exist at the surface of 

 the earth, exposed to our atmosphere, are continually under this pressure, and 

 that every square inch on their surface constantly sustains a force of about 15 

 pounds. Thus the body of a man the surface of which amounts to 2,000 

 square inches, will sustain a pressure from the surrounding air to the enor- 

 mous amount of 30,000 pounds. 



It might at first view be expected that this great force to which all bodies 

 are subject, would produce manifest effects, so as to crush, compress, or break 

 them, whereas we find bodies of most delicate texture unaffected by it. Thus 

 a close bag, made of the finest silver paper, and partially filled with air, is ap- 

 parently subject to no external force. Its sides do not collapse. This arises 

 partly from the circumstance of the pressure on every side and in every direction 

 being equal, and, therefore, producing mechanical equilibrium. It is obvious 

 that a body which is driven in every possible direction, upward and downward, 

 laterally and obliquely, with equal forces, will not move in any one direction, 

 for to suppose such a motion would be to assume that the quantity of pressure 

 in that direction exceeds the quantity of pressure in other directions. But 

 still, though a body may not be driven in any direction by the atmospheric 

 pressure, it may happen that its parts are crushed and compressed. 



We do not, however, find this to happen. This arises from the fact, 'that the 

 elastic force of the air is equal to its pressure ; and since the internal cauties 

 of a body, such as the thin bag above-mentioned, are filled with air, whi< h is 

 confined within them, that air has precisely the same tendency to swell the 

 bag, and to keep the parts asunder, as the external pressure of the atmosphere 

 has to make them collapse. 



In the same manner we may account for the fact that animals move freely in 

 the air without being sensible of the enormous pressure to which their bodies 

 are subjected. The internal parts of their bodies are filled with fluids, both in 

 the liquid and gaseous states, which offer a pressure from within exactly equiv- 

 alent to the external pressure of the air. This may be easily rendered mani- 

 fest by applying to the skin the mouth of a close vessel to which an exhausting 

 syringe is attached. By this instrument, which will be described hereafter, 

 the air may be rarefied in the vessel, and the atmospheric pressure conse- 

 quently partially removed from the skin. Immediately the force of the fluid 



