AIR AND LIFE. 139 



the weight of the atmosphere, wherever considered, varies every day 

 more or less, often appreciably within tbe limits of a few hours and 

 even minutes. This could not happen, of course, if the weight of the 

 strata of air did not vary also. The weight of the whole atmosphere 

 increases or decreases because the weight of the air, considered at 

 any region vertically above the point where the observation is made, 

 increases or decreases. These variations of weight, or pressure, are 

 indicated by the barometer — devised in 1613 by Torricelli, pupil and 

 friend of Galileo — and the oscillations of that instrument are only indi- 

 cations of the differences of the weight or pressure of tlie air. 



jSTow, as the pressure is increased at low stations, and diminished at 

 high ones, and, as there is a very definite and regular connection between 

 differences of altitude and barometrical indications, it is conceivable 

 that the barometer may, to some extent, and with certain limitations, 

 yield information as to the altitude at which an observer finds himself. 

 It is sufficient to mention the fact; the methods by which it is estab- 

 lished would require too long an exposition. 



Since air is material and has a weight of its own, however varia- 

 ble, it must press upon all organisms. It is not difficult to estimate 

 with some precision the weight of the superincumbent air. For each 

 square centimeter of our skin, the pressure exerted is exactly that 

 of a vertical column of mercury 1 square centimeter in section, and 

 of the same height as the barometrical column at that moment. If 

 the barometer stands at 760 millimeters, the pressure is exactly that 

 of 76 cubic centimeters of mercury, and as each cubic centimeter 

 weighs 13.6 grams, the sum total, per square centimeter, is 1 kilo- 

 gram 33 grams, or about 15 pounds per square inch. Taking the skin 

 surface of the average adult to be something like 1J square meters 

 (15,000 square centimeters), the weight with which the atmosphere 

 presses on each of us amounts to 15,450 kilograms; but under ordi- 

 nary circumstances we do not feel this enormous load, because the 

 pressure is exerted in all directions; from within outward as well 

 as from without inward, from below upward as well as from above 

 downward. To perceive this pressure, it must be removed from one 

 side, as when the hand is placed over the open end of a cylinder in 

 which a vacuum is being formed; then one feels the strong pressure 

 pushing the hand toward the opposite end of the cylinder. Of course 

 the pressure exerted upon the body and all objects, is lessened as 

 the altitude increases, or the barometer falls; and, reciprocally, if the 

 barometer rises, or if the body be at a low station — in a mine for 

 instance — the pressure is higher. The total weight of the atmosphere, 

 at sea level, under normal circumstances, averages some 5,000,000,- 

 000,000,000,000 kilograms — that is, the millionth of the weight of our 

 planet itself; or, to use other terms, the weight of a continuous 

 stratum of mercury 76 centimeters high, and covering the entire sur- 

 face of the globe, both sea and land. This is a fairly high figure for a 



