112 DENSITY OF THE ATMOSPHERE. SECT. XV. 



acid gas in 1000 parts of atmospheric air. These pro- 

 portions are found to be the same at all heights hitherto 

 attained by man. The air is an elastic fluid resisting 

 pressure in every direction, and is subject to the law of 

 gravitation. As the space in the top of the tube of a 

 barometer is a vacuum, the column of mercury sus- 

 pended by the pressure of the atmosphere on the sur- 

 face of the cistern is a measure of its weight. Conse- 

 quently every variation in the density occasions a cor- 

 responding rise or fall in the barometrical column. The 

 pressure of the atmosphere is about fifteen pounds on 

 every square inch; so that the surface of the whole 

 globe sustains a weight of 11,449,000,000 hundreds of 

 millions of pounds. Shell-fish which have the power of 

 producing a vacuum, adhere to the rocks by a pressure 

 of fifteen pounds upon every square inch of contact. 



Since the atmosphere is both elastic and heavy, its 

 density necessarily diminishes in ascending above the 

 surface of the earth ; for each stratum of air is com- 

 pressed only by the weight above it. Therefore the 

 upper strata are less dense, because they are less com- 

 pressed than those below them. Whence it is easy to 

 show, supposing the temperature to be constant, that if 

 the heights above the earth be taken in increasing 

 arithmetical progression that is, if they increase by 

 equal quantities, as by a foot or a mile, the densities of 

 the strata of air, or the heights of the barometer which 

 are proportionate to them, will decrease in geometrical 

 progression. For example, at the level of the sea, if the 

 mean height of the barometer be 29-922 inches, at the 

 height of 18,000 feet it will be 14-961 inches, or one 

 half as great; at the height of 36,000 feet, it will be one 

 fourth as great; at 54,000 feet, it will be one eighth, 

 and so on, which affords a method of measuring the 

 heights of mountains with considerable accuracy, and 

 would be very simple, if the decrease in the density of 

 the air were exactly according to the preceding law. 

 But it is modified by several circumstances, and chiefly 

 by changes of temperature, because heat dilates the 

 air and cold contracts it, varying ] F of the whole bulk 

 when at 32, for every degree of Fahrenheit's ther- 

 mometer. Experience shows that the heat of the air 



