SECT, xv.] CONSTITUTION OF THE ATMOSPHERE. 131 



SECTION XV. 



Analysis of the Atmosphere Its Pressure Law of Decrease in Density Law 

 of Decrease in Temperature Measurement of Heights by the Barometer 

 Extent of the Atmosphere Barometrical Variations Oscillations Trade- 

 WindsMonsoons Rotation of Winds Laws of Hurricanes. 



THE atmosphere is not homogeneous. It appears from 

 analysis that of 100 parts 79 are azotic gas, and 21 oxy- 

 gen, the great source of combustion and animal heat. Be- 

 sides these there are three or four parts of carbonic acid 

 gas in 1000 parts of atmospheric air. These proportions 

 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 suspended by the pressure 

 of the atmosphere on the surface of the cistern is a measure 

 of its weight. Consequently every variation in the density 

 occasions a corresponding 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 compressed only by the 

 weight above it. Therefore the upper strata are less dense, 

 because they are less compressed 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 



K 2 



