SECT. XV. ANALYSIS OF THE ATMOSPHERE. 117 



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- Winds Cloud-Ring Monsoons Rotation of 

 Winds Laws of Hurricanes. 



THE atmosphere is not homogeneous. It appears from analysis 

 that, of 100 parts, 99'5 consist of nitrogen and oxygen gases 

 mixed in the proportions of 79 to 21 of volume, the remainder 

 consists of 0'05 parts of carbonic acid and on an average 0'45 of 

 aqueous vapour. 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 that in the cistern is a 

 measure of its weight. Consequently every variation in the 

 density occasions a corresponding rise or fall in the barometrical 

 column. At the level of the sea in latitude 42, and at the 

 temperature of melting ice, the mean height of the barometer is 

 29*922 or 30 inches nearly. 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 11,671,000,000 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. 



The atmosphere when in equilibrio is an ellipsoid flattened at 

 the poles from its rotation with the earth. In that state its 

 strata are of uniform density at equal heights above the level of 

 the sea ; but since the air is both heavy and elastic, 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 



