CHAPTER II. 

 THE ATMOSPHERE. 



ALMOST all the vital phenomena important from an agricultural aspect 

 occur in contact with, and largely by the action of, the air around. 

 Moreover, air plays an important part in the formation of soils and 

 profoundly affects the temperature and climate of the earth. It is 

 therefore obvious that a knowledge of the composition and properties 

 of the atmosphere is absolutely essential in order that the chemistry of 

 the processes involved in the life of animals and plants may be clearly 

 understood. 



The reader will already possess some acquaintance with the 

 chemical nature of atmospheric air and with the properties of its chief 

 constituents. 



In this chapter, therefore, is given only a short summary of what is 

 known of the extent, variations in composition, and functions of the 

 constituents of air. 



With reference to the extent, it is found that air exerts an average 

 pressure of about 14-75 Ib. per sq. in. (1033 grammes per sq. cm.) at 

 the sea-level. This pressure is due to the earth's attraction for the air 

 above and is, of course, a direct measure of the weight of the atmosphere. 

 There rests, therefore, on every square foot of surface 14-75 x 144 Ib. 

 of air; or upon an acre the total weight offair would be 41, 300 tons. 

 Allowing for the space occupied by land above the sea-level, Herschel 

 has calculated that the mass of the atmosphere is about T o oVo~o ^ tnat 

 of the earth. 



Although it is possible, as shown above, to estimate fairly accur- 

 ately the weight of the atmosphere, there is very little information 

 available from which the height to which the atmosphere extends can 

 be calculated. 



It can easily be shown that if the atmosphere were homogeneous 

 its height would be between 5 and 6 miles ; but, as is well known, the 

 density rapidly diminishes with the height above the sea-level. At 

 a height of 5520 metres (i.e., about 18,110 ft.) the pressure is half what 

 it is at the sea-level, while at 11,040 metres (or 36,220 ft.) it is reduced to 

 one-fourth of the sea-level pressure, and so on. In the case of small 

 elevations it may be said that, roughly, an ascent of 900 ft. lowers the 

 barometric pressure by an inch. If P and p be the corrected heights, 

 in inches, of the barometers at two stations and T and t be the respec- 

 tive atmospheric temperatures in C, then the difference in level of 

 the two stations in feet H, is given by the formula 



H =60360 (log P-lpgp) (l + 

 (17) 



