6 AGRICULTURE. 



of heat from the earth, become unbearable for 

 all living beings were it not for its currents, 

 caused by the expansion of the heated air which 

 renders it lighter and causes it to rise, while the 

 cooler air above, being heavier, descends by the 

 force of gravity.* 



2. Movements of tJie Atmosphere. It is to 

 these movements due, primarily, to the coun- 

 teraction of the sun's energy by the force of 

 gravity that we owe the formation of clouds 

 and the condensation of their moisture ; the dis- 

 tribution of gases to act upon the rock surface, 

 or to be consumed by living beings; the circu- 

 lation of air in the soil, so essential to plant life; 

 the transportation of plant food and of seeds ; 

 and the maintenance of the relative composition 

 of the whole atmosphere. It is through these 

 movements that the air travels to the sea and 

 back again, bringing moisture for the thirsty 

 life. 



The winds play an important part in the for- 

 mation of soil : () in the disintegration of 

 rocks, by pelting them with sand or rain, thus 

 mechanically wearing them away by friction ; 

 (<$) by keeping them bare, so that they are ex- 

 posed to other atmospheric forces ; (V) by stir- 



* " Professor Langley, after a long and careful experiment 

 at the base and summit of Mount Whitney, California, concludes 

 that had our earth no atmosphere its surface temperature under 

 the equator at noon would be 328 F." The Soil, King, p. 13. 



