294 CELESTIAL DYNAMICS. 



work on one square metre of the earth's surface would be 

 equal to 0-36, and the total effect for the whole globe 180 bil- 

 lions of horse-powers. A not inconsiderable portion of this 

 enormous quantity of vis viva is consumed in the production 

 of atmospheric actions, in consequence of which numerous 

 motions are set up in the earth's atmosphere. 



In spite of their great variety, the atmospheric currents 

 may be reduced to a single type. In consequence of the une- 

 qual heating of the earth in different degrees of latitude, the 

 colder and heavier air of the polar regions passes in an under 

 current towards the equator ; whereas the heated air of the 

 tropics ascends to the higher parts of the atmosphere, and 

 flows from thence towards the poles. In this manner the air 

 of each hemisphere performs a circuitous motion. 



It is known that these currents are essentially modified by 

 the motion of the earth on its axis. The polar currents, with 

 their smaller rotatory velocity, receive a motion from east to 

 west contrary to the earth's rotation, and the equatorial cur- 

 rents one from west to east in advance of the axial rotation 

 of the earth. The former of these currents, the easterly 

 winds, must diminish the rotatory effect of the globe, the lat- 

 ter, the westerly winds, must increase the same power. The 

 final result of the action of these opposed influences is, as re- 

 gards the rotation of the earth, according to well-known me- 

 chanical principles, =0 ; for these currents counteract each 

 other, and therefore cannot exert the least influence on the 

 axial rotation of the earth. This important conclusion was 

 proved by Laplace. 



The same law holds good for every imaginable action 

 which is caused either by the radiant heat of the sun, or by 

 the heat which reaches the surface from the earth's interior, 

 whether the action be in the air, in the water, or on the land. 

 The effect of every single motion produced by these means on 

 the rotation of the globe, is exactly compensated by the effect 

 of another motion in an opposite direction ; so that the result- 



