1 50 ELEMENTS OF SCIENCE 



scale, what takes place, on a minute scale, in every room 

 where there is a fire, or which is in any way unequally 

 heated. But the currents towards the poles after a time 

 become so much cooler, while the equatorially rushing 

 air becomes so much warmer, that they have generally 

 been supposed to change places at about latitude 30. 

 Thence the air from the equator generally forms the 

 lower current, till it approaches the pole, where it again 

 rises amidst the irregularities of wind known as " polar 

 gales "(Fig. 22). 



FIG. 22. 



The action of different degrees of heat has great effect 

 in determining the motion of the winds, but most con- 

 spicuous effects are also produced by another cause, as 

 follows : The earth, as every schoolboy knows, revolves 

 on its axis from west to east once in every twenty-four 

 hours. Now, just as the body of every traveller in a 

 carriage or a boat, participates in the velocity of the 

 vehicle which carries him,* so the atmosphere more or 

 less fully participates in the velocity of that part of the 

 earth's surface it covers. But from the globular shape of 



* See ante, p. 58. 



