THE POLAR GLACIERS. 31 



II. 



The element of all others most sensitive to the changes and 

 impulses of every kind of force is the earth's atmosphere. It is 

 in a state of constant disturbance, and seems to be obedient to no 

 laws or regularity. Yet, unstable as the winds appear, they are 

 really, in their general movements, among the most orderly and 

 effective agents in Nature. This is shown in a remarkable man- 

 ner by their agency in impelling the great ocean-streams ; and 

 thence arises their important influence on glacial phenomena. In 

 order to make this evident, it will be necessary to explain in brief 

 the general laws of their circulation. 



The earth turns on its axis from west to east, and with it ro- 

 tates daily the enormous envelope of the atmosphere. The velocity 

 of rotation at the equator is something over 1,000 miles an hour; 

 at thirty degrees distance it is about 150 miles an hour less. In 

 higher latitudes it is still less ; and at the poles nothing. There- 

 fore, whenever the air moves north or south on the surface of the 

 earth, it will carry with it a less or greater velocity of rotation 

 than the places it passes over, and will turn into an easterly or 

 westerly wind, according as it approaches or recedes from the 

 equator. In the region of the sun's greatest heat, the air, rare- 

 fied and lightened, is continually rising, and cooler currents come 

 in on both sides to take the place of the ascending volume. As 

 these side-currents come from a distance of about thirty degrees 

 from the equator, they have, at starting, an eastward velocity 

 many miles an hour less than the localities they will eventually 

 reach. Consequently they will appear to lag behind in all the 

 course of their progress to the equator that is, they will have a 

 westerly motion united with their north or south movements. 

 These are the great trade-winds, blowing constantly from the 

 northeast on this side, and the southeast on the other side of the 

 equator. 



But the heated air which has risen in immense volumes in the 

 tropics, spreads out to the north and the south in the upper re- 

 gions, passes entirely over the trade-winds, and comes down to 

 the earth in the temperate zones. It however continues to have 



