178 THE POPULAR SCIENCE MONTHLY. 



and they tend more than anything else to that miserable " breaking- 

 down afterward" of which I have already spoken. Chambers's 

 Journal. 



-- 



THE POLAR GLACIERS. 



By C. C. MEERIMAN. 



II. 



THE element of all others most sensitive to the changes and im- 

 pulses of ever) 7 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 manner by their agency in 

 impelling the great ocean-streams, and therefore their important in- 

 fluence 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 rotates 

 daily the enormous envelope of the atmosphere. The velocity of rota- 

 tion 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 lati- 

 tudes it is still less; and at the poles nothing. Therefore, 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, rarefied 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 and 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 regions, 

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

 the temperate zones. It, however, continues to have the velocity 

 toward the east which it acquired at the equator, and, when it strikes 

 the slower-moving latitudes, it will be traveling much faster than the 

 regions it comes down upon. Hence the westerly winds that prevail 

 almost constantly in the middle latitudes. 



