§ 30, 31. THE SEA AND THE ATMOSPHERE. 9 



from north and south two sets of meridional currents blowing at 

 the surface of the earth toward the equator ; in the upper regions 

 we should have had them flowing back again to the place whence 

 they came. On the other hand, were the heating and cooling in- 

 fluences just referred to to cease, and the earth to fail in impress- 

 ing its own motion on the atmosphere, we should have a furious 

 hurricane rushing round the globe at the rate of 1000 miles an 

 hour — tornadoes of ten times the speed of the most violent now 

 known to us, sweeping every thing before them. A combination 

 of the two influences, modified by the friction of the earth, which 

 tends to draw the air after it, gives us the trade- winds, which, at 

 the speed of from ten to twenty miles an hour, sweep round the 

 equatorial region of the globe unceasingly. 



30. " Impressed with the motion of the air, constantly sweeping 

 Currents. its surfacc in one direction, and obeying the same 

 laws of motion, the great sea itself would be excited into currents 

 similar to those of the air, were it not walled in by continents 

 and subjected to other control. As it is, there are constant cur- 

 rents flowing from the torrid toward the frigid zone to supply the 

 vast amount of vapor there drained off, while other whirlpools 

 and currents, such as the gigantic Gulf Stream, come to perform 

 their part in the same stupendous drama. The waters of this vast 

 ocean river are, to the north of the tropic, greatly warmer than 

 those around ; the climate of every country it approaches is im- 

 proved by it, and the Laplander is enabled by its means to live 

 and cultivate his barley in a latitude which, every where else 

 throughout the world, is condemned to perpetual sterility. There 

 are other laws which the great sea obeys which peculiarly adapt 

 it as the vehicle of interchange of heat and cold betwixt those 

 regions where either exists in excess. 



81. " In obedience to these laws water warmer than ice attacks 

 Icebergs. the basis and saps^ the foundations of the icebergs 

 — themselves gigantic glaciers, which have fallen from the mount- 

 ains into the sea, or which have grown to their present size in 

 the shelter of bays and estuaries, and by accumulations from 

 above. Once forced from their anchorage, the first storm that 

 arises drifts them to sea, where the beautiful law which renders 

 ice lighter than the warmest water enables it to float, and drifts 

 southward a vast magazine of cold to cool the tepid water which 



