THE ATMOSPHERE. 31 



never so wayward, is as obedient to law and as subservient to or- 

 der as were the morning stars when they " sang together?" 



149. There are at least two forces coneerned in driving the 

 wind through its circuits. We have seen (§ 124) whence that 

 force is derived which gives easting to the winds as they approach 

 ^he equator, and westing as they approach the poles, and allusion, 

 without explanation, has been made (§ 136) to the source whence 

 they derive their northing and their southing. The trade-winds 

 are caused, it is said, by the inter-tropical heat of the sun, which, 

 expanding the air, causes it to rise up near the equator ; it then 

 flows off in the upper currents north and south, and there is a rush 

 of air at the surface both from the north and the south to restore 

 the equilibrium — hence the trade-winds. But to the north side 

 of the trade-wind belt in the northern, and on the south side in 

 the southern hemisphere, the prevailing direction of the winds is 

 not toward the source of heat about the equator, but exactly in 

 the opposite direction. In the extra-tropical region of each hem- 

 isphere the prevailing winds blow from the equator toward the 

 poles. It therefore at first appears paradoxical to say that heat 

 makes the easterly winds of the torrid zone blow toward the equa- 

 tor, and the westerly winds of the temperate zones to blow toward 

 the poles. Let us illustrate : 



150. The jprhmiin mobile of the extra-tropical winds toward 

 the equator is, as just intimated, generally ascribed to heat, and 

 in this wise, viz. : Suppose, for the moment, the earth to have no 

 diurnal rotation ; that it is at rest ; that the rays of the sun have 

 been cut off from it ; that the atmosphere has assumed a mean 

 uniformity of temperature, the thermometer at the equator and the 

 thermometer at the poles giving the same reading ; that the winds 

 are still, and that the whole aerial ocean is in equilibrium and at 

 rest. Now imagine the screen which is supposed to have shut 

 off the influence of the sun to be removed, and the whole atmos- 

 phere to assume the various temperatures in the various parts of 

 the world that it actually has at this moment, what would take 

 place, supposing the uniform temperature to be a mean between 

 that at the equator and that at the poles ? Why, this would take 

 place : a swelling up of the atmosphere about the equator by the 



