76 - THE PHYSICAL GEOGRAPHY OF THE SEA. 



sand eddies in the main stream ; yet, nevertheless, the general di- 

 rection of the whole is not, disturbed nor affected by those counter 

 currents ; so with the atmosphere and the variable winds which 

 we find here in this latitude. 



Have I not, therefore, very good grounds for the opinion (^ 92) 

 that the ''wand in his circuits," though apparently to us never so 

 wayward, is as obedient to law and as subservient to order as 

 were the morning stars when they " sang together?" 



109. There are at least two forces concerned in driving the 

 wind through its circuits. We have seen (§ 97 and § 98) whence 

 that force is derived which gives easting to the winds as they ap- 

 proach the equator, and westing as they approach the poles, and 

 allusion, without explanation, has been made (§ 105) 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 equihbrium — 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 ex- 

 actly m the opposite direction. In the extra-tropical region of 

 each hemisphere the prevailing winds blow from the equator to* 

 ward the poles. It therefore at first appears paradoxical to say 

 that heat makes the easterly winds of the torrid zone blow toward 

 the equator, and the westerly winds of the temperate zones to 

 blow toward the poles. Let us illustrate : 



110. The primum 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 di- 

 urnal 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 w^hich is supposed to have 

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

 mosphere to assume the various temperatures in the various parts 



