76 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
sand eddies in the main stream ; yet, nevertheless, the general di- 
rection of the whole is not distm'bed nor affected by those counter 
cm-rents ; 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 ''wind 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 in 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 which 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 
