78 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
without any change in volume, this quality would also be the 
source of at least two systems of currents in the air, viz., an up- 
per and a lower. The two agents combined, viz., that which 
changes level or volume, and that which changes specific gravity, 
give us the general currents under consideration. Hence we. say 
that the primum mobile of the air is derived from change of speci- 
fic gravity induced by the freezing temperature of the polar re- 
gions, as well as from change of specific gravity due the expand- 
ing force of the sun's rays within the tropics. 
112. Therefore, fairly to appreciate the extent of the influence 
due the heat of the sun in causing the winds, it should be recol- 
lected that we may with as much reason ascribe to the inter- 
tropical heat of the sun the northwest winds, which are the pre- 
vailing winds of the extra-tropical regions of the southern hemi- 
sphere, or the southwest winds, which are the prevailing winds 
of the extra-tropical regions of the northern hemisphere, as we 
may the trade-winds, which blow in the opposite directions. Par- 
adoxical, therefore, as it seems for us to say that the heat of the 
sun causes the winds between the parallels of 25° or 30° north 
and south to blow toward the equator, and that it also causes the 
prevailing winds on the polar sides of these same parallels to blow 
toward the poles, yet the paradox ceases when wei come to rec- 
ollect that by the process of equatorial heating and polar cooling 
which is going on in the atmosphere, the specific gravity of the 
air is changed as well as its level. Nevertheless, as Halley said, 
in his paper read before the Royal Society in London in 1686, 
and as we also have said 99), "it is likewise very hard to con- 
ceive why the limits of the trade-wind should be fixed about the 
parallel of latitude 30° all around the globe, and that they should 
so seldom exceed or fall short of those bounds." 
113. Operated upon by the equilibrating tendency of the at- 
mosphere and by diurnal rotation, the wind approaches the north 
pole, for example, by a series of spirals from the southwest. If 
we draw a circle about this pole on a common terrestrial globe, 
and intersect it by spirals to represent the direction of the wind, 
we shall see that the wind enters all parts of this circle from the 
southwest, and, consequently, that a whirl ought to be created 
thereby, in which the ascending column of air revolves from right 
to left, or against the hands of a watch. At the south pole the 
