This explains why it is that we find that the bottom temperature for a given depth is frequently less in one place than it is in another, even in places of the same parallel of latitude. One or two examples may be taken to illustrate these points. The temperature off Rio Janeiro in lat. 20° S. was found by the ‘Challenger’ to be 0·6° C. at a depth of 2,150 fathoms. In a similar latitude north of the equator at a depth of 2,900 fathoms the temperature was found to be 2·2° C., and at a point near Porto Rico there is a deep hole of 4,561 fathoms with a bottom temperature of 2·2° C. Again it has been shown by the American expedition that the temperature of the water at the deepest point in the Gulf of Mexico, 2,119 fathoms, is the same as that of the bottom of the Straits of Yucatan, 1,127 fathoms, namely 4·1° C. And, passing to another part of the world altogether, we find in the small but deep sea that lies between the Philippines and Borneo that, at a depth of 2,550 fathoms, the temperature is 10·2° C. These facts then show that, although at the bottom of the deep seas the water is always very cold, the degree of coldness is by no means constant in the same latitude for the same depth. We must now return to the polar currents. We have assumed above that these currents do exist, and it is probable that by this time the reader must have seen why they are assumed to exist. The water at the bottom of the ocean is exceedingly cold. Where does this coldness come from? It is obvious that in temperate and tropical climes it does not come from the surface. Nor is it at all probable that it comes from the earth upon which the water rests; for, if it were so, the temperature for water of a given depth would always be the same. We should not find the bottom temperature of 0·4° C. at 2,900 fathoms off Rio de la Plata and a temperature of 2·2° F. in 4,561 fathoms off Porto Rico. In fact the only hypothesis that can with any show of reason be put forward to account for the temperature of the bottom of the ocean is that which derives its coldness from the Polar ice. We have at present very little evidence to enable us to judge of the force and direction of the polar currents in the two hemispheres, but the researches of the ‘Challenger’ prove almost conclusively that in the Atlantic Ocean there is a very strong predominance of the Antarctic polar current. In fact it seems very probable that the Arctic polar current, if it exist at all, is very small and confined to the eastern and western shores of the North Atlantic.