But, owing to the greater difficulty that the sea has in getting rid of its heat, the mean temperature of equilibrium of the ocean must be higher than that of the land; consequently the mean temperature of the ocean, and also of the air immediately over it, in tropical regions should be higher than the mean temperature of the land and the air over it. The greater portion of the southern hemisphere, however, is occupied by water, and why then, it may be asked, is this water hemisphere colder than the land hemisphere? Ought it not also to follow that the sea in inter-tropical regions should be warmer than the land under the same parallels; yet, as we know, the reverse is actually found to be the case. How then is all this to be explained, if the foregoing reasoning be correct? We find when we examine Professor Dove’s charts of mean annual temperature, that the ocean in inter-tropical regions has a mean annual temperature below the normal, and the land a mean annual temperature above the normal. Both in the Pacific and in the Atlantic the mean temperature sinks to 2°·3 below the normal, while on the land it rises 4°·6 above the normal. The explanation in this case is obviously this: the temperature of the ocean in inter-tropical regions, as we have already seen, is kept much lower than it would otherwise be by the enormous amount of _heat_ that is being constantly carried away from those regions into temperate and polar regions, and of _cold_ that is being constantly carried from temperate and polar regions to the tropical regions by means of ocean-currents. The same principle which explains why the sea in inter-tropical regions has a lower mean annual temperature than the land, explains also why the southern hemisphere has a lower mean annual temperature than the northern. The temperature of the southern hemisphere is lowered by the transference of heat by means of ocean-currents.