The astronomers who look at the planet Mars tell us that at the Northern and Southern Poles there are great areas of snow, very much greater than the arctic regions of the earth, for the south polar area alone occupies 11,330,000 square miles. But the geological records of the earth show that our own arctic regions once extended very much farther than they do at present, a fact which need not in any way surprise us, for as we have already remarked, snow and ice are very largely a matter of the nearness of the sea to the land. We may put the same thing in another way by saying that winter cold and summer heat depend largely on the distribution of sea and land. Thus Venice, which is not very much farther from the North Pole than Vladivostok, has an altogether different climate, and inhabitants of the Shetland Isles have a very different kind of climatic experience from those who delve by the frozen Yukon. There is another consideration which is sometimes overlooked. We do not think of the earth as a very warm body. But at its very coldest part, where the thermometer goes down to seventy or ninety degrees below freezing, it is several hundreds of degrees warmer than the space outside the earth. Midway between the earth and the moon the temperature must be 430° F. below freezing; so that if we take the surface of the earth as a whole we may say that it is between four hundred and five hundred degrees warmer than the space by which it is surrounded. Every one knows what is happening when he warms his hands at a fire. The fire being hotter than its surroundings is giving out heat towards them, and the hands catch some of this radiated heat. Similarly the earth is radiating heat, and the atmosphere round the earth catches some of it. So also do the seas. While therefore it is certain that the heat of the sun warms the earth and the air and the sea, and so gives rise to currents of air and perhaps of water, so also is it likely that the heat of the earth causes warm air to rise, and so plays its part in forming the winds, the currents of air, and the currents of water. When the earth was warmer than it is now it had more and greater effects in this direction. It caused more evaporation of the water, more clouds, and therefore more rain, and in winter more snow.