The amount of cold thus locked up in snow is enormous. To melt one cubic foot of ice requires as much heat as would raise the temperature of a cubic foot of water 176° Fahrenheit. To melt a "layer of ice only one inch and a half thick would require as much heat as would raise a stratum of air eight hundred feet thick from the freezing-point to the tropical heat of 88° Fahrenheit." It is the slowness with which ice melts which enables it to accumulate as it does, both in winter and upon high mountains and in arctic regions. Captain Scoresby relates that when near the north pole the sun would sometimes be so hot as to melt the pitch on the south side of his vessel, while water was freezing on the north side, in the shade, owing to the cooling effect of the masses of ice with which he was surrounded. Thus it will appear that a change in the direction of the moist winds blowing from the equator towards the poles might produce a Glacial epoch. If snow falls upon the ocean it cools the water, but through the currents, everywhere visible in the sea, the temperature in the water in the different parts soon becomes equalized. If, however, the snow falls upon the land, it must be melted by the direct action of the sun and wind upon the spot where it is. If the heat furnished by these agencies is not sufficient to do it year by year, there will soon be such an accumulation that glaciers will begin to form. It is clear, therefore, that the conditions producing a Glacial period are likely to prove very complicated, and we need not be surprised if the conclusions to which we come are incapable of demonstration. Theories respecting the cause of the Glacial period may be roughly classified as astronomical and geological. Among the astronomical theories, one which has sometimes been adduced is that the solar system in its movement through space is subjected to different degrees of heat at different times. According to this theory, the temperate climate which characterised the polar regions during the Tertiary period, and continued up to the beginning of the Glacial epoch, was produced by the influence of the warmer stretches of space through which the whole solar system was moving at that time; while the Glacial period resulted from the influence upon the earth of the colder spaces through which the system subsequently moved.