To explain these interesting diversities of behavior in the several polar caps we shall have to go back a little in general physics in order to get a proper take off. It is a curious concomitant of the law of gravity that the amount of heat received by a planet in passing from any point of its path to a point diametrically opposite is always the same no matter what be the eccentricity of the orbit. Thus, a planet has as many calories falling upon it in travelling from its vernal equinox to its autumnal as from the autumnal to the vernal again, although the time taken in the one journey be very different from that of the other. This is due to the fact that the angle swept over by the radius vector, that is, the imaginary bond between it and the sun, is at all points proportional to the amount of heat received; just as it is of the gravity undergone, the two forces radiating into space as the inverse square of the distance. Thus the heat received by a point or a hemisphere, through any orbital angle, is independent of the eccentricity of the orbit. But it is not independent of the axial tilt. For the force of the sun’s rays is modified by their obliquity. The amount of heat received at any point in consequence of the tilt turns upon the position of the point, and for any hemisphere taken as a whole it depends upon the degree to which the pole is tilted to the source of heat. In consequence of being more squarely presented to its beams, the hemisphere which is directed toward the sun and therefore is passing through its summer season gets far more insolation than that which is at the same time in the depth of its winter. For a tilt of twenty-four degrees, the present received value for the axis of Mars, the two hemispheres so circumstanced get amounts of heat respectively in the proportion of sixty-three to thirty-seven. But, though the summer and winter insolation thus differ, they are the same for each hemisphere in turn. Consequently the mere amount of heat received cannot be the cause of any differences detected between the respective maxima and minima of the two polar caps. If heat were a substance which could be stored up instead of being a mode of motion, the effect produced would be in accordance with the quantity applied and the two caps would behave alike. As it is the total amount has very little to say in the matter.