First, we must remember that the nearer a planet is to the sun the greater is the amount of heat and light that it receives, the variation being proportional to the inverse square of the distance. The earth's distance from the sun being 93,000,000 miles, while Mercury's is only 36,000,000, it follows, to begin with, that Mercury gets, on the average, more than six and a half times as much heat from the sun as the earth does. That alone is enough to make it seem impossible that Mercury can be the home of living forms resembling those of the earth, for imagine the heat of the sun in the middle of a summer's day increased six or seven fold! If there were no mitigating influences, the face of the earth would shrivel as in the blast of a furnace, the very stones would become incandescent, and the oceans would turn into steam. Still, notwithstanding the tremendous heat poured upon Mercury as compared with that which our planet receives, we can possibly, and for the sake of a clearer understanding of the effects of the varying distance, which is the object of our present inquiry, find a loophole to admit the chance that yet there may be living beings there. We might, for instance, suppose that, owing to the rarity of its atmosphere, the excessive heat was quickly radiated away, or that there was something in the constitution of the atmosphere that greatly modified the effective temperature of the sun's rays. But, having satisfied our imagination on this point, and placed our supposititious inhabitants in the hot world of Mercury, how are we going to meet the conditions imposed by the rapid changes of distance--the swift fall of the planet toward the sun, followed by the equally swift rush away from it? For change of distance implies change of heat and temperature. It is true that we have a slight effect of this kind on the earth. Between midsummer (of the northern hemisphere) and midwinter our planet draws 3,000,000 miles nearer the sun, but the change occupies six months, and, at the earth's great average distance, the effect of this change is too slight to be ordinarily observable, and only the astronomer is aware of the consequent increase in the apparent size of the sun. It is not to this variation of the sun's distance, but rather to the changes of the seasons, depending on the inclination of the earth's axis, that we owe the differences of temperature that we experience. In other words, the total supply of heat from the sun is not far from uniform at all times of the year, and the variations of temperature depend upon the distribution of that supply between the northern and southern hemispheres, which are alternately inclined sunward.