In an inquiry as to the probability of the existence of life, one of the most important points to be taken into account is the amount of heat available. Now, Mars is at such a distance from the sun that on the whole it would receive about two-fifths as much solar heat as we do. This does not, however, give the amount of heat that is actually received on the surface of the planet, a considerable proportion being absorbed by the atmosphere; and since our atmosphere is so much denser and thicker than that of Mars, it follows that we lose a much larger percentage of the solar heat. To calculate the exact amount of heat absorbed by a given thickness of air is a very difficult, if not impossible, problem; but it seems likely that, taking everything into account, the inhabitant of Mars will receive more heat from the sun than we do. This would have the effect of making the evaporation very large, and if so, the Martial atmosphere would be mostly composed of water-vapour. According to Professor Langley, the true colour of the sun is blue; and its yellowness is due to the dirt always present in the air. To the inhabitants of Mars, it would most probably appear nearly white, unless, indeed, they also have volcanoes to fill the air with lava-dust. Let us now sum up the facts we have stated, and determine as far as we can what sort of man the inhabitant of Mars must be. In the first place, the force of gravitation at the surface is only just over one-third of its equivalent on the earth; a pound would therefore weigh about six ounces in Mars. If, therefore, we assume that the men are of such a size that their weight and activity are the same as ours, they would be about fourteen feet high on the average. This would make their strength very great; for not only would it be actually superior to ours, but, as every weight is so much smaller, it would be apparently proportionally increased. We should, therefore, expect to find that the Martialites have executed large engineering works; perhaps also their telescopes are much superior to ours, and we have been objects of interest for their observers. With regard to telescopes, it may be interesting to examine what is the effect of the highest magnifying power we can use. At his nearest approach, the distance from us to Mars is about thirty-seven million miles; and assuming that the highest power that can be used with advantage is twelve hundred, we approach with our telescopes to a distance of thirty thousand miles, so that houses, or towns, or indeed any artificial works, would be hopelessly invisible. With regard to the supply of heat and light, we have seen that the Martialite is not worse off than we are. To him the sun would appear as a white, or perhaps blue disc about two-thirds of the diameter that it appears to us. The Martial day differs but slightly from ours; his year, however, is much longer, being about six hundred and eighty-seven of our days, which is about six hundred and fifty Martial days.