It is only comparatively recently that it has been possible to examine, with any approach to scientific method, the question of the habitability of the planets. The ingenious theories of Lowell have drawn attention to the possibilities presented by Mars, and though the problem is still far from being settled, considerable advance has been made. We have to limit our inquiries entirely to life as we know it. Thus the planets Jupiter, Saturn, Uranus, and Neptune appear to be in a semi-molten state, which precludes any possibility of their being inhabited. In fact. Mars and Venus seem to be the only planets which are not ruled out by some unfavourable physical condition, though the suggestion has been made that the satellites of Jupiter may receive sufficient heat from their primary to render some of them habitable. Now the problem can be solved only by the appearance of some phenomenon which can be due to no conceivable cause other than living intelligent beings. This is what Lowell claims to have discovered in the case of Mars. But while in view of the extreme diversity of opinion concerning them in the astronomical world his theories cannot be considered decisive, they may be supported or opposed by a different type of argument. Such is supplied by an examination of the physical conditions on the surface of Mars. Unfortunately there is, even here, very considerable difference of opinion. The non-existence of oxygen in the Martian atmosphere, if proved, would settle the matter at once; and the same would occur if the temperature were not somewhere in the neighbourhood of that prevailing on the earth. But the question of temperature is even more intimately connected with the subject. Lowell's canal theory depends essentially on the idea that water is conveyed by artificial means from the polar regions. The theory is therefore quite untenable, unless the maximum temperature on Mars is well above the melting-point of ice. There are two outstanding determinations of the mean temperature, and the great difficulty is that they appear to be absolutely contradictory. The first is due to Poynting, who, from a discussion of the general properties of solar radiation, finds the mean temperature of Mars to be-38[degrees]C.; the second is that of Lowell himself, and leads to the value +8[degrees]C. The discrepancy, though less than 50[degrees]C, affects the whole question, for if Poynting's value is correct, it is certain that on Mars ice will never melt. It is easy enough, under certain assumptions, to calculate the mean temperature of Mars from that of the earth. Assuming the two planets to be similar in their behaviour towards solar radiation, and ignoring the central heat, we have that the energy received from the sun is inversely proportional to the square of the distance, while the energy given out is, by Stefan's Law, directly proportional to the fourth power of the absolute temperature. But since the temperature does not vary much, the energy received must balance that radiated out, so that the fourth power of the absolute temperature is inversely proportional to the square of the distance from the sun; or, in other words, the absolute temperature is inversely proportional to the square root of the distance. Taking the ratio of the distances as 1.5237, and the mean temperature of the earth as 15[degrees]C, we get that of Mars as -39[degrees]C, practically that obtained by Poynting. Now we have here made the assumption that Mars and the earth are similar in their behaviour towards solar radiation. This is what Lowell denies. The solar radiation on a planet may be either reflected or absorbed. The reflected radiation plays no part in raising the temperature. Strange to say, remarks Lowell, important fact had never been taken into account till the present investigation of the subject, which led to a completely different outcome from what had previously been supposed. I do not think that this statement is quite just to Poynting, in whose paper the idea certainly occurs. …