In the case of Mercury, I have added another figure, 332°. Mercury always turns the same side to the sun, and the hottest point of this side would reach a temperature of 397°; its mean temperature, according to my calculation, is 332°, while the other side, turned away from the sun, cannot be at a temperature much above absolute zero, -273°. I have made a similar calculation for the moon, which turns so slowly about its axis (once in twenty-seven days) that the temperature on the side illuminated by the sun remains almost as high (106°) as if the moon were always turning the same face to the sun. The hottest point of this surface would attain a temperature of 150°, while the poles of the moon and that part of the other side which remains longest without illumination can, again, not be much above absolute zero temperature. This estimate is in fair agreement with the measurements made of the lunar radiation and the temperature estimate based upon it. The first measurement of this kind was made by the Earl of Rosse. He ascertained that the moon disk as illuminated by the sun—that is to say, the full moon—would radiate as much heat as a black body of the temperature 110° Cent. (230° F.). A later measurement by the American Very seems to indicate that the hottest point of the moon is at about 180°, which would be 30° higher than my estimate. In the cases of the moon and of Mercury, which do not possess any atmosphere to speak of, this calculation may very fairly agree with the actual state of affairs. The temperature of the planet Venus would be about 65° Cent. (149° F.) if its atmosphere were perfectly transparent. We know, however, that dense clouds, probably of water drops, are floating in the atmosphere of this planet, preventing us from seeing its land and water surfaces. According to the determinations made by Zöllner and others, Venus would reflect not less than 76 per cent. of the incident light of the sun, and the planet would thus be as white as a snow-ball. The rays of heat are not reflected to the same extent. We may estimate that the portion of heat absorbed by the planet is about half the incident heat. The temperature of Venus will therefore be reduced considerably, but it is partly augmented again by the protective action of this atmosphere. The mean temperature of Venus may, hence, not differ much from the calculated temperature, and may amount to about 40° (104° F.). Under these circumstances the assumption would appear plausible that a very considerable portion of the surface of Venus, and particularly the districts about the poles, would be favorable to organic life.