Passing to the earth, we find that the temperature-reducing influence of the clouds must be strong. They protect about half of the earth’s surface (52 per cent.) from solar radiation. But even with a perfectly clear sky, not all the light from the sun really reaches the earth’s surface; for finely distributed dust is floating even in the purest air. I have estimated that this dust would probably absorb 17 per cent. of the solar heat. Clouds and dust would therefore together deprive the earth of 34 per cent. of the heat sent to it, which would lead to a reduction of the temperature by about 28°. Dust and the water-bubbles in the clouds also prevent the radiation of heat from the earth, so that the total loss of heat to be charged to clouds and dust will amount to about 20° (36° F.). It has now been ascertained that the mean temperature of the earth is 16° (61° F.), instead of the calculated 6.5° (43.7° F.). Deducting the 20° due to the influence of dust and clouds, we obtain -14° (7° F.), and the observed temperature would therefore be higher than the calculated by no less than 30° (54° F.). The discrepancy is explained by the heat-protecting action of the gases contained in the atmosphere, to which we shall presently refer (page 51). There are but few clouds on Mars. This planet is endowed with an atmosphere of extreme transparency, and should therefore have a high temperature. Instead of the temperature of -37° (35° F.), calculated, the mean temperature seems to be +10° (+50° F.). During the winter large white masses, evidently snow, collect on the poles of Mars, which rapidly melt away in spring and change into water that appears dark to us. Sometimes the snow-caps on the poles of Mars disappear entirely during the Mars summer; this never happens on our terrestrial poles. The mean temperature of Mars must therefore be above zero, probably about +10°. Organic life may very probably thrive, therefore, on Mars. It is, however, rather sanguine to jump at the conclusion that the so-called canals of Mars prove its being inhabited by intelligent beings. Many people regard the “canals” as optical illusions; Lowell’s photographs, however, do not justify this opinion.