But Mars compares badly with the Earth in another respect. It receives less light and heat from the Sun in the proportion of three to seven. This we may express by saying that Mars, on the whole, is almost as much worse off than the Earth as a point on the Arctic Circle is worse off than a point on the Equator. The mean temperature of the Earth is taken as about 60° of the Fahrenheit thermometer (say, 15° Cent.); the mean temperature of Mars must certainly be considerably below freezing-point, probably near 0° F. Here on our Earth the boiling-point of water is 212°, and, since the mean temperature is 60° and water freezes at 32°, it is normally in the liquid state. On Mars it must normally be in the solid state--ice, snow, or frost, or the like. But with so rare an atmosphere water will boil at a low temperature, and it is not impossible that under the direct rays of the Sun--that is to say, at midday of the torrid zone of Mars--ice may not only melt, but water boil by day, condensing and freezing again during the night. NEWCOMB, the foremost astronomer of his day, concluded "that during {66} the night of Mars, even in the equatorial regions, the surface of the planet probably falls to a lower temperature than any we ever experienced on our globe. If any water exists, it must not only be frozen, but the temperature of the ice must be far below the freezing point.... The most careful calculation shows that if there are any considerable bodies of water on our neighbouring planet, they exist in the form of ice, and can never be liquid to a depth of more than one or two inches, and that only within the torrid zone and during a few hours each day." With regard to the snow caps of Mars, Newcomb thought it not possible that any considerable fall of snow could ever take place. He regarded the white caps as simply due to a thin deposit of hoar frost, and it cannot be deemed wonderful that such should gradually disappear, when it is remembered that each of the two poles of Mars is in turn presented to the Sun for more than 300 consecutive days. Newcomb's conclusion was: "Thus we have a kind of Martian meteorological changes, very slight indeed, and seemingly very different from those of our Earth, but yet following similar lines on their small scale. For snowfall substitute frostfall; instead of (the barometer reading) feet or inches say fractions of a millimetre, and instead of storms or wind substitute little motions of an air thinner than that on the top of the Himalayas, and we shall have a general description of Martian meteorology." We conclude, then, that Mars is not so inert a world as the Moon, but, though some slight changes of climate or weather take place upon it, it is quite unfitted for the nourishment and development of the different forms of organic life.