[[Categoria:Pagine che usano RigaIntestazione|Scientia - Vol. VII.djvu{{padleft:273|3|0]]to the total mass of the planet for both the Earth and Mars, the atmospheric pressure on Mars would be about one-seventh that on the Earth, or 2 lbs to the square inch. But in spite of this low pressure at the surface, at about 15 miles, or 24 kilometres, above the surface of the two planets, the Martian atmosphere is as dense as ours, and above that level, is the denser of the two. It is probable that the average height at which meteors become incandescent is double as great on Mars as with ourselves. The Martian atmosphere, though much less dense than the terrestrial, is therefore distinctly deeper.

But since we see the surface of Mars so clearly, although the atmosphere is so deep, it is evident that the above estimate of its density must be a maximum. In all probability the pressure at the surface is considerably under 2 lbs to the square inch; it certainly cannot be greater.

With an atmospheric pressure of 2 lbs to the square inch, water will boil at about 44° centigrade. But this means a great diminution of the range of temperature, through which it is possible for water to remain in the liquid state. The mean temperature of the Earth is usually taken as about 15° centigrade, whilst the range from freezing to boiling point is 100°. The two facts taken together imply that the liquid form is the one in which water is usually found upon our planet, though the nearness of the mean to the freezing point suggests that, in the polar zones, it must normally take the forms of ice and snow.

Now the mean temperature of Mars must be considerably below that of the Earth, seeing that it receives, surface for surface, but three-sevenths as much of the solar heat But. even on the Earth, on high plateaux and mountains, the range in the daily temperature of the soil is enormous, amounting sometimes to 100° centigrade. The range must be even greater still for Mars, and the opportunity for water to remain in the liquid form, must be much diminished; its normal form must be snow or vapour. It will be easily congealed, easily evaporated. Day by day, at least in the tropical and sub-tropical regions, evaporation will proceed readily; night after night severe frosts will succeed. At dawn probably therefore the air is dry; during the afternoon, it may be saturated. This heating of the region in daylight, and refri-