Pagina:Scientia - Vol. VII.djvu/274

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266 scientia

geration of that in night, will set up the chief winds known to Mars, — its morning and evening breezes, — and the chief atmospheric change will be the condensation of the water vapour beginning a little before sunset; and the melting and evaporation of the frost produced during the night, beginning soon after sunrise. It is clear that the frozen condition of the water of Mars must extend further into the area in daylight along the arc of sunrise, than along the arc of sunset; the sun will take some time after rising to make its power felt. Both terminators have been observed to be fringed by a white line but the white fringe to the sunrise terminator has been noticed by several observers to be distinctly broader than that of the sunset terminator.

So far we have looked at the tropical regions of Mars.

When we turn our attention to the poles, a most important circumstance has to be considered, namely, that for a period, very nearly equal to an entire year of the Earth, each pole of Mars, in turn, is subjected continuously to the solar rays. Indeed the northern pole is so exposed for more than a terrestrial year. There is no need for surprise that under such conditions, the polar snowcaps, and especially the northern one, should sometimes disappear altogether. But that they do diminish to a degree so far surpassing the diminution of the terrestrial polar caps, is a clear indication that the amount of moisture present on the planet is relatively very small; and the narrowness of the white fringe to the sunrise and sunset terminators, is a further confirmation of this fact.

We have already seen that the chief atmospheric circulation of the planet must be that which goes on between the sunlit hemisphere and the hemisphere in darkness. Looking at Mars from the view-point of the Sun — and our own view-point does not greatly differ from this — we may expect that there will be a general cold current close to the ground, blowing inward from the circumference of the disc towards the centre, but prevailing only for a relatively small distance from the circumference. In the centre of the disc there will be ascendin currents, flowing towards the circumference. This general scheme of circulation will of course, not be symmetrical; the hottest part of the day with Mars, as with the Earth, will be in the afternoon.