Page:Popular Science Monthly Volume 4.djvu/199

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N order successfully to observe Mars, two conditions are requisite: First, the earth's atmosphere must be clear at the point of observation; and, second, the atmosphere of Mars must be also free from clouds—for that planet, like the earth itself, is surrounded by an aerial atmosphere which from time to time is obscured by clouds just like our own. These clouds, as they spread themselves out over the continents and seas, form a white veil which either entirely or partially conceals from us the face of the planet. Hence the observation of Mars is not so easy a matter as it might at first appear. Then, too, the purest and most transparent terrestrial atmosphere is commonly traversed by rivers of air, some warm, some cold, which flow in different directions above our heads, so that it is almost impossible to sketch a planet like Mars, the image seen in the telescope being ever undulating, tremulous, and indistinct. I believe that, if we were to reckon up all the hours during which a perfect observation could be had of Mars, albeit his period of opposition occurs every two years, and although telescopes were invented more than two and a half centuries ago, the sum would not amount to more than one week of constant observation.

And yet, in spite of these unfavorable conditions, the Planet of War is the best known of them all. The moon alone, owing to its nearness to us, and the absence of atmosphere and clouds, has attracted more particular and assiduous study; and the geography (selenography rather) of that satellite is now satisfactorily determined. That hemisphere of the moon which faces us is better known than the earth itself; its vast desert plains have been surveyed to within a few acres; its mountains and craters have been measured to within a few yards; while on the earth's surface there are 30,000,000 square kilometres (sixty times the extent of France), upon which the foot of man has never trod, which the eye of man has never seen. But, after the moon, Mars is the best known to us of all the heavenly bodies. No other planet can compare with him. Jupiter, which is the largest, and Saturn the fullest of curious interest, are both far more important than Mars, and more easily observed in their ensemble, owing to their size; but they are enveloped with an atmosphere which is always laden with clouds, and hence we never see their face. Uranus and Neptune are only bright points. Mercury is almost always eclipsed, like a courtier, by the rays of the sun. Venus alone may compare with Mars; she is as large as the earth, and consequently has twice the