Page:Popular Science Monthly Volume 35.djvu/51

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N the whole planetary empire of the sun there is but one body, if we except the moon, whose actual surface can be satisfactorily examined even with the most powerful telescope. The broad disk of Jupiter presents a most inviting and splendid sight; but it is apparent that we are not looking at the solid shell of a planet, but at a vast expanse of thick clouds, surrounding and concealing the planetary core, and reflecting the sunlight from their shifting surfaces. Saturn presents a somewhat similar appearance, modified by greater distance. Uranus and Neptune are so nearly beyond the present reach of telescopes, so far as the phenomena of their disks are concerned, that we know almost nothing of their surface appearances. Some observations of Uranus, however, indicate that it presents the same equatorial parallelism of exterior markings that characterizes Jupiter and Saturn; and so we may infer that what we faintly discern on its disk are the outlines of cloud-masses, enveloping the planet, and drawn out by the effects of its rotation into belts and streaks. Coming to the nearer planets, we find that Venus, superbly brilliant to the naked eye, and consequently, it might naturally be thought, a promising object for telescopic scrutiny, is nevertheless the most disappointing of all the planets when viewed with a telescope. The splendor of its luminosity in itself forms an obstacle to the study of its surface, where flitting glimpses of shadowy forms and brilliant spots only serve to excite the keenest curiosity. With respect to Mercury, our knowledge is equally unsatisfactory. The surface of the moon, of course, has been well studied, as such maps as those of Beer and Mädler, Neison and Schmidt sufficiently attest. But, after all, the absence of the faintest indication of life robs the wonderful lunar landscapes of a large share of the interest that would otherwise attach to them.

Finally, we look at Mars, and here at last we find a globe whose true surface we can inspect, and which at the same time possesses an atmosphere and other concomitants of vital organization. Since Mars has been selected by more than one astronomer as the probable abode of life (and perhaps the only one besides the Earth in the solar system), and especially since a discussion of the markings seen upon the planet necessarily involves the physical features upon which the theory of Mars's fitness for inhabitation rests, it will be well to recall here the principal facts that have been ascertained respecting that interesting orb.

The diameter of Mars is 4,300 miles, or only some 240 miles