the surface to be seen at different times, and maps of Mars have been constructed, the exact position of the features of the planet being determined by their latitude and longitude, as in the case of the Earth.

262. But although we see in Mars so many things that remind us of our planet, and show us that the extreme temperatures of the two planets are not far from equal, a distinction must be drawn between them. In consequence of the great eccentricity of the orbit of Mars, the lengths of the various seasons are not so equal as with us, and, owing to the longer year, they are of much greater extent. In the northern hemisphere of the planet they are as under ::

days. hrs.

191 8

181 O

149 8

147 O

As we must reverse the seasons for the southern hemisphere, spring and summer, taken together, are 76 days longer in the northern hemisphere than in the southern.

LESSON XX.-THE OTHER PLANETS COMPARED WITH THE EARTH (continued). JUPITER: HIS BELTS AND MOONS. SATURN GENERAL SKETCH OF HIS

SYSTEM.

263. Let us now pass on to Jupiter, by far the largest planet in the system, and bright enough sometimes, in spite of its great distance, to cast a shadow like Venus. The first glance at the drawing (Plate X. Fig. 1) will show us that we have here something very unlike Mars;

and this is the case. The planet Jupiter is surrounded by an atmosphere so densely laden with clouds, that of the actual planet itself we know nothing.

What are generally known as the belts of Jupiter are dusky streaks which cross a brighter background in directions generally parallel to the planet's equator. And for the most part, the largest belts are situated on either side of it, in exactly the same way as the two belts of Trade-Winds on the Earth lie on either side of the belt of Equatorial Calms and rains. Outside these, again, we get representatives of the Calms of Cancer and Capricorn, although these are not so regularly seen, the portion of the planet's surface polewards of the two belts being liable to great changes of appearance, sometimes in a very short time. The portions of the atmosphere representing the terrestrial calm-belts sometimes exhibit a beautiful rosy tint, the equatorial one especially.

264. The variations of this cloudy atmosphere lend great variety to the appearance of the planet at different times; the belts are sometimes seen in large numbers, and extend almost to the poles. Besides the belts, sometimes bright spots, sometimes dark ones, are seen, which have enabled us to determine the period of the planet's rotation, which, as we have seen, is very rapid—so rapid, that on the equator an observer would be carried round at the rate of 467 miles a minute, instead of 17 as on the Earth. We can easily understand that this rapid rotation would break the cloudy surface into belts more than with us, or as is the case of Mars; in the latter planet, indeed, no trace of cloud-belts has as yet been detected; their absence is perhaps due to its slow rotation and small size.

265. Although all astronomers do not agree that the surface of the planet is never seen, there are many strong reasons why it should not be seen. In the first place,

Mars and the Earth, whose atmospheres are nearly alike, have nearly the same densities (Art. 145), while, in the case of Jupiter and Saturn-the belts of which latter planet, as far as we can observe them, resemble Jupiter's—the density, as calculated on the idea that what we see is all planet, is only about one-fifth that of the Earth; and as the density of the Earth is 5 times that of water, it follows that the densities of the two planets in question are not far off that of water.

266. Now, if we suppose that the apparent volume of Jupiter (and similarly of Saturn) is made up of a large shell of cloudy atmosphere and a kernel of planet, there is no reason why the density of the real Jupiter (and of the real Saturn) should vary very much from that of the Earth or Mars, and this would save us from the waterplanet hypothesis. Moreover, a large shell of cloudy atmosphere is precisely what our own planet was most probably enveloped in, in one of the early stages of its history (Art. 208).

267. In addition to the changing features of Jupiter itself, the telescope reveals to us four moons, which as they course along rapidly in their orbits, and as these orbits lie nearly in the plane of the planet's orbit, lend a great additional interest to

the picture. In the various positions in their orbits the satellites sometimes appear at a great distance from the primary; sometimes they come between us and the planet, appearing now as bright and now as dark spots on its surface. other times they pass between the planet and the Sun, throwing their shadows on the planet's disc, and causing, in fact, eclipses of the Sun. They also enter

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