occurred on the 30th of August, and the snow-zone was observed to be smallest on the 11th of October, or fortytwo of our days after the highest position of the Sun. This very rapid melting may be ascribed to the inclination of the axis, which is greater than with us; to the greater eccentricity of the planet's orbit; and to the fact that the summer-time of the southern hemisphere occurs when the planet is near perihelion.

261. For a reason that will be easily understood when we come to deal with the effect of the Earth's revolution round the Sun on the apparent positions and aspects of the planets, we sometimes see the north pole, and sometimes the south pole of Mars, and sometimes both; when either pole only is visible, the features, which appear to pass across the planet's disk in about 12 hours-that is, half the period of the planet's rotation-describe curves with the concave side towards the visible pole. When both poles are visible they describe straight lines, exactly as in the case of the Sun (Art. 106). These changes enable all 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

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 such is the fact. 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 TradeWinds 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, some

* Proctor's "Saturn and his System," from which some of the statements concerning Saturn are taken, may be consulted by the teacher.