Two singular white spots are to be seen at the north and south poles, which we believe to be polar ice and snow. Somebody looking at our earth in like manner from a distance, would doubtless perceive two such white snow-spots. These two polar caps are seen to vary with the seasons. When the north pole of Mars is turned towards the sun, the white spot there grows smaller; and at the same time, the south pole of Mars being turned away from the sun, the white spot there grows larger. Again, when the south pole is towards the sun, and the north pole away from the sun, the white spot at the south is seen to be the smallest, and the white spot at the north is seen to be the largest. This is exactly what takes place in the summers and winters of our north and south poles. The markings of Mars have been so carefully studied, that at last a map has been made of the planet--a map of a world, never less than forty millions of miles away! Names have been given to the continents and oceans--such as Dawes Continent, Herschel Continent, De La Rue Ocean, Airy Sea, Huggins Inlet, and so on. Land and water seem to be very differently arranged on Mars from what they are on earth. Here we have about three times as much water as land, and to get from one continent to another without crossing the sea is in some cases impossible. But a traveler there might go most conveniently to and fro, hither and thither, to all parts of his world, either on land or on water, without any change. If he preferred water, he would never need to set foot on land; and if he preferred land, he would never need to enter a boat. The two are so curiously mingled together, narrow necks of land running side by side with long, narrow sea-inlets, that Atlantic and Pacific Oceans are unknown. Some have wondered whether the reddish color of the land may be caused by grass and trees being red instead of green. Very strange if so it were. But in that case, no doubt the inhabitants of Mars would find green just as trying to their eyesight, as we should find red trying to ours. CHAPTER XVII. JUPITER. Passing at one leap over the belt of tiny asteroids, about which we know little beyond their general movement, and the size and weight of a few among them, we reach at once the giant planet Jupiter: Mighty Jupiter, hurrying ever onward, with a speed, not indeed equal to that of Mercury or of our earth, yet eighty times as rapid as the speed of a cannon-ball! Think of a huge body, equal in bulk to twelve hundred earths, equal in weight to three hundred earths, rushing ceaselessly through space, at the rate of seven hundred thousand miles a day!