So when people talk about Mars, and about what may be seen on Mars, remember this--that at the very best _we can only see Mars as we should see a world 35 thousands of miles away_! On the Earth even a hundred miles seems a long distance. From the top of a mountain one can see to a hundred miles no doubt, in clear weather; but very little can be made out at such a distance. Yet a hundred miles would be only a small piece of one country. It takes ten hundreds to make a thousand, and a thousand miles off seems to us very far indeed. Nobody on the Earth can ever be farther from us than about 8,000 or about 12,006 miles off, that is, through the middle of the Earth 8,000 miles, or reckoning round the outside rather over 12,000. You know how distant Australia seems from us. But Mars at his nearest, and looked at through the largest of telescopes, is still only seen as a world _three times farther away_ than the very farthest off country upon this whole Earth from you or me. Of course it is very wonderful that a planet thirty-five millions of miles away can be actually seen through a telescope as if it were only thirty-five thousands of miles away. Still, at the best, thirty-five thousands of miles is a pretty good distance. Although we cannot find out half or a quarter of what we want to know about Mars, still we do know a good deal. The big telescopes tell us much, and another instrument, called a “spectroscope,” tells us yet more. But in this small book I cannot even try to explain to you what a “spectroscope” is.[1] [1] See “Sun, Moon, and Stars,” page 307. We know that Mars has some sort of air, perhaps rather like our Earth air, only more thin. We know that water floats in that air, as water floats unseen in our air. As for climates, one might expect Mars to be terribly cold at such a distance from the Sun. He cannot have half the quantity of light or heat that we have. Yet, somehow, there seem to be signs that Mars is not a very much colder world than our Earth is. At the north pole and the south pole of Mars tiny white caps, or patches, are seen; and these are most likely made of ice and snow. We on the Earth have always ice and snow at our two poles; and people on another world, a long way off, might perhaps see our polar ice and snow as white caps, or patches. Sometimes clouds are seen to flit across Mars, white clouds, like the white clouds which cover Venus. This only means that they are white outside, on the _upper_ surface, where the Sun shines. They may be gray below, like so many of our gray Earth clouds, though we also often see clouds white and shining in sunlight. And when a man gets up a high mountain above the clouds, and looks down upon them, he sees their upper surface, white as snow and beautifully bright.