The science that tells of these things and how men have come to know about them is Astronomy, and to books of astronomy the reader must go to learn more about the sun and stars. The science and description of the world on which we live are called respectively Geology and Geography. The diameter of our world is a little under 8000 miles. Its surface is rough; the more projecting parts of the roughness are mountains, and in the hollows of its surface there is a film of water, the oceans and seas. This film of water is about five miles thick at its deepest part--that is to say, the deepest oceans have a depth of five miles. This is very little in comparison with the bulk of the world. About this sphere is a thin covering of air, the atmosphere. As we ascend in a balloon or go up a mountain from the level of the sea-shore the air is continually less dense, until at last it becomes so thin that it cannot support life. At a height of twenty miles there is scarcely any air at all--not one hundredth part of the density of air at the surface of the sea. The highest point to which a bird can fly is about four miles up--the condor, it is said, can struggle up to that; but most small birds and insects which are carried up by aeroplanes or balloons drop off insensible at a much lower level, and the greatest height to which any mountaineer has ever climbed is under five miles. Men have flown in aeroplanes to a height of over four miles, and balloons with men in them have reached very nearly seven miles, but at the cost of considerable physical suffering. Small experimental balloons, containing not men, but recording instruments, have gone as high as twenty-two miles. It is in the upper few hundred feet of the crust of the earth, in the sea, and in the lower levels of the air below four miles that life is found. We do not know of any life at all except in these films of air and water upon our planet. So far as we know, all the rest of space is as yet without life. Scientific men have discussed the possibility of life, or of some process of a similar kind, occurring upon such kindred bodies as the planets Venus and Mars. But they point merely to questionable possibilities.