It is believed to be made partly of gases, and perhaps partly of meteors. 13. Is a comet heavy, or light? Very light indeed, compared with its great size. CHAPTER XVIII. LITTLE METEORS. Meteors are the very smallest bodies of which we know, that float and rush about in the sky. Besides being the smallest they are also the most abundant. Their numbers are not only past counting, but past our power to imagine. We cannot see them as they speed hither and thither through the skies, travelling either alone or in tens of millions. Each one indeed gives forth its tiny light, borrowed from the Sun. But those dim gleams are far too weak to reach us here on Earth. The only time when they can be seen by us is when they come by accident into our air. Then indeed we do see them--not by the gentle shining which they catch from the Sun, but by one brilliant flash of light as they are destroyed. It is the rush through our thick air which destroys the meteors. The air always tries to hold back anything moving fast through it. A meteor far away in the sky is a hard and cold little body--very cold indeed, out in the terrible cold of Space. It has no light of its own to give forth. And in the sky a meteor goes very fast, rushing round the Sun. When it first gets into our air it keeps up that great speed. The air tries to hold it back; and the rubbing of the air against it heats the outside of the little meteor so intensely that it glows with bright light. It becomes in fact “white-hot.” The outside melts and pours away in a stream of shining dust, which to us looks like a tail of light. The dust soon cools, and drops gently down upon the ground. Before the meteor has rushed twenty or thirty miles it is generally done for. All of it has gone off in bright dust, and nothing is left of the tiny heavenly body except that dust. This is what you see when you look at a “shooting star” after dark. Of course you have seen shooting stars very often. If not, you should begin to look out for them as soon as possible. A shooting star is no star at all, really. It is only a little meteor, or meteorite, which has travelled for ages in the skies, and which has at last happened to come too near to our Earth. The strong pull of the Earth’s attraction has dragged it into the air, and so it has perished. Hundreds and thousands of meteors are ever dropping earthward. If it were not for our soft protecting air we should be under a regular cannonade from the sky; but happily most of the cannon-balls are used up long before they can reach the ground. On the Moon, where there is no protecting air, one would have to undergo a fearful battering. Now and again a meteor is large enough not to be _all_ destroyed in its rush downwards. A good part is melted, and runs away as a little tail of brightness, but both speed and heat grow less before the whole is gone.