Mountains, as every one knows, are colder than the plains below. No one cares to stay very long on a mountain-top, for fear of catching cold. It may be worth while to consider why they are cold. Perhaps you answer, "Because they are so high." That is true, but not a complete answer to our question. We must look at the matter a little more closely. The earth is a warm body surrounded by space in which the cold is inconceivably intense; but just as we protect our bodies against cold with garments, so the earth is wrapped up in an atmosphere which serves more or less to keep in the heat. All warm bodies give out heat as luminous bodies give out light; but the rays of heat, unlike those of light, are quite invisible to our eyes, so that we are unaware of them. These "dark heat-rays," as they are called, do not make any impression on the retina, because our eyes are not capable of responding to them as they do to the ordinary rays of light. But there is a delicate little instrument known as the thermopile, which responds to, and so detects these invisible rays; and if our eyes were sensitive to such vibrations as these, we should see heat-rays (which like light and sound are due to vibrations) streaming from every object, just as light does from a candle-flame. Those parts of the earth which are least covered or protected by the atmosphere lose heat most rapidly,--in the same way that on a frosty day one's fingers become cold unless covered up. Now, there is less air over mountains; and in those higher regions above the peaks what air there is, is more rarefied, and therefore less capable of stopping the heat-rays coming from the earth. Professor Tyndall has shown that water-vapour in the air has a great power of stopping dark heat-rays; and the lower regions, which contain more vapour, stop or absorb a good deal of heat which would otherwise escape into space.