Page:Astronomy for Everybody.djvu/128

rays of the vertical sun. The heat which the sun will radiate to them will be sufficient to warm the water about three and a half or four degrees Centigrade, or not very far from seven degrees Fahrenheit, in one minute. It follows that if we suppose a thin spherical shell of water, one centimetre thick, of the same radius as the earth's orbit, and having the sun in its centre, that shell of water will be heated with the rapidity just mentioned. The heat which it receives will be the total amount radiated by the sun. We can thus define how much heat the sun loses every minute, day and year.

A very simple calculation will show that if the sun were of the nature of a white-hot ball it would cool off so rapidly that its heat could not last more than a few centuries. But it has in all probability lasted millions of years. Whence, then, comes the supply? The answer of modern science to this question is that the heat radiated from the sun is supplied by the contraction of size as heat is lost. We all know that in many cases when motion is destroyed heat is produced. When a cannon shot is fired at the armour plate of a ship of war, the mere stroke of the shot makes both plate and shot hot. The blacksmith can make iron hot by hammering it.

These facts have been generalized into the statement that whenever a body falls and is stopped in its fall by friction, or by a stroke of any sort, heat is produced. From the law governing the case, we know that the water of Niagara, after it strikes the bottom of the falls, must be about one quarter of a degree warmer than it was during the fall. We also know that a hot body contracts