The nearer a comet gets to the Sun the faster it travels; some comets have been known to move a thousand times as fast as a rifle-ball, and this is the more surprising when we consider that the great, flaming tail goes along with it at the same terrific rate of speed. The tails of comets do some very strange things; for example we should rather expect the tail of a comet to always follow its head like a skyrocket, but while it does so when the comet is headed toward the Sun, when the comet is passing round and shooting into space the tail turns away from the Sun, and this causes it to move ahead of the comet, as shown in Fig. 130. This shows that while the head of a comet obeys the laws of gravitation, the tail does not do so, but acts as if it was electrified like the Sun, when of course it would be repelled by it. Fig. 131 is a picture of Halley’s comet of 1910. Although a great astronomer once said that there are more comets in the sky than fishes in the sea, there have only been 1,000 comets recorded since the beginning of written history. [Illustration: FIG. 132.—METEORITE OF IRON ETCHED WITH ACID.] =Meteors and Meteorites.=—If you will look at almost any part of the sky on a clear night when there is no Moon you will no doubt see a bright flash like a rocket. But if you will scan the sky during the dark nights of August and November you will be very apt to see dozens of these shooting stars or meteors. Now, _meteors_, _fireballs_, _shooting stars_ and _meteorites_ are all one and the same thing to start with and they have their beginnings when some comet goes to pieces. After a comet breaks up, the pieces of stone and iron which form it still travel round in the same orbit. Some of these pieces may get within range of the Earth’s force of gravitation when they are drawn to it, and on striking the air they are intensely heated by the friction, and if they are small they burn up before reaching the ground. The smaller meteors burn up almost instantly and the shining tails they leave last only a few seconds. Fireballs, which are simply large meteors, leave burning trails which can sometimes be seen for several minutes. Shooting stars are merely meteors which are not very bright, while meteorites are meteors which have fallen to the Earth before they have had time to burn up. Many meteorites have been found, but ninety-five out of every hundred are of the stony kind, the others being of the iron kind. The way to tell a meteorite from a common stone is by examining its surface. A true meteorite is covered with a black, shiny, burnt crust, caused by the intense heat to which it was subjected as it fell through the air. The test for an iron meteorite is to grind and polish a part of its surface and then cover it with a dilute solution of nitric acid, when markings like those shown in Fig. 132 will be etched upon it. [Illustration: FIG. 133.—THE MILKY WAY.]