S6 The Outline of Science 



Most of them weigh only an ounce or two, and are invisible. 

 Some of them wdgh a ton or more, but even against these 

 large masses the air acts as a kind of "torpedo-net." They 

 generally burst into fragments and fall without doing dam- 

 age. 



It is clear that "empty space" is, at least within the limits 

 of our solar system, full of these things. They swarm like fishes 

 in the seas. Like the fishes, moreover, they may be either solitary 

 or gregarious. The solitary bit of cosmic rubbish is the meteorite, 

 which we have just examined. A "social" group of meteorites 

 is the essential part of a comet. The nucleus, or bright central 

 part, of the head of a comet (Fig. 19) consists of a swarm, some- 

 times thousands of miles wide, of these pieces of iron or stone. 

 This swarm has come under the sun's gravitational influence, and 

 is forced to travel round it. From some dark region of space 

 it has moved slowly into our system. It is not then a comet, for 

 it has no tail. But as the crowded meteors approach the sun, 

 the speed increases. They give off fine vapour-like matter and 

 the fierce flood of light from the sun sweeps this vapour out in an 

 ever-lengthening tail. Whatever way the comet is travelling, 

 the tail always points away from the sun. 



A Great Comet 



The vapoury tail often grows to an enormous length as the 

 comet approaches the sun. The great comet of 1843 had a tail 

 two hundred million miles long. It is, however, composed of 

 the thinnest vapours imaginable. Twice during the nineteenth 

 century the earth passed through the tail of a comet, and noth- 

 ing was felt. The vapours of the tail are, in fact, so attenuated 

 that we can hardly imagine them to be white-hot. They may 

 be lit by some electrical force. However that may be, the comet 

 dashes round the sun, often at three or four hundred miles a 

 second, then may pass gradually out of our system once more. 

 It may be a thousand years, or it may be fifty years, before 



