x THE STARRY HEAVENS 403 



moreover shown by their small weight. 

 Enormous as they are I remember Sir G. 

 Airy saying that there was probably more 

 matter in a cricket ball than there is in a 

 comet. No one, however, now doubts that 

 the weight must be measured in tons ; but 

 it is so small, in relation to the size, as to 

 be practically inappreciable. If indeed they 

 were comparable in mass even to the planets, 

 we should long ago have perished. The 

 security of our system is due to the fact that 

 the planets revolve round the Sun in one 

 direction, almost in circles, and very nearly 

 in the same plane. Comets, however, enter 

 our system in all directions, and at all angles ; 

 they are so numerous that, as Kepler said, 

 there are probably more Comets in the sky 

 than there are fishes in the sea, and but for 

 their extreme tenuity they would long ago 

 have driven us into the Sun. 



When they first come in sight Comets 

 have generally no tail ; it grows as they 

 approach the Sun, from which it always 

 points away. It is no mere optical illusion ; 

 but while the Comet as a whole is attracted 



