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 
