to the absence of vapour and the extreme
transparency of the air."[71]

The past history of the Leonids, which Le Verrier has traced out with
great probability, if not proved, is very interesting. They did not, he
considers, approach the Sun until 126 A.D., when, in their career
through the heavens, they chanced to come near to Uranus. But for the
influence of that planet they would have passed round the Sun, and then
departed again for ever. By his attraction, however, their course was
altered, and they will now continue to revolve round the Sun.

There is a remarkable connection between star showers and comets, which,
however, is not yet thoroughly understood. Several star showers follow
paths which are also those of comets, and the conclusion appears almost
irresistible that these comets are made up of Shooting Stars.

We are told, indeed, that 150,000,000 of meteors, including only those
visible with a moderate telescope, fall on the earth annually. At any
rate, there can be no doubt that every year millions of them are
captured by the earth, thus constituting an appreciable, and in the
course of ages a constantly increasing, part of the solid substance of
the globe.

THE STARS

We have been dealing in the earlier part of this chapter with figures
and distances so enormous that it is quite impossible for us to realise
them; and yet we have still others to consider compared with which even
the solar system is insignificant.

In the first place, the number of the Stars is enormous. When we look at
the sky at night they seem, indeed, almost innumerable; so that, like
the sands of the sea, the Stars of heaven have ever been used as
effective symbols of number. The total number visible to the naked eye
is, however, in reality only about 3000, while that shown by the
telescope is about 100,000,000. Photography, however, has revealed to us
the existence of others which no telescope can show. We cannot by
looking long at the heavens see more than at first; in fact, the first
glance is the keenest. In photography, on the contrary, no light which
falls on the plate, however faint, is lost; it is taken in and stored
up. In an hour the effect is 3600 times as great as in a second. By
exposing the photographic plate, therefore, for some hours, and even on
successive nights, the effect of the light is as it were accumulated,
and stars are rendered visible, the light of which is too feeble to be
shown by any telescope.