t seen the people terrified at the
sight of a flaming comet? Has not the end of the world by the agency of
comets been often enough predicted? These predictions are so to speak
periodic; they crop up each time that the return of these cosmical
formations is announced by the astronomers, and always meet with a
certain number of timid souls who are troubled as to our destinies.

* * * * *

To-day we know that these wanderers are subject to the general laws
that govern the universe. The great Newton announced that, like the
planets, they were obedient to universal attraction; that they must
follow an extremely elongated curve, and return periodically to the
focus of the ellipse. From the basis of these data Halley calculated the
progress of the comet of 1682, and ascertained that its motions
presented such similarity with the apparitions of 1531 and 1607, that he
believed himself justified in identifying them and in announcing its
return about the year 1759. Faithful to the call made upon it,
irresistibly attracted by the Orb of Day, the comet, at first pale, then
ardent and incandescent, returned at the date assigned to it by
calculation, three years after the death of the illustrious astronomer.
Shining upon his grave it bore witness to the might of human thought,
able to snatch the profoundest secrets from the Heavens!

This fine comet returns every seventy-six years, to be visible from the
Earth, and has already been seen twenty-four times by the astonished
eyes of man. It appears, however, to be diminishing in magnitude. Its
last appearance was in 1835, and we shall see it again in 1910, a little
sooner than its average period, the attraction of Jupiter having this
time slightly accelerated its course, while in 1759 it retarded it.

The comets thus follow a very elongated orbit, either elliptic, turning
round the Sun, or parabolic, dashing out into space. In the first case,
they are periodic (Fig. 52), and their return can be calculated. In the
second they surprise us unannounced, and return to the abysses of
eternity to reappear no more.

[Illustration: FIG. 52.--The orbit of a Periodic Comet.]

Their speed is even greater than that of the planets, it is equivalent
to this, multiplied by the square root of 2, that is to say by 1.414.
Thus at the distance of the Earth from the Sun this velocity = 29,500
meters (18 miles) per second, multiplied by the above number, that is,
41,700 m