evail as to whether it is a periodic comet,
or one of those that only visit our system once. If it be periodic, as
suspected, it is the same as appeared when Julius Caesar was killed, and
which likewise appeared in the years 531 and 1106 A.D. Should it appear
in 2255, our posterity will probably regard it as a memorial of Newton.

[Illustration: FIG. 98.--Parabolic and elliptic orbits. The _a b_
(visible) portions are indistinguishable.]

The next comet discussed in the light of the theory of gravitation was
the famous one of Halley. You know something of the history of this.
Its period is 75-1/2 years. Halley saw it in 1682, and predicted its
return in 1758 or 1759--the first cometary prediction. Clairaut
calculated its return right within a month (p. 219). It has been back
once more, in 1835; and this time its date was correctly predicted
within three days, because Uranus was now known. It was away at its
furthest point in 1873. It will be back again in 1911.

[Illustration: FIG. 99.--Orbit of Halley's comet.]

Coming to recent times, we have the great comets of 1843 and of 1858,
the history of neither being known. Quite possibly they arrived then for
the first time. Possibly the second will appear again in 3808. But
besides these great comets, there are a multitude of telescopic ones,
which do not show these striking features, and have no gigantic tail.
Some have no tail at all, others have at best a few insignificant
streamers, and others show a faint haze looking like a microscopic
nebula.

All these comets are of considerable extent--some millions of miles
thick usually, and yet stars are clearly visible through them. Hence
they must be matter of very small density; their tails can be nothing
more dense than a filmy mist, but their nucleus must be something more
solid and substantial.

[Illustration: FIG. 100.--Various appearances of Halley's comet when
last seen.]

I have said that comets arrive from the depths of space, rush towards
and round the sun, whizzing past the earth with a speed of twenty-six
miles a second, on round the sun with a far greater velocity than that,
and then rush off again. Now, all the time they are away from the sun
they are invisible. It is only as they get near him that they begin to
expand and throw off tails and other appendages. The sun's heat is
evidently evaporating them, and driving away a cloud of mist and
volatile matter. This is when they can be seen. The comet is most