on_, mixed with that of sodium and
other elements. It should, however, be noted that comets have
occasionally been seen which possess several tails of these various
types.

We will now touch upon a few of the best known comets of modern times.

The comet of 1680 was the first whose orbit was calculated according to
the laws of gravitation. This was accomplished by Newton, and he found
that the comet in question completed its journey round the sun in a
period of about 600 years.

In 1682 there appeared a great comet, which has become famous under the
name of Halley's Comet, in consequence of the profound investigations
made into its motion by the great astronomer, Edmund Halley. He fixed
its period of revolution around the sun at about seventy-five years, and
predicted that it would reappear in the early part of 1759. He did not,
however, live to see this fulfilled, but the comet duly returned--_the
first body of the kind to verify such a prediction_--and was detected on
Christmas Day, 1758, by George Palitzch, an amateur observer living near
Dresden. Halley also investigated the past history of the comet, and
traced it back to the year 1456. The orbit of Halley's comet passes out
slightly beyond the orbit of Neptune. At its last visit in 1835, this
comet passed comparatively close to us, namely, within five million
miles of the earth. According to the calculations of Messrs P.H. Cowell
and A.C.D. Crommelin of Greenwich Observatory, its next return will be
in the spring of 1910; the nearest approach to the earth taking place
about May 12.

On the 26th of March, 1811, a great comet appeared, which remained
visible for nearly a year and a half. It was a magnificent object; the
tail being about 100 millions of miles in length, and the head about
127,000 miles in diameter. A detailed study which he gave to this comet
prompted Olbers to put forward that theory of electrical repulsion
which, as we have seen, has since been so carefully worked out by
Bredikhine. Olbers had noticed that the particles expelled from the head
appeared to travel to the end of the tail in about eleven minutes, thus
showing a velocity per second very similar to that of light.

The discovery in 1819 of the comet known as Encke's, because its orbit
was determined by an astronomer of that name, drew attention for the
first time to Jupiter's comet family, and, indeed, to short-period
comets in general. This comet revolves around the sun in the shorte