variety, and there is, in consequence, a disposition
in the tails of the second type to a more indefinite outline than in the
hydrogen tails. Cases have been recorded in which several tails have
been seen simultaneously on the same comet. The most celebrated of these
is that which appeared in the year 1744. Professor Bredichin has devoted
special attention to the theory of this marvellous object, and he has
shown with a high degree of probability how the multiform tail could be
accounted for. The adjoining figure (Fig. 74) is from a sketch of this
object made on the morning of the 7th March by Mademoiselle Kirch at the
Berlin Observatory. The figure shows eleven streaks, of which the first
ten (counting from the left) represent the bright edges of five of the
tails, while the sixth and shortest tail is at the extreme right.
Sketches of this rare phenomenon were also made by Cheseaux at Lausanne
and De L'Isle at St. Petersburg. Before the perihelion passage the
comet had only had one tail, but a very splendid one.

[Illustration: Fig. 74.--The Comet of 1744.]

It is possible to submit some of the questions involved to the test of
calculation, and it can be shown that the repulsive force adequate to
produce the straight tail of Type I. need only be about twelve times as
large as the attraction of gravitation. Tails of the second type could
be produced by a repulsive force which was about equal to gravitation,
while tails of the third type would only require a repulsive force about
one-quarter the power of gravitation.[33] The chief repulsive force
known in nature is derived from electricity, and it has naturally been
surmised that the phenomena of comets' tails are due to the electric
condition of the sun and of the comet. It would be premature to assert
that the electric character of the comet's tail has been absolutely
demonstrated; all that can be said is that, as it seems to account for
the observed facts, it would be undesirable to introduce some mere
hypothetical repulsive force. It must be remembered that on quite other
grounds it is known that the sun is the seat of electric phenomena.

As the comet gradually recedes from the sun the repulsive force becomes
weaker, and accordingly we find that the tail of the comet declines. If
the comet be a periodic one, the same series of changes may take place
at its next return to perihelion. A new tail is formed, which also
gradually disappears as the comet regains the dept