arabola is too slight to be established by observations. The periods
of the comets are therefore unknown except that they must be counted by
centuries and possibly by thousands of years.

Another fact which increases the complexity of the question is the
well-established connexion of comets with meteoric showers. The shower
of November 13-15, now known as the Leonids, which recurred for several
centuries at intervals of about one-third of a century, are undoubtedly
due to a stream of particles left behind by a comet observed in 1866.
The same is true of Biela's comet, the disintegrated particles of which
give rise to the Andromedids, and probably true also of the Perseids, or
August meteors, the orbits of which have a great similarity to a comet
seen in 1862. The general and well-established conclusion seems to be
that, in addition to the visible features of a comet, every such body is
followed in its orbit by a swarm of meteoric particles which must have
been gradually detached and separated from it. (See METEOR.)

The source of the repulsive force by which the matter forming the tail
of a comet is driven away from the sun is another question that has not
yet been decisively answered. Two causes have been suggested, of which
one has only recently been brought to light. This is the repulsion of
the sun's rays, a form of action the probability of which was shown by
J. Clerk Maxwell in 1870, and which was experimentally established about
thirty years later. The intensity of this action on a particle is
proportional to the surface presented by the particle to the rays, and
therefore to the square of its diameter, while its mass, and therefore
its gravitation to the sun, are proportional to the cube of the
diameter. It follows that if the size and mass of a particle in space
are below a certain limit, the repulsion of the rays will exceed the
attraction of the sun, and the particle will be driven off into space.
But, in order that this repulsive force may act, the particles, however
minute they may be, must be opaque. Moreover, theory shows that there is
a lower as well as an upper limit to their magnitude, and that it is
only between certain definable limits of magnitude that the force acts.
Conceiving the particle to be of the density of water, and considering
its diameter as a diminishing variable, theory shows that the repulsion
will balance gravity when the diameter has reached 0.0015 of a
millimetre. As the diameter i