duce? This is the idea which Encke put forward. Even if the greater
part of space be utterly void, so that the path of the filmy and almost
spiritual comet is incapable of feeling resistance, yet in the
neighbourhood of the sun it was supposed that there might be some medium
of excessive tenuity capable of affecting so light a body. It can be
demonstrated that a resisting medium such as we have supposed would
lessen the size of the comet's path, and diminish the periodic time.
This hypothesis has, however, now been abandoned. It has always appeared
strange that no other comet showed the least sign of being retarded by
the assumed resisting medium. But the labours of Backlund have now
proved beyond a doubt that the acceleration of the motion of Encke's
comet is not a constant one, and cannot be accounted for by assuming a
resisting medium distributed round the sun, no matter how we imagine
this medium to be constituted with regard to density at different
distances from the sun. Backlund found that the acceleration was fairly
constant from 1819 to 1858; it commenced to decrease between 1858 and
1862, and continued to diminish till some time between 1868 and 1871,
since which time it has remained fairly constant. He considers that the
acceleration can only be produced by the comet encountering periodically
a swarm of meteors, and if we could only observe the comet during its
motion through the greater part of its orbit we should be able to point
out the locality where this encounter takes place.

We have selected the comets of Halley and of Encke as illustrations of
the class of periodic comets, of which, indeed, they are the most
remarkable members. Another very remarkable periodic comet is that of
Biela, of which we shall have more to say in the next chapter. Of the
much more numerous class of non-periodic comets, examples in abundance
may be cited. We shall mention a few which have appeared during the
present century. There is first the splendid comet of 1843, which
appeared suddenly in February of that year, and was so brilliant that it
could be seen during full daylight. This comet followed a path which
could not be certainly distinguished from a parabola, though there is no
doubt that it might have been a very elongated ellipse. It is frequently
impossible to decide a question of this kind, during the brief
opportunities available for finding the place of the comet. We can only
see the object during a very small arc o