er, in perfect accordance with
the best results derived from the attraction of Jupiter on his
satellites and the other planets.

We have hitherto discussed the adventures of Encke's comet in cases
where they throw light on questions otherwise more or less known to us.
We now approach a celebrated problem, on which Encke's comet is our only
authority. Every 1,210 days that comet revolves completely around its
orbit, and returns again to the neighbourhood of the sun. The movements
of the comet are, however, somewhat irregular. We have already explained
how perturbations arise from Mercury and from Jupiter. Further
disturbances arise from the attraction of the earth and of the other
remaining planets; but all these can be allowed for, and then we are
entitled to expect, if the law of gravitation be universally true, that
the comet shall obey the calculations of mathematics. Encke's comet has
not justified this anticipation; at each revolution the period is
getting steadily shorter! Each time the comet comes back to perihelion
in two and a half hours less than on the former occasion. Two and a half
hours is, no doubt, a small period in comparison with that of an entire
revolution; but in the region of its path visible to us the comet is
moving so quickly that its motion in two and a half hours is
considerable. This irregularity cannot be overlooked, inasmuch as it has
been confirmed by the returns during about twenty revolutions. It has
sometimes been thought that the discrepancies might be attributed to
some planetary perturbations omitted or not fully accounted for. The
masterly analysis of Von Asten and Backlund has, however, disposed of
this explanation. They have minutely studied the observations down to
1891, but only to confirm the reality of this diminution in the periodic
time of Encke's comet.

An explanation of these irregularities was suggested by Encke long ago.
Let us briefly attempt to describe this memorable hypothesis. When we
say that a body will move in an elliptic path around the sun in virtue
of gravitation, it is always assumed that the body has a free course
through space. It is assumed that there is no friction, no air, or other
source of disturbance. But suppose that this assumption should be
incorrect; suppose that there really is some medium pervading space
which offers resistance to the comet in the same way as the air impedes
the flight of a rifle bullet, what effect ought such a medium to
pro