et is
approximately decided by the sun's attraction, and they can discriminate
the different adjustments which that place is to receive in consequence
of the disturbances produced by the other planets. The capabilities of
the planets for producing disturbance are greatly increased when the
disturbed body follows the eccentric path of a comet. It is frequently
found that the path of such a body comes very near the track of a
planet, so that the comet may actually sweep by the planet itself, even
if the two bodies do not actually run into collision. On such an
occasion the disturbing effect is enormously augmented, and we therefore
turn to the comets when we desire to illustrate the theory of planetary
perturbations by some striking example.

Having decided to choose a comet, the next question is, _What_ comet?
There cannot here be much room for hesitation. Those splendid comets
which appear so capriciously may be at once excluded. They are visitors
apparently coming for the first time, and retreating without any
distinct promise that mankind shall ever see them again. A comet of this
kind moves in a parabolic path, sweeps once around the sun, and thence
retreats into the space whence it came. We cannot study the effect of
perturbations on a comet completely until it has been watched during
successive returns to the sun. Our choice is thus limited to the
comparatively small class of objects known as periodic comets; and, from
a survey of the entire group, we select the most suitable to our
purpose. It is the object generally known as Encke's comet, for, though
Encke was not the discoverer, yet it is to his calculations that the
comet owes its fame. This body is rendered more suitable for our purpose
by the researches to which it has recently given rise.

In the year 1818 a comet was discovered by the painstaking astronomer
Pons at Marseilles. We are not to imagine that this body produced a
splendid spectacle. It was a small telescopic object, not unlike one of
those dim nebulae which are scattered in thousands over the heavens. The
comet is, however, readily distinguished from a nebula by its movement
relatively to the stars, while the nebula remains at rest for centuries.
The position of this comet was ascertained by its discoverer, as well as
by other astronomers. Encke found from the observations that the comet
returned to the sun once in every three years and a few months. This was
a startling announcement. At that t