rtain changes upon its velocity. As the comet recedes, the disturbing
influence of Mercury rapidly abates, and ere long becomes insensible.
But time cannot efface from the orbit of the comet the effect which the
disturbance of Mercury has actually accomplished. The disturbed orbit is
different from the undisturbed ellipse which the comet would have
occupied had the influence of the sun alone determined its shape. We are
able to calculate the movements of the comet as determined by the sun.
We can also calculate the effects arising from the disturbance produced
by Mercury, provided we know the mass of the latter.

Though Mercury is one of the smallest of the planets, it is perhaps the
most troublesome to the astronomer. It lies so close to the sun that it
is seen but seldom in comparison with the other great planets. Its orbit
is very eccentric, and it experiences disturbances by the attraction of
other bodies in a way not yet fully understood. A special difficulty has
also been found in the attempt to place Mercury in the weighing scales.
We can weigh the whole earth, we can weigh the sun, the moon, and even
Jupiter and other planets, but Mercury presents difficulties of a
peculiar character. Le Verrier, however, succeeded in devising a method
of weighing it. He demonstrated that our earth is attracted by this
planet, and he showed how the amount of attraction may be disclosed by
observations of the sun, so that, from an examination of the
observations, he made an approximate determination of the mass of
Mercury. Le Verrier's result indicated that the weight of the planet was
about the fourteenth part of the weight of the earth. In other words, if
our earth was placed in a balance, and fourteen globes, each equal to
Mercury, were laid in the other, the scales would hang evenly. It was
necessary that this result should be received with great caution. It
depended upon a delicate interpretation of somewhat precarious
measurements. It could only be regarded as of provisional value, to be
discarded when a better one should be obtained.

The approach of Encke's comet to Mercury, and the elaborate
investigations of Von Asten and Backlund, in which the observations of
the body were discussed, have thrown much light on the subject; but,
owing to a peculiarity in the motion of this comet, which we shall
presently mention, the difficulties of this investigation are enormous.
Backlund's latest result is, that the sun is 9,700,000 t