profound effect on the system.

It is of the utmost interest to investigate the extent to which one
planet can affect another in virtue of their mutual attractions. Such
investigations demand the exercise of the highest mathematical
gifts. But not alone is intellectual ability necessary for success
in such inquiries. It must be united with a patient capacity for
calculations of an arduous type, protracted, as they frequently have
to be, through many years of labour. Le Verrier soon found in these
profound inquiries adequate scope for the exercise of his peculiar
gifts. His first important astronomical publication contained an
investigation of the changes which the orbits of several of the
planets, including the earth, have undergone in times past, and which
they will undergo in times to come.

As an illustration of these researches, we may take the case of the
planet in which we are, of course, especially interested, namely, the
earth, and we can investigate the changes which, in the lapse of
time, the earth's orbit has undergone, in consequence of the
disturbance to which it has been subjected by the other planets. In
a century, or even in a thousand years, there is but little
recognisable difference in the shape of the track pursued by the
earth. Vast periods of time are required for the development of the
large consequences of planetary perturbation. Le Verrier has,
however, given us the particulars of what the earth's journey through
space has been at intervals of 20,000 years back from the present
date. His furthest calculation throws our glance back to the state
of the earth's track 100,000 years ago, while, with a bound forward,
he shows us what the earth's orbit is to be in the future, at
successive intervals of 20,000 years, till a date is reached which is