the extension of our knowledge of the nature of the heavenly bodies by
the employment of spectroscopy. It has not only thrown wonderful
light upon the physical and chemical constitution of the sun, fixed
stars, and nebulæ, and comets, but it holds out a prospect of
obtaining definite evidence as to the nature of our so-called
elementary bodies.

[Sidenote: its relation to geology.]

The application of the generalisations of thermotics to the problem of
the duration of the earth, and of deductions from tidal phenomena to
the determination of the length of the day and of the time of
revolution of the moon, in past epochs of the history of the universe;
and the demonstration of the competency of the great secular changes,
known under the general name of the precession of the equinoxes, to
cause corresponding modifications in the climate of the two
hemispheres of our globe, have brought astronomy into intimate
relation with geology. Geology, in fact, proves that, in the course of
the past history of the earth, the climatic conditions of the same
region have been widely different, and seeks the explanation of this
important truth from the sister sciences. The facts that, in the
middle of the Tertiary epoch, evergreen trees abounded within the
arctic circle; and that, in the long subsequent Quaternary epoch, an
arctic climate, with its accompaniment of gigantic glaciers, obtained
in the northern hemisphere, as far south as Switzerland and Central
France, are as well established as any truths of science. But, whether
the explanation of these extreme variations in the mean temperature of
a great part of the northern hemisphere is to be sought in the
concomitant changes in the distribution of land and water surfaces of
which geology affords evidence, or in astronomical conditions, such as