nt has to do in each case with
the nineteenth century. We shall see at once that this is a time both
of rapid progress and of great differentiation. We have heard almost
nothing hitherto of such sciences as paleontology, geology, and
meteorology, each of which now demands full attention. Meantime,
astronomy and what the workers of the elder day called natural
philosophy become wonderfully diversified and present numerous
phases that would have been startling enough to the star-gazers and
philosophers of the earlier epoch.

Thus, for example, in the field of astronomy, Herschel is able, thanks
to his perfected telescope, to discover a new planet and then to reach
out into the depths of space and gain such knowledge of stars and
nebulae as hitherto no one had more than dreamed of. Then, in rapid
sequence, a whole coterie of hitherto unsuspected minor planets is
discovered, stellar distances are measured, some members of the starry
galaxy are timed in their flight, the direction of movement of the solar
system itself is investigated, the spectroscope reveals the chemical
composition even of suns that are unthinkably distant, and a tangible
theory is grasped of the universal cycle which includes the birth and
death of worlds.

Similarly the new studies of the earth's surface reveal secrets of
planetary formation hitherto quite inscrutable. It becomes known that
the strata of the earth's surface have been forming throughout untold
ages, and that successive populations differing utterly from one another
have peopled the earth in different geological epochs. The entire point
of view of thoughtful men becomes changed in contemplating the history
of the world in which we live--albeit the newest thought harks back to
some extent to those days when the inspired thinkers of early Greece
dreamed out the wonderful theories with which our earlier chapters have
made our readers familiar.

In the region of natural philosophy progress is no less pronounced and
no less striking. It suffices here, however, by way of anticipation,
simply to name the greatest generalization of the century in physical
science--the doctrine of the conservation of energy.

I. THE SUCCESSORS OF NEWTON IN ASTRONOMY

HEVELIUS AND HALLEY

STRANGELY enough, the decade immediately following Newton was one of
comparative barrenness in scientific progress, the early years of the
eighteenth century not being as productive of great astronomers as the
late