new physical-celestial science is the
leading fact in recent astronomical history. It was out of the regular
course of events. In the degree in which it has actually occurred it
could certainly not have been foreseen. It was a seizing of the prize by
a competitor who had hardly been thought qualified to enter the lists.
Orthodox astronomers of the old school looked with a certain contempt
upon observers who spent their nights in scrutinising the faces of the
moon and planets rather than in timing their transits, or devoted
daylight energies, not to reductions and computations, but to counting
and measuring spots on the sun. They were regarded as irregular
practitioners, to be tolerated perhaps, but certainly not encouraged.

The advance of astronomy in the eighteenth century ran in general an
even and logical course. The age succeeding Newton's had for its special
task to demonstrate the universal validity, and trace the complex
results, of the law of gravitation. The accomplishment of that task
occupied just one hundred years. It was virtually brought to a close
when Laplace explained to the French Academy, November 19, 1787, the
cause of the moon's accelerated motion. As a mere machine, the solar
system, so far as it was then known, was found to be complete and
intelligible in all its parts; and in the _Mecanique Celeste_ its
mechanical perfections were displayed under a form of majestic unity
which fitly commemorated the successive triumphs of analytical genius
over problems amongst the most arduous ever dealt with by the mind of
man.

Theory, however, demands a practical test. All its data are derived from
observation; and their insecurity becomes less tolerable as it advances
nearer to perfection. Observation, on the other hand, is the pitiless
critic of theory; it detects weak points, and provokes reforms which may
be the beginnings of discovery. Thus, theory and observation mutually
act and react, each alternately taking the lead in the endless race of
improvement.

Now, while in France Lagrange and Laplace were bringing the
gravitational theory of the solar system to completion, work of a very
different kind, yet not less indispensable to the future welfare of
astronomy, was being done in England. The Royal Observatory at Greenwich
is one of the few useful institutions which date their origin from the
reign of Charles II. The leading position which it still occupies in the
science of celestial observation was