ndaries. The unification of the physical
sciences is perhaps the greatest intellectual feat of recent times. The
process has included astronomy; so that, like Bacon, she may now be said
to have "taken all knowledge" (of that kind) "for her province." In
return, she proffers potent aid for its increase. Every comet that
approaches the sun is the scene of experiments in the electrical
illumination of rarefied matter, performed on a huge scale for our
benefit. The sun, stars, and nebulae form so many celestial laboratories,
where the nature and mutual relations of the chemical "elements" may be
tried by more stringent tests than sublunary conditions afford. The laws
of terrestrial magnetism can be completely investigated only with the
aid of a concurrent study of the face of the sun. The solar spectrum
will perhaps one day, by its recurrent modifications, tell us something
of impending droughts, famines, and cyclones.

Astronomy generalises the results of the other sciences. She exhibits
the laws of Nature working over a wider area, and under more varied
conditions, than ordinary experience presents. Ordinary experience, on
the other hand, has become indispensable to her progress. She takes in
at one view the indefinitely great and the indefinitely little. The
mutual revolutions of the stellar multitude during tracts of time which
seem to lengthen out to eternity as the mind attempts to traverse them,
she does not admit to be beyond her ken; nor is she indifferent to the
constitution of the minutest atom of matter that thrills the ether into
light. How she entered upon this vastly expanded inheritance, and how,
so far, she has dealt with it, is attempted to be set forth in the
ensuing chapters.

FOOTNOTES:

[Footnote 1: The denomination "physical astronomy," first used by
Kepler, and long appropriated to this branch of the science, has of late
been otherwise applied.]

[Footnote 2: _Histoire de l'Astronomie au xviii^e Siecle_, p. 267.]

PART I

PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE NINETEENTH CENTURY

CHAPTER I

_FOUNDATION OF SIDEREAL ASTRONOMY_

Until nearly a hundred years ago the stars were regarded by practical
astronomers mainly as a number of convenient fixed points by which the
motions of the various members of the solar system could be determined
and compared. Their recognised function, in fact, w