1852 by the detection of
a common period affecting at once the spots on the sun and the magnetism
of the earth, was extended and accelerated by the discovery of spectrum
analysis. The nature of that change is concisely indicated by the
heading of the present chapter; we would now ask our readers to
endeavour to realise somewhat distinctly what is implied by the
"foundation of astronomical physics."

Just three centuries ago, Kepler drew a forecast of what he called a
"physical astronomy"--a science treating of the efficient causes of
planetary motion, and holding the "key to the inner astronomy."[402]
What Kepler dreamed of and groped after, Newton realized. He showed the
beautiful and symmetrical revolutions of the solar system to be governed
by a uniformly acting cause, and that cause no other than the familiar
force of gravity, which gives stability to all our terrestrial
surroundings. The world under our feet was thus for the first time
brought into physical connection with the worlds peopling space, and a
very tangible relationship was demonstrated as existing between what
used to be called the "corruptible" matter of the earth and the
"incorruptible" matter of the heavens.

This process of unification of the cosmos--this levelling of the
celestial with the sublunary--was carried no farther until the fact
unexpectedly emerged from a vast and complicated mass of observations,
that the magnetism of the earth is subject to subtle influences,
emanating, certainly from some, and presumably from all of the heavenly
bodies; the inference being thus rendered at least plausible, that a
force not less universal than gravity itself, but with whose modes of
action we are as yet unacquainted, pervades the universe, and forms, it
might be said, an intangible bond of sympathy between its parts. Now for
the investigation of this influence two roads are open. It may be
pursued by observation either of the bodies from which it proceeds, or
of the effects which it produces--that is to say, either by the
astronomer or by the physicist, or, better still, by both concurrently.
Their acquisitions are mutually profitable; nor can either be considered
as independent of the other. Any important accession to knowledge
respecting the sun, for example, may be expected to cast a reflected
light on the still obscure subject of terrestrial magnetism; while
discoveries in magnetism or its _alter ego_ electricity must profoundly
affect solar inqu