layers, which will be of varying thickness about a
spot, will account for all the shades of darkness seen in the penumbra.
Ascending currents from the solar surface will elevate certain regions,
and may increase the solar activity near by, and will thus give rise to
faculae, which HERSCHEL shows to be elevated above the general surface.
It will not be necessary to give a further account of this theory. The
data in the possession of the modern theorist is a thousand-fold that to
be derived from HERSCHEL'S observations, and, while the subject of the
internal construction of the sun is to-day unsettled, we know that many
important, even fundamental, portions of his theory are untenable.
A remark of his should be recorded, however, as it has played a great
part in such theories:

"That the emission of light must waste the sun, is not a difficulty
that can be opposed to our hypothesis. Many of the operations of
Nature are carried on in her great laboratory which we cannot
comprehend. Perhaps the many telescopic comets may restore to the
sun what is lost by the emission of light."

Arguments in favor of the habitability of both sun and moon are
contained in this paper; but they rest more on a metaphysical than a
scientific basis, and are to-day justly forgotten.

_Researches on the Motion of the Sun and of
the Solar System in Space._

In 1782 HERSCHEL writes, in regard to some of his discoveries of double
stars:

"These may serve another very important end. I will just mention it,
though it is foreign to my present purpose. Several stars of the
first magnitude have been observed or suspected to have a proper
motion; hence we may surmise that our sun, with all its planets and
comets, may also have a motion towards some particular point of the
heavens. . . . If this surmise should have any foundation, it will show
itself in a series of some years in a kind of systematical parallax,
or change, due to the motion of the whole solar system."

In 1783 he published his paper _On the Proper Motion of the Solar
System_, which contained the proofs of his surmises of a year before.
That certain of the stars had in fact a _proper_ motion had been well
established by the astronomers of the eighteenth century. After all
allowances had been made for the effects of precession and other
displacements of a star's position which were produced by motions