quite independent of each other. An elastic fluid, now known as the
_photosphere_, is in course of continual formation on the dark rugged
surface of the solar mass; and rising, on account of its specific
lightness, it forms the _pores_ in the stratum of reflecting clouds;
then, combining with other gases, it produces the irregularities or
furrows in the luminous cloud-region. When the ascending currents are
powerful, they create those appearances which astronomers designate the
_nuclei_, the _penumbrae_, the _faculae_.

Such was Herschel's explanation of the mode of formation of the solar
spots; and allowing it to be well-founded, we must expect to find--what
is, indeed, the case--that the Sun does not always and regularly pour
forth equal quantities of light and heat. It is true that Herschel's
hypothesis has been modified by later astronomers; but his is the credit
of having directed them into the right course of inquiry and
observation.

* * * * *

The physical constitution of the Moon was a subject which also engaged
the attention of our indefatigable enthusiast. As early as 1780 he
attempted the measurement of the lunar mountains, and came to the
conclusion that few of them exceeded 2600 feet in height. Later
research, however, has proved these figures to be inadequate. Next he
addressed himself to a study of the lunar volcanoes, three of which he
declared to be in a state of ignition; two of them apparently on the
decline, the third still active. He was so convinced of the reality of
the phenomenon, that on the 20th of April 1787 he wrote:--"The volcano
burns with greater violence than it did last night." The real diameter
of the volcanic light he estimated at 16,400 feet. Its intensity he