existence of very active ascending currents, but
direct experience seems to contradict it.

The following is the way in which a learned man, Sir David Brewster,
appreciates this view of Herschel's: "It is not conceivable that
luminous clouds, ceding to the lightest impulses and in a state of
constant change, can be the source of the sun's devouring flame and of
the dazzling light which it emits; nor can we admit besides, that the
feeble barrier formed by planetary clouds would shelter the objects that
it might cover, from the destructive effects of the superior elements."

Sir D. Brewster imagines that the non-luminous rays of caloric, which
form a constituent part of the solar light, are emitted by the dark
nucleus of the sun; whilst the visible coloured rays proceed from the
luminous matter by which the nucleus is surrounded. "From thence," he
says, "proceeds the reason of light and heat always appearing in a state
of combination: the one emanation cannot be obtained without the other.
With this hypothesis we should explain naturally why it is hottest when
there are most spots, because the heat of the nucleus would then reach
us without having been weakened by the atmosphere that it usually has to
traverse." But it is far from being an ascertained fact, that we
experience increased heat during the apparition of solar spots; the
inverse phenomenon is more probably true.

Herschel occupied himself also with the physical constitution of the
moon. In 1780, he sought to measure the height of our satellite's
mountains. The conclusion that he drew from his observations was, that
few of the lunar mountains exceed 800 metres (or 2600 feet). More recent
selenographic studies differ from this conclusion. There is reason to
observe on this occasion how much the result surmised by Herschel
differs from any tendency to the extraordinary or the gigantic, that
has been so unjustly assigned as the characteristic of the illustrious
astronomer.

At the close of 1787, Herschel presented a memoir to the Royal Society,
the title of which must have made a strong impression on people's
imaginations. The author therein relates that on the 19th of April,
1787, he had observed in the non-illuminated part of the moon, that is,
in the then dark portion, three volcanoes in a state of ignition. Two of
these volcanoes appeared to be on the decline, the other appeared to be
active. Such was then Herschel's conviction of the reality of the
phenom