enon, that the next morning he wrote thus of his first
observation: "The volcano burns with more violence than last night." The
real diameter of the volcanic light was 5000 metres (16,400 English
feet). Its intensity appeared very superior to that of the nucleus of a
comet then in apparition. The observer added: "The objects situated near
the crater are feebly illuminated by the light that emanates from it."
Herschel concludes thus: "In short, this eruption very much resembles
the one I witnessed on the 4th of May, 1783."

How happens it, after such exact observations, that few astronomers now
admit the existence of active volcanoes in the moon? I will explain this
singularity in a few words.

The various parts of our satellite are not all equally reflecting. Here,
it may depend on the form, elsewhere, on the nature of the materials.
Those persons who have examined the moon with telescopes, know how very
considerable the difference arising from these two causes may be, how
much brighter one point of the moon sometimes is than those around it.
Now, it is quite evident that the relations of intensity between the
faint parts and the brilliant parts must continue to exist, whatever be
the origin of the illuminating light. In the portion of the lunar globe
that is illuminated by the sun, there are, everybody knows, some points,
the brightness of which is extraordinary compared to those around them;
those same points, when they are seen in that portion of the moon that
is only lighted by the earth, or in the ash-coloured part, will still
predominate over the neighbouring regions by their comparative
intensity. Thus we may explain the observations of the Slough
astronomer, without recurring to volcanoes. Whilst the great observer
was studying in the non-illuminated portion of the moon, the supposed
volcano of the 20th of April, 1787, his nine-foot telescope showed him
in truth, by the aid of the secondary rays proceeding from the earth,
even the darkest spots.

Herschel did not recur to the discussion of the supposed actually
burning lunar volcanoes, until 1791. In the volume of the _Philosophical
Transactions_ for 1792, he relates that, in directing a twenty-foot
telescope, magnifying 360 times, to the entirely eclipsed moon on the
22d of October, 1790, there were visible, over the whole face of the
satellite, about a hundred and fifty very luminous red points. The
author declares that he will observe the greatest reserve r