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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 described as superior to that of the nucleus of a comet then flashing across our system.  The objects situated near the crater were fully illuminated by the glare of its burning matter.

It may seem strange that, after observations so exact and minute, few astronomers now admit the existence of active volcanoes in the Moon.  The reasons for their incredulity are thus stated:—­

The various parts of the Moon do not all reflect with the same intensity.  Here, that intensity may be dependent on the form; elsewhere, on the nature of the materials.  Those persons who have examined the lunar orb with telescopes, know how very considerable the difference arising from these two causes may be,—­with how much keener and stronger a radiance one point of the Moon will sometimes shine than those around it.  Well, it would seem to be obvious that the ratio of intensity between the brilliant parts and the faint parts must always be the same, whatever the origin of the illuminating light.  In that portion of the lunar sphere which receives the glow and glory of the sun, we know that some points exist, the brightness of which is extraordinary compared with the feeble flickering gleam of those around them.  And these same points, when seen in the dim reflection of the Earth, will still predominate in intensity over the neighbouring regions.  In this way Arago and others explain the observations of Herschel, without admitting the existence of active volcanoes in the Moon.  That volcanoes there are, is a familiar fact; but they would seem to have exhausted their activity in long-past ages.  The lunar surface is now a dreary waste of rugged lava and ashes, covered with the matter ejected from craters once in a state of furious eruption.  The Moon, in fact, is a world which has burned itself out.  How strange the thought that in a far-back period the inhabitants of Earth, had Earth then been inhabited, might have seen the glare of countless volcanoes diffused, lurid and threatening, over the face of their satellite!  How strange the thought that the once active fires should all have died away, and the Moon have thus been prepared for the better reception and reflection of the solar radiance in order to illuminate the nights of Earth!