* * * * *
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.
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