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!
The planets, needless to say, were the objects of Herschel's assiduous
attention. Mercury was the one which least interested him; but he
ascertained the perfect circularity of its disc.
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