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certain observations of Mr. Schroeter, of Lilienthal. He observed the
moon when two days and a half old, in the evening soon after sunset,
before the dark part was visible, and continued to watch it until it
became visible. The two cusps appeared tapering in a very sharp faint
prolongation, each exhibiting its farthest extremity faintly illuminated
by the solar rays, before any part of the dark hemisphere was
visible. Soon afterward, the whole dark limb became illuminated. This
prolongation of the cusps beyond the semicircle, I thought, must have
arisen from the refraction of the sun's rays by the moon's atmosphere. I
computed, also, the height of the atmosphere (which could refract light
enough into its dark hemisphere to produce a twilight more luminous than
the light reflected from the earth when the moon is about 32 degrees
from the new) to be 1,356 Paris feet; in this view, I supposed the
greatest height capable of refracting the solar ray, to be 5,376 feet.
My ideas on this topic had also received confirmation by a passage in
the eighty-second volume of the Philosophical Transactions, in which
it is stated that at an occultation of Jupiter's satellites, the third
disappeared after having been about 1" or 2" of time indistinct, and the
fourth became indiscernible near the limb.(*4)
"Cassini frequently observed Saturn, Jupiter, and the fixed stars,
when approaching the moon to occultation, to have their circular figure
changed into an oval one; and, in other occultations, he found no
alteration of figure at all. Hence it might be supposed, that at some
times and not at others, there is a dense matter encompassing the moon
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