ae, 
J. D. Dana on the Volcanoes of the Moon. 341 
The various pit craters differ in shade of color, or rather 
in the degree of light they reflect; and ten different degrees 
are distinguished in the work of Beer and Midler. 1 to 3, he 
says, may be described as gray, 4 to 5 light gray, 6 to 7 
white, 8 to 10 shining white. The so-called seas, though but 
slightly depressed, are sometimes very much lighter than the 
surrounding surface. In some instances, as these authors state, 
two pits, side by side, alike in size and features, so differ in bril- 
lianey that one» is wholly obscured in the full moon, while the 
other still shines: the two are seen together again as soon as 
their shadows reappear. he brightest craters are Aristarchus, 
Werner and Proclus. Aristarchus is 7629 feet in depth. It has 
@ point of greatest brilliancy, besides two or three separate cir- 
cular spots remarkably light. Werner has a single brilliant point. 
Proclus has brilliant walls, yet is dark at bottom. 
_ Sir Wm. Herschel published the first account of existing volean- 
ic action in the moon. In a notice of three lunar volcanoes,* he 
says that two of them, on April 19, 1787, were either nearly ex- 
tnet or about to break out, while the third was in actual. erup- 
tion. April 20, he observed that the active one burned with 
greater violence ; and he estimated that the fiery area was above 
miles in diameter. All the adjacent parts of the crater seem- 
ed to be illuminated by the eruption.t The other two volcanoes, 
he says, resembled large pretty faint nebule, that are gradually 
much brighter at the middle, but no well defined luminous spot 
could be distinguished. Herschel alludesalso to an eruption seen 
by him previously, in 1783. . 
_ Such are the general facts, which call for explanation, to wit: 
the existence of circular pit craters, 5 to 150 miles in diameter, and 
five to twenty-four thousand feet in depth ;—the great number of 
these pit craters, and their peculiar features ;—the depressions of a 
Similar character of still larger area;—and the various degrees of il- 
»* Phil. Trans. for 1787, p. 229. nso 
tltis supposed that this crater was that called Aristarchus, or the Mons Porphy- 
rites of Hevelius. Aristarchus is described as apparently in action in 1821, by H. 
Kater, in the Philosophical Transactions for 1821, p..1303 also by Rev. M. Ward, 
it nearly the same time, in the M 
_ 1573, also the following year by Rev. Fearon Fallows, in the Philosophical Trans- 
actions for ] 237. Dr. Olbers observed Aristarchus ne ' 
Kater BF gd eile the light to the reflection of the earth’s light by its: 
