Mr J. D. Dana on the Volcanoes of the Moon, 17 



clearly sustain it. They contain cones and circular areas 

 like the better defined pits.* 



The light streaks alluded to form radiating lines around 

 large cones, and especially about Eider, Kepler, Copernicus, 

 and Aristarchus. They are from one to five hundred miles 

 in length, and cross ridges and depressions, without inter- 

 ruption. They coalesce about the summit of Kepler, so that 

 the whole surface appears nebulous. 



The various pit-craters differ in shade of colour, or rather 

 in the degree of light they reflect ; and ten different degrees 

 are distinguished in the work of Beer and Madler. 1 to 3, 

 he says, may be described as grey, 4 to 5 light grey, 6 to 7 

 Avhite, 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 brilliancy 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. The brightest 

 craters are Aristarchus, Werner, and Proclus. Aristarchus 

 is 7629 feet in depth. It has a point of greatest brilliancy, 

 besides two or three separate circular spots remarkably light. 

 Werner has a single brilliant point. Proclus has brilliant 

 walls, yet it is dark at bottom. 



Sir William Herschel published the first account of exist- 

 ing volcanic action in the moon. In a notice of three lunar 

 volcanoes,t he says that two of them, on April 19, 1787, were 

 either nearly extinct or about to break out, while the third 

 was in actual eruption. April 20, he observed that the active 

 one burned with greater violence ; and he estimated that the 

 fiery area was above three miles in diameter. All the adja- 

 cent parts of the crater seemed to be illuminated by the 

 eruption. I The other two volcanoes, he says, resembled 



* The " seas," according to M. Rozet, have escarpments of 45 degrees, some 

 of which are 400 metres in height. In the interior there are annular cavities, 

 perfect rings in shape, the diameter of which attains sometimes to 100,000 

 metres. 



t Phil. Trans, for 1787, p. 229. 



X It is supposed that this crater was that called Aristarchw, or the Mons Por- 

 VOTi. XLIII. NO. LXXXV.— .lULY 1847. B 



