VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 255 



likely as any to furnish data for determining the nature of the spheroid or figure 

 of the earth. 



I ennbrace this opportunity of mentioning a circumstance, wholly unknown to 

 me at the time my paper was composed. From what has been said, in the 

 foregoing pages, it will probably be inferred, that I considered the pro- 

 posed mode of determining the differences of longitude by the observations of 

 the pole star, made with a very accurate instrument, rather as new, not know- 

 ing that the same idea, or one nearly the same, had before occurred to the Rev. 

 Mr. Michell, and been treated on by him in his very ingenious paper in the 

 Philosophical Transactions, vol. 56, for the year 1766. That I must have 

 read that valuable performance about the time of its publication is not to be 

 doubted ; but in the lapse of so many years, every trace of it had gone from my 

 remembrance, otherwise I would have most certainly referred to it in the proper 

 place, and with the attention that it so well deserves. However, without en- 

 tering here into particulars, it will obviously appear, that the one has not been 

 borrowed from the other. 



XX. Of three Folcanos in the Moon. By Wm. Herschel. LL. D., F. R. S. 



p. 229. 



April 19, 1787, lO'^ 36*" sidereal time, I perceive 3 volcanos in different 

 places of the dark part of the new moon. Two of them are either already 

 nearly extinct, or otherwise in a state of going to break out; which perhaps 

 may be decided next lunation. The 3d shows an actual eruption of fire, or lu- 

 minous matter. I measured the distance of the crater from the northern limb 

 of the moon, and found it 3' 57''-3. Its light is much brighter than the 

 nucleus of the comet which M. Mechain discovered at Paris the 10th of this 

 month. 



April 20, 1787, 10^ 2™ sidereal time, the volcano burns with greater vio- 

 lence than last night. I believe its diameter cannot be less than 3"', by com- 

 paring it with that of the Georgian planet; as Jupiter was near at hand, I 

 turned the telescope to his 3d satellite, and estimated the diameter of the burn- 

 ing part of the volcano to be equal to at least twice that of the satellite. Hence 

 we may compute that the shining or burning matter must be above 3 miles in 

 diameter. It is of an irregular round figure, and very sharply defined on the 

 edges. The other 2 volcanos are much farther towards the centre of the moon, 

 and resemble large, pretty faint nebulae, that are gradually much brighter in 

 the middle; but no well defined luminous spot can be discerned in them. These 

 3 spots are plainly to be distinguished from the rest of the marks on the moon; 

 for the reflection of the sun's rays from the earth is, in its present situation, 

 sufficiently bright, with a ten-feet reflector, to show the moon's spots, even 



