September 7th, 1820. 29 



of altitude was to be observed, some appearing considerably 

 the most elevated ground in the planet, while others scarcely 

 extended beyond its dark sphericity. Throughout the whole 

 extent of the circumference, and during every period of the 

 eclipse, these appearances were nearly uniform ; the mountain- 

 ous inequalities exhibiting less diversity both in figure and 

 position, than a priori would have been inferred. 



This irregularity of surface, as it constitutes one of the most 

 striking analogies in the conformation of the earth and its 

 attendant planet, so it has naturally been the subject of consi- 

 derable discussion, and the existence of lunar mountains is not 

 less generally admitted than their magnitudes have been 

 variously estimated. While they were thus conspicuous there- 

 fore, it seemed probable that an attempt to ascertain their 

 elevation, might be made with success. A different method of 

 investigation, however, from any of those hitherto employed, 

 was obviously necessary, and their progress over the margin of 

 the sun's western limb at length suggested, that from the rela- 

 tion between the motion of a planet in its orbit, and the space 

 passed over in a given time, the requisite data might be derived. 

 Since the earth and moon revolve about the same centre, and 

 with a common velocity, both, as regards this motion, in the 

 present instance, may be considered as at rest. The former thus 

 becomes a fixed station, whence the observer may view the 

 progress of the latter over the surface of the sun, as she is carried 

 along by the horary motion in her own orbit. Hence it appeared, 

 that if during the eclipse any remarkable point in the body 

 of the sun were assumed, and the number of seconds accu- 

 rately determined, which elapsed from the apex of a mountain 

 on the moon's circumference coming in contact with this point, 

 to the arrival of the base at the same point; the orbicular velocity 

 of the moon, as compared with the observed time, would give 

 the distance described, that is, the perpendicular height of the 

 mountain. It is plain, that this method could be proceeded on 

 in the case of those mountains only, the whole height of which 

 extended beyond the moon's circumference ; and that two 

 points in the solar body might be employed for this purpose, 



