VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 239 



cusp, it appeared very faint and dwindling, equalled however that of our Pic of 

 TenerifFe. Nor can it be conceived why this glimmering light broke off so 

 suddenly at both the cusps, without a progressive diminution. It can hardly be 

 supposed that similar grey, prominent, flat areas, of the same form and dimen- 

 sions, and comparatively of a faint light, which, while in the dark, hemisphere, 

 they derive immediately from the sun, exist on all parts of the moon ; more 

 especially as at the places observed, the limb happened to exhibit throughout 

 an exact spherical form, without the least sensible inequality ; and as in both the 

 bordering regions of the northern and southern hemispheres, especially in the 

 latter, no such grey, prominent planes are any where discernible. It may then 

 be asked, why did this faint glimmering light appear at both cusps, along equal 

 arcs of the limb, of equal length and breadth, and of the same pyramidal form ? 

 and why did its farther extremity blend itself with the terrestrial light of the 

 dark hemisphere, which, according to a great number of my selenotopographic 

 observations, is by no means the case, even with those grey prominent areas 

 which, being at some distance on the dark side of the terminating border, are yet 

 illuminated immediately by the sun ? 



These therefore could certainly not derive their light immediately from the 

 sun; whence this appearance, like the similar ones on the planet Venus, can 

 only be ascribed to the solar rays reflected by the atmosphere of the moon on 

 those planes, producing on them a very faint, gradually diminishing, glimmer- 

 ing light, which at last loses itself in the reflected terrestrial light, in the same 

 manner as our twilight blends itself with the light of the moon. Every circum- 

 stance of the above observation seems to confirm this supposition ; and hence 

 the observation itself, which, though single, was however a most fortunate and 

 complete one, must appear of no small degree of importance, since it not only 

 confirms the observations and inferences on the long contested lunar atmosphere 

 contained in my Selenotop. Fragm., but also furnishes us with many more lights 

 concerning the atmosphere of planets in general, than had been afforded us by 

 all those observations collectively. This, and the mathematical certainty that 

 the phenomenon is, in fact, nothing but a real twilight in the lunar atmosphere, 

 Mr. S. further evinces by theoretical deductions, derived from long mathema- 

 tical calculations. 



From which calculations it appears, that the lower and more dense part of 

 the lunar atmosphere, that part, namely, which has the power of reflecting this 

 bright crepuscular light, is only 1356 Paris feet in height; and hence it will 

 easily be explained how, according to the different librations of the moon, 

 ridges of mountains even of a moderate height, situated at or near the termi- 

 nating border, may partially interrupt, or at times wholly prevent this crepus- 

 cular light, either at one or the other cusp, and sometimes at both. I cannot 



