VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 241 



and computations made concerning the twilights of different planets. There is 

 no doubt but that the faintest twilight of Venus, as seen either before or after 

 the rising and setting of the sun across our twilight, is much brighter than that 

 of the moon ; and it appears also from computation, that the denser part of the 

 atmosphere of Venus measures at least 15000 Paris feet in height, and spreads 

 its twilight 67 geogr. miles into the dark hemisphere, while the denser part of the 

 lunar atmosphere, whose height does not exceed 1356 feet, produces a faint 

 twilight not above 10^ geogr. miles in breadth. Thus, as my successful obser- 

 vations on the twilight of Venus led me to the discovery of that of the moon, so 

 did these latter reciprocally confirm the former : and thus, which ever way we 

 contemplate the subject, must we be struck with the coincidence that prevails 

 throughout. 



4. But if the lunar atmosphere be comparatively so rare, it follows that the 

 inflection of light produced by it cannot be very considerable ; and hence does 

 the computation of M. du Sejour, according to which, the inflection of the 

 solar rays which touch the moon, amounts to no more than 4^4-, receive an 

 additional degree of authenticity.* Besides which, 



5. As the true extent of the brightest lunar twilight amounts to 2° 34', the 

 obliquity of the ecliptic in the moon only to 1° 29' ; the inclination of the orbit 

 of the moon, on the contrary, to 5° 15', and its synodic period, during which 

 it performs a revolution round its axis is = 2Q'' 12*'; it follows, that its brightest 

 twilight, to where it loses itself in the light reflected by the almost fully illumi- 

 nated disc of our earth, must, at least at its nodes, last 5^ 3™, and that it will 

 be still longer at other parts of the orbit, according to the situation of the nodes. 



6. And lastly, it being a well-known fact,-|- that the fixed stars, as they ap- 

 proach the moon, diminish in splendour at the most only a very few seconds 

 before their occultations, it was natural for me, after the successful observations 

 I had made on the twilight of the moon, to pay particular attention to this 

 circumstance. On the 25th February, at 6^ p. m. the sky being very clear, the 

 limb of the dark part of the moon appeared uncommonly distinct ; and only a 

 few seconds of a degree from its edge was seen a telescopic star, of about the 

 10th or 12th magnitude. I counted full 20' before its occultation, and 18-i of 

 these, without the least perceptible diminution of light. The star however 

 began now gradually to fade, and after the remaining l"i, during which 1 ob- 

 served it with all possible attention, it vanished in an instant. This observation 

 agrees perfectly with the above computations. Though it be proved that the 

 inferior dense part of the lunar atmosphere reflects a stronger light than that which 

 the dark hemisphere receives from an almost fully illuminated disc of our earth ; 

 and though, considering the inferiority of gravitation on the surface of the 

 moon, there be no doubt that this dense part, together with the superior gra- 



• See De Lalande's Astron. §. 1992— 1994. t SeeSelenot Fragm. |. 531, wiUi its note.— Orig. 

 VOL. XVII. I I 



