236 PHILOSOPHICAL TRANSACTIONS, [aNNO 17 g2. 



4. We compare, according to the above-mentioned observations, the alter- 

 nate relative appearances of the cusps of Venus reciprocally with each other. On 

 the Qth and 12th of March, 1790, when the southern cusp extended, not in 

 the true spherical curve of the limb of the planet, but in a somewhat hooked 

 direction, into the dark, hemisphere, the pale bluish ash-coloured streak appeared 

 only at the point of the northern cusp, whence it proceeded, in a true spherical 

 curve, along thedark limb of the planet. On the 10th of March, on the other 

 hand, when the southern cusp did not penetrate so far into the dark hemisphere, 

 the pale streak was perceived at both points, though somewhat more sensibly at 

 the northern than at the southern; and such also were the appearances after the 

 inferior conjunction. 



On the atmosphere of the moon. — Referring to my Selenotopographic Fragments 

 for the proofs I there adduced of the real existence of a lunar atmosphere, which 

 had been so frequently doubted; I shall also appeal to the same work for the ob- 

 servations I formerly made on several of its relative properties, compared with the 

 same in our atmosphere, such as its greater dryness, rarity, and clearness, which 

 however do not prevent its refracting the solar rays, having pointed out the cir- 

 cumstance, that the mountains in the dark hemisphere of the moon, near its 

 luminous border, which are of sufficient height to receive the light of the sun, 

 are the more feebly illuminated the more distant they are from that border: from 

 which proofs of a refracting atmosphere, I also deduced the probability of the 

 existence of a faint twilight, which however my long series of observations had 

 not yet fully evinced. 



As one fortunate discovery often leads to another, I had no sooner succeeded 

 in my observations on the twilight of Venus, than I directed my attention to 

 that of the moon, and applied the calculations and inferences I there made, to 

 some appearances I had already noticed on this satellite. It occurred, that if in 

 fact there were a twilight on the moon, as there is on Venus and our earth, it 

 could not, considering the greater rarity of its atmosphere, be so considerable: 

 and that the vestiges of it, allowing for the brightness of the luminous part of 

 the moon, the strong light that is thence thrown on the field of the telescope, 

 and in some measure the reflected light of our earth, could only be traced on 

 the limb, particularly at the cusps; and even this only at the time when our own 

 twilight is not strong, but the air very clear, and when the moon, in one of its 

 least phases, is in a high altitude, either in the spring, following the sun 2 days 

 after a new moon, or in the autumn, preceding the sun in the morning, with the 

 same aspect: in short, that the projection of this twilight will be the greater and 

 more perceptible the more falcated the phase, and the higher the moon above the 

 horizon, and out of our own twilight. This struck me the more, as I recol- 



