234 PHILOSOPHICAL TRANSACTIONS. [aNNO 1792. 



with regard to Venus and the moon, not be very feasible; but this, I think, I 

 may with great confidence infer from my long series of observations, that Venus 

 has an atmosphere in some respects similar to that of our earth, and which must 

 far exceed that of the moon in its density, or power to weaken the rays of 

 the sun. 



Thus far had I proceeded in my observations, when the heavens favoured me 

 with the following ones, which may prove the more interesting, as they not only 

 confirm those hitherto made, but also lead to some further inferences concerning 

 the atmosphere of Venus. Among all the favourable circumstances for observing 

 the planet Venus, none could be more so than those I had in the months of 

 March and April, but especially from the Qth to the l6th of March, 1790, when, 

 besides the almost constant and unusual serenity of the sky, the planet, which 

 was then in Aries, at 7° and 8° n. declination, was so high above the horizon, 

 that, notwithstanding its approaching inferior conjunction on the 18th of March, 

 at 4 p. M. I could still view it on the 1 6th, and should certainly have observed it 

 during the conjunction, had not the weather become hazy on the 17th. 



Under these very fortunate circumstances, I fell accidentally (not having, after 

 10 years of constant attention to Venus been able to devise any new mode of ob- 

 serving) on the 9th, 10th, 11th, and 12th of March, on an observation, which 

 I repeatedly confirmed, and which, on account of its singularity, and the light 

 it will probably throw on the physical constitution of this planet, will certainly 

 be ever thought important; especially as it may not in many years be repeated 

 under so favourable a combination of incidents. 



On the 9th of March, 1790, immediately after sun-set, and till S*" 45"", I saw 

 Venus with a 7-feet reflector, magnifying 75, and 161 times, very distinctly, 

 and uncommonly splendid. The southern cusp did not appear precisely of its 

 usual circular form, but rather inflected in the shape of a hook beyond the lu- 

 minous semicircle into the dark hemisphere of the planet. This however, after 

 my former observations, was not new to me: but a far more striking phenome- 

 non, which I had never seen before, excited my admiration, and particular at- 

 tention. The northern cusp was terminated in the same narrow tapering manner 

 as the southern, but did not extend in its bright luminous state into the dark 

 hemisphere. From its point however, the light of which, though gradually 

 fading, was yet of sufficient brightness, a streak of glimmering bluish light pro- 

 ceeded into the dark hemisphere, which, though intermittent as to intensity, was 

 yet permanent as to duration, and though very faint, could yet be plainly seen 

 with both the above-mentioned magnifjing powers. Like the luminous line 

 then seen on Saturn, its light seemed to twinkle in various detaciied points, and 

 appeared throughout not only very faint, when compared with the light at the 

 point of the cusp, but also of a very peculiar kind of faintness, verging towards 



