332 PHILOSOPHICAL TRANSACTIONS. [anNO i7Q3. 



tleman who has seen tlie mountains in Venus, has made observations on flat 

 spherical forms on Saturn, it is to be regretted that he has not attended to the 

 revolution of this planet on its axis, which could not remain an hour unknown 

 to him when he saw these forms. Last night, May 31, 1793, for instance, I 

 saw 2 small dark spots on Jupiter ; I shall not call them flat spherical forms, 

 because their flatness, as well as their sphericity, must be hypothetical ; indeed 

 these 2 terms seem to me to contradict each other. These were evidently re- 

 moved, in less than an hour, in such a manner as to point out, very nearly, the 

 direction and quantity of the rotation of this planet. 



Before I remark on the rest of the extraordinary relations above-mentioned 

 I will give a short extract of my observations on Venus, with such deductions as 

 it seems to me that we are authorized to make from them. Thus, 



April 17, i777f the disc of Venus was exceedingly well defined, distinct, and 

 bright, but no spot was visible by which I could judge of her diurnal motion. 

 The same telescope shows the spots on Mars extremely well. 7-feet reflector. 



April l6, 1777. Tiie disc well defined, and bright, but no spot. 10-feet 

 reflector. 



In this manner Dr. H. sets down a number of similar observations ; whence 

 he infers, that Venus has a motion on an axis ; and that she has an atmosphere 

 he considers evident, from the changes he took notice of, which could not be on 

 the solid body of the planet. 



Then follow many other observations on the same, with some on the diameter 

 of Venus. After all. Dr. H. adds, a few very evident results may be drawn from 

 the foregoing observations. 



With regard to the rotation of Venus on an axis, it appears that we may be 

 assured of this planet's having a diurnal motion, and though the real time of it 

 is still subject to considerable doubts, it can hardly be so slow as 24 days. Its 

 direction, or rather the position of the axis of Venus, is involved in still greater 

 uncertainty. 



The atmosphere of Venus is probably very considerable ; which appears not 

 only from the changes that have been observed in the faint spots on its surface, 

 but may also be inferred from the illumination of the cusps, when this planet is 

 near its inferior conjunction ; where the enlightened ends of the horns reach far 

 beyond a semicircle. I must here take notice, that the author we have before 

 quoted on tiiis subject, has the merit of being the first who has pointed out this 

 inference, but he has overlooked the penumbra arising from the diameter of the 

 sun ; which has certainly a considerable share in the eft'ect of the extended illu- 

 mination, and in his angle of 15" IQ' will amount to more than 1° 1 l' -17".6_ 

 His measures are also defective ; as probably the mirror of his 7-feet reflector, 

 which was a very excellent one, was by that time considerably tarnished, and 



