510 PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



preceding. Notwithstanding Venus was got near the horizon, and had some 

 tremulous motion from the fine vapours, the sky being otherwise clear, yet her 

 image was free from false light, and sufficiently distinct, with power l6o of the 

 7-feet Schr. a reflector which hardly ever fails me. I was quite surprized to 

 perceive most evidently, at the first sight, that the above-mentioned remarkable 

 phase had changed as remarkably within 2 hours 1 5 minutes ; and that, even 

 while the instrument was screwing to its t'ocus, in all parts of the field, the 

 northern horn constantly appeared pointed ; whereas the more slender point of 

 the southern horn had vanished, and this horn had become rounded, as it was 

 on the 26th, 27th, and 28th of February, and the 13th of March. Comparing 

 this observation with those here named, it becomes very remarkable and decisive, 

 by confirming my former approximated estimate of the period of rotation. On 

 the days just mentioned I had, at the hours noted down, observed a somewhat 

 similar change in the southern horn, conformably to such a period of rotation ; 

 but had never seen it again in all the numerous observations I made since the 

 13th of March, at hours when, according to the rotation, it should not appear. 

 But now it was seen again at lO*" 15"" in the evening. From ll'' in the fore- 

 noon of the 13th of March, to the 2d of April at lo'' 13™ in the evening, there 

 are 20'' ll'' and 15*", which, with a period of rotation of 23*' 21™, divide into 

 21.005 revolutions, exact to the inconsiderable fraction of -f-^^. 



Hitherto the circumstances had not been favourable enough for a repetition of 

 the measurement, and therefore I was eager for a better observation. But May 

 20, Venus was covered with clouds. However, at length I succeeded in a mea- 

 surement : May 21, at 8^ 30™, p. m. 6 days before the inferior conjunction, and 

 consequently just the same time as in the year 1790. Venus being rather too 

 low for the 13-feet, and for the 7-feet Hersch. I employed the 7-feet Schr. ; 

 and found the crepuscular light beautiful, and sufficiently distinct. It extended 

 from the proper points of the horns a considerable way, on the edge of the dark 

 hemisphere ; and equally far on both sides, having the appearance of a very 

 dim, constantly decreasing light. But I must remark, that in the present more 

 unfavourable situation of Venus, it did not affect the eye as a bluish-grey light, 

 which was its appearance March 12, 1790, but only as a dim grey light. Ac- 

 cording to my usual projection-measure, in which each decimal line of the pro- 

 jection table is equal to 4" of space, I found the apparent diameter of the planet, 

 after repeated trials, = 15 lines = 60*; the projection of the crepuscular light 

 running into the dark hemisphere = 2.5 lines = 10", and fully so, being rather 

 more than less. 



Next folloiv Remarks on the Mountains and Rotation of F'enus. 

 These new observations clear up and confirm, in correspondence with my 

 older ones, on the mountains and rotation, that the planet Venus has very con- 



