on the Planet Venus. 
213 
May 19, 1793. Correction of the clock, — 3' 34", 7. 
n h 45'. 7-feet reflector ; power 287. Both horns perfectly 
alike, in shape and illumination. Not the least appearance of 
any mountains. The horns are exceedingly slender. 
i2 h o'. I do not see any diminution of light on the edge of 
the horns, but what may be accounted for from their slender- 
ness ; being brought to very fine points, that lose themselves 
by their minu eness. 
I saw it in great perfection, with a newly polished, plain 
speculum, which excels my former one in sharpness. 
May 20, 1793. Correction of the clock, — 3' 35",8. 
i2 h 20'. No spot or unevenness in the light of Venus upon 
either cusp, or in any other part, that could in the least make 
me suspect a mountain. 
I measured the diameter of Venus, and projection of the 
cusps beyond an hemisphere, by my disk-micrometer. This 
was not done by an illumination, as described in the appara- 
tus, (Phil. Trans. Vol. LXXIII. p. 4.) when I used it for a 
nocturnal planet ; for, day-light being sufficiently strong, 
there was no occasion to light the lamps. On the measuring 
disk were drawn concentric circles ; and also a diameter, hav- 
ing several lines parallel to it, in one of the semicircles. If 
there had been time, I should have prepared a straight edge, he, 
moveable parallel to the diameter a d. See Tab. XXII. fig. 2. 
First measure, with the double eye-glass ; power about 30. 
Diameter of Venus 2390. Projection 500. But the power is 
too low to be accurate. 
Second measure ; power 215. Diameter of Venus 4800. Pro- 
jection 620. Here the projection is probably as much too small 
as the former was too large ; but the planet is too low for re- 
2 F 
MDCCXCII I. 
