214 
Dr. Herschei/s Observations 
peating the measures. A mean of both may, perhaps, not be 
far from the truth ; which gives, diameter 3595 ; projection 
560. 
Here 1797,5 being radius, and 560 sine, we find the angle 
a c b, or d c e, equal to 18 0 9' 8", 2, 
A few very evident results may be drawn from the forego- 
ing observations. 
With regard to the rotation of Venus on an axis, it ap- 
pears that we may be assured of this planet's having a diurnal 
motion, and though the real time of it is still subject to con- 
siderable doubts, it can hardly be so slow as 24 days. Its di- 
rection, 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 ob- 
served 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 this subject, has the 
merit of being the first who has pointed out this inference, 
but he has overlooked the penumbra arising from the diame- 
ter of the sun ; * which has certainly a considerable share in 
* He mentions it upon another occasion, and says in a note, p. 313, that “ this whole 
penumbra, which, according to the greatest apparent diameter of Venus, extends 
from 59 to 60", (for what reason he fixes upon these quantities does not appear) 
“ measures, in the direction perpendicular to the line of the cusps, only o" 36.” But 
if, according to him, the apparent diameter of the sun be 44', (which is less than it 
ought to be) the penumbra must certainly extend likewise upon the surface of Venus 
