on the Planet Venus. 
217 
inequalities on its surface, which may be, for what we can say 
to the contrary, very considerable. 
The real diameter of Venus, I should think, may be in- 
ferred with great confidence, from the measures I took with 
the 20-feet reflector, in the morning of the 24th of Novem- 
ber, 1791; which, when reduced to the mean distance of the 
earth, give 18", 79 for the apparent diameter of this planet. 
This result is rather remarkable, as it seems to prove that 
Venus is a little larger than the earth, instead of being a little 
less, as has been supposed ; yet, upon the nicest scrutiny, I 
cannot find fault with the measures. The planet was put be- 
tween the two wires of the micrometer, which were outward 
tangents ; and they were, after each measure, shut, so as to 
meet with the same edge, and in the same place where the 
planet was measured. In this situation the proper deduction, 
for not being central measures, was pointed out by the index 
plate. The transits of the 25th were corrected for a small con- 
cavity of the wires, which being pretty thick and stubborn, 
were not strained sufficiently to make them quite straight, the 
amount of which was also ascertained by an examination of 
the division where the wires closed at the ends, and where 
they closed in the centre. The zero was, with equal precau- 
tion, referred to a point at an equal distance from the contact 
of the wires on each side ; for they are at liberty to pass over 
each other, without occasioning any derangement. The shake , 
or play, of the screw is less than 3-tenths of a division. 
The two planets, however, are so nearly of an equal size, 
that it would be necessary to repeat our measures of the dia- 
meter of Venus, in the most favourable circumstances, and 
