on the Planet Venus. 
175 
adopted hitherto, according to M. de la Lande, namely 16", 7, 
it follows by calculation, that on the 12th March, 1790, when I 
found the apparent diameter 59 to 60 seconds, it should have 
been, by M. de la Lande, 58", 58, but by this new determi- 
nation, 65", 91 ; and on the 21st May, 1793, when I found it 
greater in proportion, probably because the planet was lower, 
and had therefore more irradiation, namely 60 seconds, it 
should by M. de la Lande have been only 56", 75, but by the 
new determination 63", 85; consequently, according to the lat- 
ter, I must have overlooked 4 seconds on the 21st May, 1793, 
and on the 12th March, 1790, when Venus appeared to my 
eye particularly distinct, fully 6 seconds. Both, and especially 
the last, seem to me contrary to all probability. 
As the author, since the year 1780, has measured the dia- 
meter 7 different days, so have I before me no less than 24 
different measurements, made since the year 1788 only: in 
these I took the apparent diameter of Venus, sometimes when 
she was at a greater, and sometimes at a less distance ; not 
only repeating the measurement each time, but often 6, 7, or 
more times, with different telescopes, magnifying powers, and 
projection micrometers. If, out of so considerable a number 
of observations, the mean of the measurements made at each 
time be taken, and reduced to the mean distance of the earth 
from the sun, and then the mean of all these reductions be 
found, this must give the apparent diameter of Venus, at the 
mean ' distance, as exactly as possible. Having so great a 
number of measurements, I must reserve this subject for a par- 
ticular memoir: yet I think it my duty previously to an- 
nounce, that in so many observations, I have always found 
