[ 3+4 ] 
lition of the planet in a direction perpendicular to the 
axis of the orbit of the planet, therefore the paral- 
laxes of longitude, in time, are, in this cafe, to be 
addea to the obferved time of the total duration ; in 
confequence of which the obferved time of total du- 
ration, be »h A b -f cD are = to the chord deferibed 
by the planet in its paffage over the Sun 3 and if the 
femidiameters of the Sun and Venus are known, 
their dinerence is known, which is m: to the line 
AS 3 AF, from what has been laid is alfo known, 
therefore S i" may be found. But this S F is not the 
apparent lead: didance of the centers, for if we com- 
pute the parallax of latitude for the apparent middle 
of the tran fit, we fhall find it greater than MF, 
which MF is only a mean between the parallaxes of 
latitude at the ingrefs and egrefs. Let therefore the 
difference between MF and the parallax of lati- 
tude computed for the middle of the tranfit be add- 
ed to S F, and the fum will be = to the apparent 
lead: didance of the centers nearly 3 and if from this 
fum we fubtradt the parallax of latitude, computed 
for the middle of the tranfit, the remainder will be 
the geocentric lead diftance of the centers nearly. 
A true and more ready method to find the geocentric 
lead: diflance of the centers, confequently the ap- 
parent lead: difiance of the centers at any place, 
where the total duration has been obferved. 
Reduce the total duration obferved to the center, 
reduce the central femi-duration, in time, into fpacej 
then in the right-angled triangle SMA [Fig. 2.] or 
S M a, we have the two ddes S A or S<rr, and A M 
or 
