[ 621 3 
by the obfervations of the internal contad at the 
egrefs. 
The obfervations at Tobolfk and Cajaneburg I look 
upon as very good ones, they differing in the total 
duration only fix feconds ; this error in obfervation, 
from what is gone before, may be very eafily allowed. 
The obfervation of the internal contad at the ingrefs 
at Stockolm is, I believe, too foon, and the uncertain- 
ty of this obfervation may be eafily granted, when it 
is confidered, that the Sun was only 3 or 4 degrees 
above the horizon at that time, and we find a differ- 
ence of 22" between the obfervers at Upfal, where 
the fun was about the fame altitude. This, I appre- 
hend, was owing to the undulation on the limb of 
the Sun, occafioned by the vapours near the horizon : 
but the fame reafon cannot be given for the obferva- 
tion at Tornea, where the Sun was about io d degrees 
high, at the time of the internal contad at the in- 
grefs, unlefs there was an undulation at that altitude 
alfo, which may have been the cafe, though not 
mentioned. I have reafon to believe that the error 
of obfervation at Tornea is at the egrefs, where in- 
deed the two obfervers differ confiderably. 
The parallax of the Sun being thus found, by the - 
obfervations of the internal contad at the egrefs, . 
= 8", 52 on the day of the tranfit, the mean ho- - 
rizontal parallax of the Sun is " 8", 65. 
I cannot help taking notice on this occafion, of a > 
method employed by fome adronomers, of determin- 
ing the diameter of Venus by the duration of the 
egrefs over the Sun’s limb: for I am fully fatisfied, 
that the beft eye, aflided by the bed telefcope, 
and in the bed and cleared air, could not fee the. 
very 
