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an error of 2 o n in time is committed in the obfer- 
vations of the ingrefs and egrefs at both the places 
compared, this error of 20" in time will caule an 
error of 1" in the refult of the Sun’s parallax, and 
in the comparifons of thofe places where the differ- 
ence of duration is lefs, will occafion a greater error, 
and therefore the determination of the parallax, by 
this method, cannot be depended on to any great ex- 
adlnefs, becaufe of the fmall differences of the dura- 
tions compared. In this method, however, we are 
free from the uncertainty arifing from the difference 
of longitude not being exactly known. 
I now proceed to the determination of the Sun s 
parallax by the lead diftance of the centers. There 
are come to my hands only two meafurements of the 
greateft diftance of the limbs of the Sun and Venus, 
one at Tobolfk and the other at Rodrigues. I fhall 
only confider the meafurement at Rodrigues, becaufe 
there feems to me to be fome miftake in the meafure- 
ment at Tobolfk. If we fuppofe the Sun s parallax 
~ 8", 5, then the apparent middle of the tranfit hap- 
pened at Rodrigues at 9’* 37' 30 '. There is a mea- 
furement by M. Pingre of the greateft diftance of the 
limbs at 9 h 38' i3 // , which is fo near the middle of 
the tranfit that we may fafely take this quantity, viz. 
5' 54", 6 *, for the greateft diftance of the limbs of 
the Sun and Venus, and efpecially as it is marked an 
exadt obfervation. This meafurement, therefore, 
gives the apparent leaft diftance of the centers of the 
* I all along confider the obfervation of M. Pingre at Rod- 
rigues as it is printed, from his own letter, in the Philofophical 
Tranfa&ions. 
Sun 
