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in Tome meafure compenfates for the fmalnefs of the 
bafe. 
The fame irrefragable argument, made ufe of in 
the apparent lead; diftance of the centers, meafured 
at Rodrigues, to prove that the parallax of the Sun 
is very nearly = 8 ', 5, may like wife be deduced from 
the apparent lead diftance of the centers, computed 
from the total durations dbferved at thefe 1 2 places, 
but with more certainty ; becaule the determination 
of the apparent lead: diftances of the centers from 
the obferved total durations may be depended on to 
a very great precifion, but the fame cannot be faid 
with regard to the apparent lead; diftance of the cen- 
ters meafured at Rodrigues : For M. Pingre tells us 
that he ufed a very good micrometer fitted to a re- 
fracting telefcope of nine feet focus, the objeCt-giafs 
of which was but an indifferent one ; and we are 
very certain, that in meafuring, with a micrometer 
of this fort, dark objeCts on a white field or ground, 
if the image is any way indiftindt, the angle meafur- 
ed will be lefs than the true angle, and 'vice vcrfa 
when a bright objedt is meafured on a dark ground j 
as a proof of this remark, we find that M. Pingre 
meafured and found the diameter of Venus, when 
on the Sun, — 54", 7, whereas we are certain 
that it was above 58", and therefore we may prefume 
that the meafurements of the greateft diftance of the 
limbs might be greater than the true diftance, and as 
a further proof of the uncertainty of the meafure- 
ments made with this inftrument we find that M. 
Pingre makes the diftance of the limbs greateft, fe- 
deral minutes after it was paft the greateft. 
I fhalt 
