[ 160 ] 
nefs to the folar obfervations, which is the more ne- 
celfafy as they are the foundation of all the reft : but 
with refpedt to thofe planets, that in their periods come 
nearer to us than we to the Sun, the obfervations upon 
them will be affedted by a greater uncertainty. 
The determination of the menftrual parallax is of 
ftill more importance, as it is a neceflary confideration 
in the determination of the Sun’s parallax ; and this, 
whether deduced from Mars or Venus, as I fhall 
prefently fhew more particularly; but firft I muff 
ftate the quantity of the menftrual parallax, accord- 
ing to the heft data yet known, by a contrary procefs.; 
and, taking the mean quantity of the Sun’s parallax, 
according to the determination of Mr. Short, at %" 8, 
and the. relative gravities of the Earth and Moon, 
according to Mr. Mafkelyne, as 76 to 1, and the 
mean diftance of their centers equal to 60 L fe mi- 
diameters j we (hall then have the diftance of the 
Earth’s center from the center of gravity, at of 
the Earth’s femidiameter (that is, 4 of that femi- 
diameter within the Earth’s furface) and the menftrual 
parallax equal to _* 0 - of the Sun’s parallax ; confe- 
quently about 7" ; and the double menftrual paral- 
lax, or vacillation, arifing from the whole diameter of 
the epicycle, 14 '; the mean menftrual parallax of 
Mars in oppofition, 29"' 4 ; the greateft, 3 8 /7 4. 5 and 
that of Venus 49" ; from hence it follows, that, was 
a perfon to attempt the Sun’s parallax, by the diur- 
nal motion of the Earth, applied as a bafts to Mars 
in oppofition, as has formerly been tried ; and fhould 
the Moon be at new or full at the fame time, the 
change of place of the Earth’s center, in its own 
epicycle, would amount to an angle feen from Mars 
of 
