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planets. Where the forces of two planets are eon- 
fiderable, with refpedt to each other, as in the cafe of 
Jupiter and Saturn, it may be neceflary, in fuch com- 
putations, to have regard to the excentricity of their 
orbits; and this may be referred for a fubjedt of fu- 
ture ferutiny. But the fuppofing the orbits of the 
earth and Venus to be circular, may, in the prefent 
cafe, be admitted, without difficulty, as the forces of 
thefe two planets are fo fmall, and the excentricity of 
their orbits not confiderable. On thefe grounds, 
therefore, I have computed the variations,, which are 
the ededts of the earth’s adtion : fird, the variation of 
Venus’s difiance from the fun ; fecondiy, that of its 
place in the ecliptic thirdly, the retrograde mo- 
tion of Venus’s: nodes ; and, fourthly,,, the variation 
of inclination of its orbit to the plane of the ecliptic. 
The fimilar irregularities in the motion of the 
earth, occafioned by its gravitation- to Venus, are here 
likewife computed : but it is to be obferved, that the 
abfolute quantity of thefe irregularities is not here 
given, it being impoffible, at prefent, to do it ; be- 
caufe the abfolute force of Venus is not known to us. 
I have, therefore, dated that planet’s force by fup-. 
pofition, and have, accordingly, computed the effedts 
it mud produce; with the view, that the adrono- 
mers may compare their obfervations with the mo- 
tions fo calculated, and, from thence, difeover how 
much the real force differs from that which has been 
fuppofed. But the exadt determination of the force of 
Venus mud be obtained, by obfervations made on the 
fun’s place, at fuch times, when the effedt of the 
other planets is either null or known. 
The 
