C 702 ] 
led into this refearch by reflecting upon that queftion, 
debated among the aftronomers for fo many ages 
paft,whether the mean inclination of the two planes of 
the ecliptic and equator fuffers any change or remains 
invariable. Confldering then what caufe could pro- 
duce a change in this inclination, I eafily conceived, 
that if the adtion of Jupiter had lufficient power 
to alter the plane of the earth’s orbit, with re- 
fpeCt to its own, by making their common inter- 
fcCtion recede, in the fame manner as the fun’s aCfion 
operates on the lunar orbit, an alteration in the obli- 
quity of the ecliptic would neceffarily follow $ and 
upon clofer examination it appeared, that Jupiter really 
caufed the earth to deviate in its courfe, and gave a 
retrograde motion to the line of interfeCtion of their 
orbits ; and further, that according to the prefent fi- 
tuation of that line, its regrefs was fuch, as to have 
occafloned a gradual diminution in the obliquity of 
the ecliptic for many ages paft : by which means that 
queftion feems decided. The reafon, why the aftro- 
nomers have not hitherto been able to fettle that 
point, is, becaufe this variation proceeds at fo flow 
a rate, that the obfervations of the ancients are not 
fufficiendy ex ad: to afcertain the fmall diminution, 
that has happened fince their time. I have endea- 
voured to fix the laws, quantity and period of this 
variation. From the fame caufe are alfo computed 
a progreflive motion occafloned in the earth’s aphely, 
and a fmall regreflive one in the equinodial points : 
in all which is added the little (hare of influence, 
that belongs to Saturn. In the laft propofltion are 
deduced fome inequalities, that occur in certain ele- 
ments of the earth’s theory, that have hitherto been 
fuppofed 
