[ 2 7 6 ] 
fame principle, that all the planets muft a<fb upon 
one another, proportionally to the quantities of mat- 
ter contained in their bulk, and inverfe ratio of the 
fquares of their mutual difhnces ; but as the quan- 
tity of matter contained in each of them, is but imall 
when compared to that of the fun, fo their action 
upon one another, is not fo feniible as that of the lun 
upon the moon. Aftronomers generally contented 
themfelves with folely conlidering thofe inequalities of 
the planetary motions, that arife from the el ipticai 
figure of their orbits 5 but as they have been enabled, 
of late years, by the perfection of their inftruments, 
to make obfervations with much more accuracy than 
before, they have difcovered other variations, which 
they have not, indeed, been able yet to fettle, but 
which leem to be owing to no other caufe, but the 
mutual attraction of thofe celeftial bodies. In order, 
therefore, to afiift the aftronomers in diftinguifhing; 
and fixing thefe variations, I fhall endeavour to cal- 
culate their quantity, from the general law of gravita- 
tion, and reduce the refult into tables, that may be 
conlulted, whenever obfervations are made. 
I offer to you, at prelent, the firfc part of fuch a 
theory, in which I have chiefly confidered the effeCts 
produced by the aCtions of the earth and Venus upon 
each other. But the fame propofitions will like wife 
give, by proper fubffitutions, the effeCfs of the other 
planets upon thefe two, or of thefe two upon the 
others. To obviate, in part, the difficulty of fuch 
intricate calculations, I have fuppoled the orbits of the 
earth and Venus to be originally circular, and to fuffer 
no other alteration, but what is occalioned by their 
mutual attraction, and the attraction of the other 
planets* 
