66 
Mr. Robertson on the Precession 
as they may be inferred from the former, after making due 
allowance for the different circumstances under which these 
two remote bodies act on the protuberant parts of the earth. 
I now proceed to estimate the force with which the sun 
tends to cause the earth to revolve about a diameter of the 
equator. 
11. Let S be the centre of the sun (Fig. 7. ) C that of the 
earth ; P, L the poles, PL the axis ; and let a plane passing 
through SC, PL cut the earth in the meridian PEAOB. Let 
EQ be the diameter of the equator, and let DF, the diameter 
of the spheroid in the plane SPCL, be at right angles to SC. 
Let SC cut the meridian EPOL in A, B ; and G being sup- 
posed a particle of matter in this meridian, let GH parallel to 
SC meet DF in H, and let SG be drawn. Let M be the quan- 
tity of matter in the sun, or its absolute attracting power, and 
then ~ is its force upon the particle G, in the direction SG, 
and is its force upon a particle at C, in the direction SC. 
But a force whose power and direction is as GS is equal to a 
force whose power and direction is as GC, together with a 
force whose power and direction is as CS ; and as the force 
whose power and direction is as GC, is directed to the centre 
it has no tendency to alter the position of the axis PL, and 
therefore may be neglected in the present enquiry. Now, by 
Mechanics, SG : SC : : = the force of the sun on 
the particle G, in the direction CS or HG. Now as the dis- 
tance of the sun from the earth is indefinitely great when 
compared to the diameter DF, its force on any particle in DF 
is equal to its force on a particle at C, and therefore the sun's 
force on a particle at H is as Consequently, as the sun's 
