156 ESSAYS anp OBSERVATIONS 
THEOREM. Fic. 7. 
Let the ellinfe, whofe greater axis is AP, 
foci S, ix, 2ndcenter C, reprefent the 
orbit of a planet round the fun at 5; 
and, fuppofing the periodic time of the 
planet round the fun to be known, and 
likeways the time the planet pafled 
through the aphelion A: As the perio- 
dic time of the planet round the fun, 
is to the time elapfed fince the planet 
paffed through the point A, fo let the 
area of the ellipfe be to the fector ACB; 
join SB, and draw CD parallel to SB 
on the fame fide of AP that SB is; and 
let OD be equal to CA; join SD; Tet 
CD, SD, meet the ellipfe in E, F: The 
true place of the planet is between the 
points E, F; that is, the planet is paf- 
fed the point E, but not come to the 
point F, rs 
Let G be the place of the planet; join 
SG, and join BD meeting AP in H, and 
join 
y 
