C J4* 1 
of the Denfity and of the Square of the Velocity) 
fhall be as the Fluxion of the Logarithm of ^dircdly, 
and the Fluxion of the Curve inverfely. Hence, 
when any Figure that can be deferibed in a Void by 
a Force that varies according to any Power of 
the Dlftance from the Centre, is deferibed in a 
Medium, the Denfity of the Medium muft be in- 
verfely as the Tangent of the Figure bounded by a 
Perpendicular at the Centre to the Ray drawn from 
it to the Point of Contact. 
After giving fome Properties of the Trajectories 
that are deferibed by a Body when it gravitates in 
right Lines perpendicular to a given Surface, and their 
Application to optical Ufcs, the Author proceeds to 
confider the Motion of a Body that gravitates towards 
feveral Centres. In fuch Cafes, that Surface is faid 
to be horizontal, which is always perpendicular to the 
Dire&ion of the Gravity that refults from the Com- 
pofition of the feveral Forces j. and it is fhewn, that 
the Velocity which is acquired by defeending from 
one horizontal Surface to another, is always the 
fame (whether the Body move in right Lines, or in 
any Curves ) ; the Square of which is meafured by the 
Aggregate of feveral Areas which have the Diftances 
from the refpe&ive Centres for their Bafes, and right 
Lines proportional to the Forces at thefe Diftances for 
their Ordinates. 
The Force which aCls upon the Moon is refolved 
into a Force perpendicular to the Plane of the 
Ecliptic, and a Force parallel to it. This laft is again 
refolved into that which is parallel to the Line of 
the Syzi'gies , and that which is parallel to the Line 
•joining the Quadratures. The Firft meafures the Se- 
cond 
