C io 9 3 
fixation ought not to be founded on the fuppofed felf- 
evidence of what is partly the thing to be proved. 
But perhaps it may be faid, that the poftulatum may 
be granted merely on this account ; the center of 
gravity of the two bodies (which in this cafe is the 
middle point between them) is not fuftained ; and 
therefore the body which is on the fame fide of the 
fulcrum with the center of gravity will defcend. 
In anfwer to this I muft obferve, that this property, 
which the center of gravity has of defcending, when 
not pi aced diredtiy above or below the point of fufpenfion, 
cannot be proved to belong to it in any cafe, nor can 
we even fhew that there is only one center of gravity 
between two bodies joined by a right line, until it is 
proved in general that the center of gravity of any two 
bodies is a point fo placed between them that their 
diftances from it are inverfely as their weights : but 
this in efFedt includes the principal property of the 
lever, which therefore cannot be proved from any pre- 
vious fuppofition,that the center of gravity will defcend, 
even when the bodies are equal, and we know it is 
the middle point between them. 
I muft now proceed to confider what Sir Ifaac New- 
ton hath delivered on this fubjedt in his Principia, after 
the 2d cor. to the 3 d law of motion which Dr. Clarke ( in 
his notes on Rohault) and all the fubfequentwriters,have 
quoted as an elegant proof of the property of the lever 5 
and therefore what appears to me at prefent an ob- 
jection to this proof I fhall mention with great dif- 
fidence, and in hopes of being fet right if I am wrong. 
Sir Ifaac fuppofes two weights, as A and P. Tab. IV. 
Fig. 1. to hang by threads, from the points M and N, 
in a wheel or circular plane perpendicular to the ho- 
rizon* 
