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it is neceffary that fome caufe fhould be affigned why 
(for inhance) the endeavour of one pound to defcend 
fhall be equal to that of four pounds ; and efpecially 
as the fulcrum on which both weights a requires no 
greater force to fupport it than that of five pounds. 
From thefe considerations I infer, that the reafon 
why very unequal weights may ballance each other, 
fhould be afligned not from their having equal mo- 
menta when made to move together, but by proving 
a priori without considering their motions, that either 
the rea&ion of the fixed parts of the machine, or fome 
other caufe, fo far takes off from the weight of the 
heavier body as to leave it only juft able to fupport the 
lighter. However, as this equality of momenta which 
always accompanies an c equilibrium , affords a very ele- 
gant theorem, it ought to be taken notice of in every 
treatife of mechanics, and may ferve as an index of 
an c •equilibrium . But I would not have it applied to 
a purpofe for which it is unfit; as it has been in ano- 
ther inftance by Do&or Keil, who from thence gives 
the reafon why water hands at the fame heighth in a 
narrow tube and a wide veffel with which it commu- 
nicates. And an argument of the fame kind is applied 
hill more improperly by Dr. Rutherforth and otherSj 
to fhew why a drop of water included in a fmall 
conical tube will move towards the narrower end ; 
and yet the true ways of accounting for both thefe 
phenomena are extremely obvious and eafy. 
The fimple mechanic powers are ufually reckoned 
fix, the lever, axle and wheel, pully, wedge, inclin- 
ed plane, and fcrew. The only method I have met 
with of explaining the nature of thefe machines on 
one principle, is that which I juh now examined; and, 
as that appears to me unfatisfa&ory, I fhall confider 
