[ ”7 ] 
I {hall now endeavour to be as concife as pofiible 
in what I have to fay of the other mechanic powers j 
having, I fear, been too tedious in my account of the 
lever, which however deferves to be particularly con- 
lidered, fince to it may be reduced the ballance, the 
axle and wheel, and (according to fome writers) the 
pulley. 
The ballance I do not confider as a diftindt machine, 
becaufe it is evidently no other than a lever fitted to 
the particular purpofe of comparing weights together, 
and does not ferve for railing weights, or overcoming 
refiftances, as the other machines do. 
When a weight is to be raifed by means of an axle 
and wheel it is fafiened to a chord that goes round the 
axle, and the power which, is to raife it is hung to a 
chord that goes round the wheel. If then the power 
be to the weight as the radius of the axle to the ra- 
dius of the wheel, it will juft fupport that weight ; as 
will eafily appear from what was proved of the lever. 
For the axle and wheel may be confidered as a lever, 
whofe fulcrum is a line palling through the center of 
the wheel and middle of the axle, and whofe long 
and fhort arms are the radii of the wheel and axle 
which are parallel to the horizon, and from whofe 
extremities the chords hang perpendicularly. And 
thus an axle and wheel may be looked upon as a kind 
of perpetual lever, on whofe arms the power and 
weight always a<ft perpendicularly, tho’ the lever turns 
round its fulcrum. And in like manner when wheels 
and axles move each other by means of teeth on their 
peripheries, fuch a machine is, really, a perpetual com- 
pound lever : and, by confidering it as fuch, we may 
compute the proportion of any power to the weight 
4 
