40 Mr. Vince’s Observations on the Theory 
the lower end made conical, and an hollow conical piece to 
receive it, the upper end passing through G in a polished nut 
of iron just big enough to give it a free motion ; on the top of 
this axis there are fixed four arms a, b, c, d, having each a 
plane h, g,f, e, which may be either of pasteboard or tin, and 
are thus fixed on. A wire has one end made very flat to which 
the plane is fixed, and the other end is left round and passes 
under two small staples made of wire, fixed into the arm so 
tight that you can but just turn it, so that if you fix the plane 
in any position it will remain there without any hazard of 
changing it. Two fine silk lines are wound together round 
the axis, one leaving the axis on one side and the other on the 
opposite side, and each, passing over a pulley, is connected to 
a scale ; by this means the lines when drawn by weights put 
into the scales will give the axis a rotatory motion, and will act 
in opposite directions, and therefore if equal weights be put 
into the scales they will destroy each other’s effects, so far as 
regard the position of the axis, so that neither the friction at 
the bottom nor at the nut at the top will be at all affected by 
whatever additional weights may be thus added. In respect 
to any additional friction at the pullies by the increase of 
weight, that may be diminished so as to become insensible, 
by increasing the radius of the pullies, and making the ends 
of their axes conical and letting them turn in a conical orifice, 
so that they may rest just at their points. If we allow the 
friction at the axis to be one-fifth of the weight added, which 
is certainly a great allowance for such an axis well polished, and 
the radius of the pulley be to the radius of that conical part 
of the axis where it rests as one hundred to one, then the ef- 
fect of the friction would be only the five hundredth part of 
