VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 475 



the stroke, were annihilated, in which case the following particles would exert 

 their force uninterruptedly. This supposition is very far from being true in fact, 

 and accordingly we fiixi very little agreement between theory and experiment. 

 To experiments therefore we must have recourse for any thing satisfactory on 

 this subject. I therefore constructed the machine which is here described, by 

 which both the absolute quantity of resistance in all cases may be very accurately 

 determined, and the law of its variation under different degrees of velocity. 



AB, CD, fig. 19, are two cross pieces of wood firmly connected together, with 

 screws at each end, so that it may be fixed on any plane ; egf is a frame fixed 

 on AB ; mn a small cylindrical well polished iron axis, having the lower end made 

 conical, and a hollow conical piece to receive it, the upper end passing through 

 G in a polished nut of iron just large enough to give it a free motion ; on the top 

 of this axis there are fixed 4 arms a, b, c, d, having each a plane h, g,f, e, which 

 may be either of paste-board or tin, and are thus fixed on. A wire has one end 

 made very fiat 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 mo- 

 tion, 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 re- 

 spect to any additional friction at the pulleys by the increase of weight, that may 

 be diminished so as to become insensible, by increasing the radius of the pulleys, 

 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 -l 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 100 to 1, then the effect of the friction wonld 

 be only the 500th part of the whole weight ; and even this might be diminished 

 100 times more by using friction wheels ; but this is a degree of accuracy which 

 I think can never be required. We might also diminish the friction at the nut, 

 if required, by letting the axis on those two sides towards which the lines act rest, 

 between two friction wheels. If the arms should be very long, it may be neces- 

 sary to fix an upright piece on k, and connect the extremity of the sails to the 

 top of it by a string or wire. When this machine is applied to find the resistance 



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