76 , PHILOSOPHICAL TRANSACTIONS. ' [aNNO i776. 



if nothing were lost by the resistance of the air, the scattering of the water 

 carried up by the wheel, and thrown ofF by the centrifugal force, &c. all which 

 •tend to diminish the effect more, at what would be the maximum if these did not 

 take place, than they do when the motion is a little slower. 



Finding these matters, as well as others, to come out in the experiments, so 

 very different from the opinions and calculations of authors of the first reputa- 

 tion, who, reasoning according to the Newtonian definition, must have been 

 led into these errors from a want of attending to the proper collateral circum- 

 stances ; I thought it very material, especially for the practical artist, that he 

 should make use of a kind of reasoning in which he should not be so liable to 

 mistakes; in order therefore to make this matter perfectly clear to myself, and 

 possibly so to others, I resolved to try a set of experiments fi'om which it might 

 be inferred, what proportion or quantity of mechanical power is expended in 

 giving the same body different degrees of velocity. This scheme was put in 

 execution in the year 1759, and the experiments were then shown to several 

 friends, particularly my very worthy and ingenious friend Mr. William Russell. 



In my experimental inquiry concerning the powers of water and wind before 

 referred to, I have defined what I meant by power, as applied to practical me- 

 chanics, that is, what I now call mechanical power ; which, in terms synonimous 

 to those there used, may be said to be measured by multiplying the weight of 

 the body into the perpendicular height from which it can descend ; tlius the same 

 weight, descending from a double height, is capable of producing a double me- 

 chanical effect, and is therefore a double mechanical power. A double weight 

 descending from the same height is also a double power, because it likewise is 

 capable of producing a double effect ; and a given body, descending through a 

 given perpendicular height, is the same power as a double body descending 

 through half that perpendicular ; for, by the intervention of proper levers, they 

 will counterbalance each other, conformably to the known laws of mechanics, 

 which have never been controverted. It must however be always understood, 

 that the descending body, when acting as a measure of power, is supposed to 

 descend slowly, like the weight of a clock or a jack ; for, if quickly descending, 

 it is sensibly compounded with another law, viz. the law of acceleration by 

 gravity. 



Description of the Machine, pi. \.Jig. 1. — ab is die base of the machine placed on a table ; AC is 

 a pillar or standard ; CD is an arm on the extremity of which is tixed a plate fg, here seen edge- ways, 

 through which is a hole for receiving a small steel pivot c, tixed in the top of the upright axis cb ; 

 the lower end of which axis finishes in a conical steel point, resting on a small cup of hard steel 

 polished at b. — ni is a cylinder of white fir, which passing through fixes in a perforation in the 

 axis i and on the 2 arms thus formed, capable of sliding, are k, l, t\io cylindric weights of lead of 

 equal size, which are capable of being fixed on any part of the cylindric arms, from the axis to their 

 extremities, by means of 2 thin wedges of wood. The 2 weights tlierefore being at equal distances 



