WR. GREGORY ON MECHANIC IOWER. ijj 



furely have been but candid to produce a paflfage which ftrikes 



with great force againft the univerfality of Mr. Smeaton's 



meafure, at the fame time that it admits the utility of this 



meafure to engineers in many cafes. This paffage is as 



follows :-— 4 « When a weight of five pounds is employed to Robifonon 

 i • . r , 11 e ij Smeaton s raea« 



drag Up a weight ot three pounds, by means or a thread over f urc , 



a pulley, it defcends with a motion uniformly accelerated, 

 four feet in the firft fecond. Mr. Smeaton would call this an 

 expenditure of a mechanical power 20. The weight threfe 

 pounds is raifed four feet. Mr. Smeaton would call this a 

 mechanical effect 12. Therefore the effect produced is not 

 adequate to the power expended. But the fact is, that the 

 preffure, ftrain. or mechanical power, really exerted in this 

 experiment, is neither five nor three pounds ; the five pound 

 weight would have fallen 16 feet, but it falls only four. A 

 force has therefore acted on it fufficient to make it defcribe 12 

 feet in a fecond, with a uniformly accelerated motion, for it 

 has counteracted fo much of its weight. The thread was 

 /trained with a force equal to 3f pounds, or ^ of 5 pound!?. 

 In like manner, the three pound weight would have fallen 

 16 feet; but it was raifed four feet. Here was a change 

 precifely equal to the other. A force of 3f pounds acting on 

 a mafs whofe matter is only three, will in a fecond, caufe it 

 to defcribe 20 feet with a uniformly accelerated motion. Nov* 

 5x12 and 3 x 20, give the fame product 60. And thus we 

 lee, that the quantity of motion extmguijked or produced, and 

 not the prorfutt of the weight and height, is the true unequivocal 

 meafure of mtchanical power really expended, or the mechanical 

 ejfcc! really produced ; and that thefe two are always equal and 

 oppofite. At the fame time, Mr. Smeaton's theorem merits 

 the attention of engineers; becaufe it generally meafures the 

 opportunities that we have for procuring the exertion of 

 power. Infome fenfe, Mr. Smeaton may fay, that the quantity 

 of water multiplied by the height from which it defcends iti 

 working our machines, is the meafure of the power expended ; 

 becaufe we muft raifethis quantity to the dam again, in order 

 to have the fame ufe of it. It is expended, but not employ- 

 ed, for the water at leaving the wheel is ffill able to do fome- 

 thing." 



In oppofi'ion to all this, Mr. H. I fuppofe, would fay that ^J^ "* 

 this is not a cafe in point, becaufe, " if the weight defcends Smeaton botk 



quickly c° ncede the 



pouit 



in debate. 



