MR. GREGORY ON HORSE POWERS. 151 



HI redilinear paths. Judicious experiments having thefe 

 purpofes in view, would certainly be beneficial, as they 

 would enable us to tell what advantages might be expeded 

 from the labours of this ufeful quadruped in different circum- 

 ilances. But with refpe(5l to the adoption of *' horjl j^ov-er" The adoption of 



.,,• r • n- ,• .1 c i\ • an arbitrary unit 



as a unit of force in eltimating the power oi Iteam engines, ^^.^^j^ ^1,^ p^^^^ 

 Sec. I confefs that if it were as well known, and as un- of an horfc. 

 variable as the length of the day of the equator, I iliould feel 

 an averfion to applying it to any fuch purpofe. It is a com- 

 mon meafure arbitrarily adopted, which has no neceflary con- 

 nedion with the fubjedt that is referred to it, which does not 

 in any refped facilitate the computation of the powers of an 

 engine, and which may, without proper caution, lead to 

 confiderable errors in the conclufions deduced from it. 



Before I clofe this letter, already perhaps too long, I beg Smeaton's 

 permifiion to fay a few words refpeding the meafure which is chankd power' 

 generally employed to determine the mechanical effe<5l pro- and effeft. He 

 duced. This is the meafure of the defervedly celebrated p^ji^'of'tJe^ 

 Mr. John Smeaton, who fays that '* the weight of a body weij'ht into the 

 multiplied by the height through which it defcends, while height pafled 

 1 • • u- • *u 1 r f .1 through in 



driving a machme, is the only proper meaiure ot the power either cafe ; 



expended; and that the weight multiplied by the height 

 through which it is uniformly raifed is the only proper meafure 

 of the effecl produced." Mr, Smeaton was led to the ufe of 

 this meafure by his profeffional |(abits ; and many who in this 

 refped pay too great a deference to his authority, have adopted 

 this meafure as univerfal and preferable to any other. Taking 

 this as a popular meafure eafy to recollect, and fimple in its 

 application, it undoubtedly has its ufes; but in many in- 

 flances it is inadequate to the purpofe for which li is propofed. 

 The late Profetfor Robifon h^s fome excellent obfervations on reaified by 

 this fubjea, in the article Machinery, Sup, Encyclop^dia^^ll^l[ 

 Britan, where he lays down the juft meafure to which the 

 Icientific inveftigator will generally have recourfe, ** We 

 take, fays he, for the meafure, (as it is the effed) of ♦ 

 exerted mechanical power, the quantity of motion which 

 it produces (or whofe accumulation it prevents) by its uniform 

 exertion during fome given time. We fay uniform exer- 

 tion, not becaufe this jgj^formity is neceflary, but only 

 becaufe, if any varlatic^i^ of the exertion has taken place, it 

 inuft be known in order to jud^e of4jj|^)ower." 



; JK|y^'# ^' A finglQ 



