MECHANIC power;" '^67 



Relating to (he preniire of the roati juft mentioned, he R^ys^ Me.^fure of 



«' but farther we know, that this prelTure is the exertion *,^'^'^^^'^'i'°^^'^ 



we have no other notion of our own force, and our notion oi 



j;:;rav'ity, of elailicity, or any other natural iorce is the .fame* 



We alfo know, that the continuance bf this exertion fatigues 



and exhaufts our ftrength as connpletely as the mod violent 



motion. A dead pul! as it is called of a horfe at a poft' fixed 



in the ground, is a ufual trial of his ilrength. No man can 



hold out his arm horizontally for much more than a quarter of 



an hour, and the exertion. of the laft minutes gives the mojflr 



di/lreffing fatigue, and difables the flioulder for action for a 



confiderable time after, , This is. therefore an gfpetuiUure. qf 



mechanical paver in the Jirid primiivoe Jhnjh.of'i^ ii'QKd, ,Of 



this expenditure we have an exac^ and adequate effe^ and 



meafure in the quantity of motion produced, that is in the 



produdl of the quantity of matter by the velocity generated 



in it by this exertion. And it mufl be particularly noticed, 



that the meafure is applicable even to cafes where no motipji 



is produced by the exertion ; that is^ if we know that the 



exertion which is juft unable to ftart a block of ftone lying on 



a fmooth pavement, but would ftart it if increafed by the 



fmalleft addition, and if we know that this would generate 



in a fecond 32 feet of velocity in 100 pounds of matter, we 



are certain that it was a prefture equal to the weight of this 



iOO pounds. It is a good meafure, though not immediate, 



and may be ufed without danger of miftake when we have no 



other.'' 



I fliould not have quoted fo much of this feftion, if it had 

 not been that I think it contains an unequivocal interpretation 

 of the writers notion of the true meafure of mechanic power, and 

 at once exhibiting, in my proud opinion, the fallacy of the 

 dodrine in toio. What ! ftiall mere mufcular exertion, 

 whether of horfe or man, be efteemed even an auxiliary to 

 get the conception of the nature of the thing ; how much lefs 

 then ftiall it be fet forth as the thing itfelf ? In perfed con- 

 Ibnance'with this writer, Mr. Gregory, page 152, Philofophical 

 Journal, Vol. XI. fays, fuppofe that a horfe while ftanding 

 ftill, fuftains by means of a rope and fimple fixed pulley, a 

 mafs of an hundred weight, and thus keeps it fufpended at 

 the top of a well for the fpace of a minute ; neither the 

 animal nor the weight moves : but ftiall we fay, in conformity 



* Article Machinery, Supplement, 4th fee. 



as 



