WOOI.F"'S'ROTATOKT apparatus; 135 



effeS, whatever be its njeafure ; the ad^ion will be ef- 



i'ddual to fcven-tenths of" the direct or entire ad ion of tiie 



arm. B in the tangent to the great circle ; 2. that when the two in whtd>th* 



arm C is either perpendicularly u<p or dow^n, the regulating ,^^'f^'*"^^y 



weight alone a6ls in the fame tangent ; 3. that when C lies and two In which 



horizontal, the efTedive force will be the excefs of the the effeft is pro. 



r 1 • 1 II ri -1 ^- aucedbythe 



power ot the engine beyond that ot the weight, bince excefs of the 

 .therefore we can do nothing with regard to the four firfl po^^"" beyond 

 mejationed pofitions, we have only to confider the four laft, * ^^'^ ' 

 and to contrive that the moving force (hall be the fame at 

 the top and bottom, as at the tw^o fides ; or in other words, 

 that the weight alone ihall be equal to the excefs of the 

 power of the engine beyond that weight; which is the The weight 

 fame thing as faying, that the weight mufl be equal to half "^"^ ^^ half the 

 iiye power of tlie engine. 



We fee then that the adion on the wheel C, will vary Variation one 

 from fevenrtenths to five-tenths, four times in every revo- J-^j^ f^''^ ^^jj 

 lution ; the extreme difference being therefore two-tenth^ adion, 

 each way from (the medium, which is about fix-tenths. It 

 may not perhaps be of any practical value to difcufs the 

 gradation by which the increafe and diminution of the op- 

 pofite adions are governed ; and the mathematical reader , 

 will eafily fee that the dired adion of each is as the cofine 

 of the angle of the obliquity of the radius it ads upon. Still 

 lefs does itfeem to be of any confequence in the prefent loofe ^ 

 view of the fubjed, to treat of any affumed obliquity of the 

 rods of communication. 



If we fuppofe a power to ad conftantly and uniformly, Goncsrmng the 

 but with alternations of oppofite diredion, in equal times ^^j^^J"^^"^" ^ 

 and through like fpaces ; and wifh to prodirce, by that 

 adion, a rotatory motion, we might in theory obtain 

 the effed by a double fpiral, known by workmen by 

 the name of a fnail: but the difficulties from fridion, Snail, 

 ftiake, and other impediments, would in many inflances 

 render the conftrudion ineligible. The crank is the mofl Ciau]^« 

 cheap, eafy, light, and pradicable means of producing ro- 

 tation from alternate right-lined motion and the contrary ; 

 and its properties are too well known to require diffcuflioii 

 in this place. Its inequalities demand a fly, whether the 

 power or the refillance be uniform or variable ; and I do 

 not fee gny flriking difference between Mr. WoolPs con- 

 trivance 



