THREE MECHANICAL STRUCTtTRBS. 51 



. part B, and the rope which pafles under the pulley C and fuf- 

 tains the weight D is wound upon each in contrary diredions. 

 Whenever, thereforCj the handle is turned, fo as to gather the 

 rope upon the larger cylinder, it will be given off by the 

 fmaller: and, for every turn of the larger or its correfpondent 

 portion of rope wound up, there will be given off a portion of 

 rope, anfwering to the circumference of the fmaller. Con- 

 fequently, the quantity of unwound rope will be kfs, after 

 fuch a turn, by a quantity equal to the difference between the 

 circumferences of the two cylinders; and the weight D will ' 

 be elevated through half that fpace. Whence 



As the circumference defcribed by the winch (or its radius) It« pioweri 



Is to half the difference between the circumferences of th« 

 cylinders (or half the difference of their radii) 



So is the power 



To the weight (in equillbrio.) 



It is evident that the peculiarity of this engine is that the half PecuUanty of 

 difference of the radii may be made as fmall as we pleafe with- ^"'^ engmc. 

 out weakening the cylinder or requiring any quick curvature 

 of the rope; neither of which requifites could be had in the 

 iimple cylinder. Its practical difadvantage appears to be that 

 a large quantity of rope will be ufed to produce a moderate 

 elevation of the weight. 



The firfl account I find of the double fcrew Plate IV. Doirtjle fcrew 

 Fig, 1. is given byMr. Wm. Hunter in the.Philofophical Tranf- oS'/evvprctfe* 

 adions for 1781. In the tketch before us, A B C reprefents down while ' 

 the fcrew of a prefs: the upper part of the fcrew A has a ^''^^^^j thnf- 

 coarfer thread than the lower B and a concave fcrew or nut, feft isproducei 

 C, receives the latter, while the former paffes through a proper ^y the different* 

 helical cavity in the frame of the prefs* In the prefent ftate 

 of the apparatus, the nut C is pinned faff to its fcrew by a 

 crofs bolt; and confequently, when the whole fcrew is made 

 to turn by the arms at the top, the nut C is carried round, and ^ 

 the effect is precifely the fame as if the prefs were governed - 

 by the fcrew A alone. But when the nut G is difengaged by 

 Drawing the crofs bolt, and likewife prevented from revolving 

 by pinning it to the moveable platform ^ the effed becomes 

 . inore compounded : that is to fay, while the whole fcrew is 

 carried downwardsby one revolution, through one thread of A, 

 the nut C is drawn upwards through one thread of C. And 



E2 the 



