74 FERGUSON'S LECTURES. 



LECT. each about two inches thick, where they touch the out- 



III 



v^v^, sides of the wedge; and let their ends be made like two 

 round flat plates, to keep the wedge from slipping off 

 edgewise from between them. Let a small cord with a 

 loop on one end of it, go over a pivot in the end of each 

 cylinder, and the cords S and T belonging to the cylin- 

 der A B go over the fixed pulleys W and X, and be fasten- 

 ed at their other ends to the bar w jc, on which any weight 

 as Z may be hung at pleasure. In like manner, let the 

 cords Q and R belonging to the cy Under B Cgo over the 

 fixed pulleys Fand U to the bar v u, on which a weight 

 Y equal to may be hung. These weights, by drawing 

 the cylinders towards one another, may be considered as 

 the resistance of the wood acting equally against oppo- 

 site sides of the wedge ; the cylinders themselves being 

 suspended near, and parallel to each other, by their 

 pivots in loops on the lines E, F, G, H; which lines may 

 be fixed to hooks in the ceiling of the room. The longer 

 these lines are, the better ; and they should never be less 

 than four feet each. The farther also the pulleys V I/ and 

 XWare from the cylinders, the truer will the experiments 

 . be : and they may turn upon pins fixed into the wall. 



In this machine, the weights Y and Z, and the weight 

 p, may be varied at pleasure, so as to be adjusted in 

 proportion of double the wedge's perpendicular height 

 to the thickness of its back ; and when they are so ad- 

 justed, the wedge will be in equilibria with the resistance 

 of the cylinders. 



The wedge is a very great mechanical power, since 

 not only wood wood but even rocks can be split by it ; 

 which would be impossible to effect by the lever, wheel 

 and axle, or pulley : for the force of the blow, or stroke, 

 shakes the cohering parts, and thereby makes them 

 separate the more easily. 



The screw, (j. The sixth and last mechanical power is the 

 screw; which cannot properly be called a smple ma- 



