OF THE MECHANICAL POWERS. 73 



equal weight and deptli with this, and only half as thick, LECT. 

 be put between the cylinders, it will require twice as much ^^^^ 

 weight to be hung at the ends of the lines which draw them 

 together to keep the wedge from going down between 

 them. That is, a wedge of 40 ounces, whose back is only 

 equal to half its perpendicular height, will require 80 

 ounces to each cylinder, to keep it in an equilibrium be- 

 tween them : and twice 80 is 160, equal to four times 40. 

 So that the power will be always to the resistance, as the 

 thickness of the back of the wedge is to twice its per- 

 pendicular height, when the cylinders move off in a line 

 at right angles to that perpendicular. 



The best way, though perhaps not the neatest, that I 

 know of, for making a wedge with its appurtenances 

 for such experiments, is as follows. Let / K L M and 

 L M N O 34 be two flat pieces of wood, each about 

 fifteen inches long and three or four in breadth, joined 

 together by a hinge at L M, and let P be a graduated 

 arch of brass, on which the said piece of wood may be 

 opened to any angle not more than 60 degrees, and then 

 fixed at the given angle by means of the two screws a b. 

 I K N will represent the back of the wedge, L M its 

 sharp edge which enters the wood, and the outsides of 

 the pieces I K L M and L M N the two sides of 

 the wedge against which the wood acts in cleav- 

 ing. By means of the said arch, the wedge may 

 be opened so as to adjust the thickness of its back 

 in any proportion to the length of either of its sides, 

 but not to exceed that length : and any weight, as 

 p, may be hung to the wedge upon the hook M, which 

 weight, together with the weight of the wedge itself, may 

 be considered as the impelling power ; which is all the 

 same in the experiment, whether it be laid upon the back 

 of the wedge, to push it down, or hung to its edge to puL 

 it down. Let A B and C D be two wooden cylinders, 



See Eng " :r "* "aire 72. 



