Mechanic Powers. 243 



V. The wedge is nearly allied to the inclined 

 plane ; indeed it may properly be considered as 

 two equally inclined planes joined together. You 

 know that its uses are to cleave or separate wood 

 or stone, or any heavy bodies that adhere toge- 

 ther. The power of the wedge is as its length 

 to the thickness of its back. To show how we 

 may calculate the force of a wedge, let a (fig. 

 101) be a wedge, which is interposed between the 

 two cylinders c and w, which are pulled against 

 the wedge by the two weights r and s, represent- 

 ing the resistance to be overcome by the force of 

 the wedge. If then r and s influence the cylin- 

 ders each with a force equal to two pounds, the 

 resistance to be overcome will be equal to four 

 pounds. Now the length of the wedge a is 

 twice the thickness of its back, and the weight 

 o, suspended to it, is two pounds. Here, then, 

 is a resistance equal to four pounds overcome by 

 a weight of two pounds, by means of a wedge, 

 the length of which is double the thickness of 

 its back. This explains sufficiently what a wedge 

 will be able to effect by simple weight or pres- 

 sure ; but we see every day, where a hard stone 

 or a piece of tough wood is to be cleft by a wedge, 

 that a ton weight would not force it in, when a 

 smart stroke of a hammer, which has not a for- 

 tieth part of that weight, will effect it at once. 

 In this case we are to have recourse to what was 

 said in the last lecture on the momentum or force 

 which is gained by the velocity of a moving 



