72 



FERGUSON'S LECTURES. 



LECT. 

 HI. 



gether, and the two half wedges joined to make one ; it 

 will require as much force to drive it down between the 

 blocks as is equal to the sum of the separate powers act- 

 ing upon the half wedges when the bar was between them. 

 To coritirm this by an experiment, let two cylinders, as 

 A B and D C, be drawn towards one another by lines 



;Y 



running over fixed pulleys, and a weight of 40 ounces 

 hanging at the lines belonging to each cylinder : and let 

 a wedge of 40 ounces' weight, having its back just as thick 

 as either of its sides is long, be put between the cylin- 

 ders, which will then act against each side with a resist- 

 ance equal to 40 ounces, whilst its own weight endea- 

 vours to bring it down and separate them. And here 

 the power of the wedge's gravity impelling it downward, 

 will be to the resistance of both the cylinders against 

 the wedge, as the thickness of the wedge is to double its 

 perpendicular height ; for there will then be an equili- 

 brium between the weight of the wedge and the resistance 

 of the cylinders against it, and it will remain at any height 

 between thorn ; requiring just as much power to push it 

 upward as to pull it downward. If another wedge of 



