TO ASCEKTAIX THE AMOUNT OP FRICTION. 83 



genious contrivance is adopted : An inclined plane, a h 

 (fig. 96), is so formed that it may be raised to any desired 

 height by means of the arc of a circle and a screw. Lay 

 a flat surface of the substance we wish to examine upon 

 this inclined plane, and another smaller piece or block of 

 the same substance upon this surface ; then raise the plane 

 until it becomes just steep enough for the block to slide 

 down by its weight. Now, by measuring the degree of 

 slope, we know at once the amount of friction. Suppose, 

 for example, the two surfaces be smoothly-planed wood: 

 it will be found that the plane must be elevated about 

 half as high as its length ; therefore we know, by the 



Fig. 90. 



properties of the inclined plane, heretofore explained, that 

 it requires a force equal to one-half the weight of the 

 wooden block to slide it over a smooth wooden surface. 

 Some kinds of wood have more friction than others, but 

 this is about the average.* 



From the result of this experiment we may learn that 

 to slide any object of wood across a floor requires an 

 amount of strength equal to one-half the weight of the 

 object. A heavy box, for instance, weighing two hundred 

 pounds, can not be moved without a force equal to one 

 hundred pounds. It also shows the impropriety of placing 



* These experiments may be made with tolerable accuraey, by hook- 

 ing a spring-balance into any object of known weight, and then observ- 

 ing the comparative force as measured by the balance, to draw it over a 

 perfectly level surface. 



