ROLLING FRICTION, 



CHAPTER VI. 



103 



SECTION I. 



The ' subject of friction has been postponed, or has 

 been merely alluded to, in treating heretofore of ma- 

 chines, to prevent the confusion of considering too many- 

 things at once. As it has often an important influence 

 on the action of machines, it is worthy of careful in- 

 vestigation. 



It is famihar to most persons, that when two sur- 

 faces slide over each other while pressing together, the 

 minute unevenness or roughness of their surfaces causes 

 some obstruction, and more or less force is required. 

 This resistance is known as friction. 



ROLLING FRICTION. 



The term is also apphed to the resistance of one body 

 rolling over another. This may be observed in various 

 degrees by rolling an ivory ball successively over a car- 

 pet, a smooth floor, and a sheet of ice ; the same force 

 which would impel it only a few feet on the carpet, 

 would cause it to move as many yards on a bare floor, 

 and a still greater distance on the ice. The two ex- 

 tremes may be seen by the force required to draw a 

 carriage on a deep sandy or loose-gravel road, and on 

 a rail-road. 



