FRICTION. 113 



_, ^ .. 255. Friction, altlious^i an obstacle in the workiner of ma- 



What are the , & • 



advantages of chinery generally, is not without some advantages. "Without 

 frictionr friction, the Stones and bricks used in building would tend to 



fall apart from one another. When nails and screws are driven into bodies, 

 ■with a view of holding thera together, it is friction alone that maintains them 

 in their places. The strength of cordage depends on the friction of the short 

 fibers of the cotton, flax, or hemp, of which it is composed, which prevents 

 them from untwisting. In walking, we are dependent on friction for our 

 tiothold upon the ground: the difficulty of walking upon smooth ice illus- 

 trates this most clearly. Without friction we could not hold any body in the 

 hand; the difficulty of holding a lump of ice is an example of this. Without 

 friction, the locomotive could not propel its load ; for if the the of the driving 

 wheel and the rail were both perfectly smooth, one would sHp upon the other 

 without affording the requisite adhesion. 



^ , . 256. Experiments seem to show that the friction of two 



How does fnc- ^ / , 



tion between surfaces 01 the same substance is generallj- greater than the 



the same and friction of two unlike substances. The friction of polished 

 different sub- -"^ 



Btances com- steel against polished steel, is greater than that of polished 



P^'®' steel upon copper, or on brass. So of wood and various 



other metals. 



257. For transporting very heavy timbers, or large castings, 



^eel"^ used 'vvheels of great size are used, as by their use the weight is 



for transport- moved with greater facility, and the roughness of the road 



▼ei<'hts? ^^^ more easily overcome than with small wheels. The reason 



of this is, that the large wheels bridge over the cavities of the 



road, instead of sinking into them ; and in surmounting an obstacle, the large 



circumference of the wheel, causes the load to rise very gradually. 



The resistance of sliding friction is much greater than that of rolling fric- 

 tion. In the wheel of a carriage there is rolling friction at the circumference 

 of the wheel, but sliding friction at the axles. In a locomotive, the so-called 

 driving wheels are turned by the force of the steam-engine ; the whole car- 

 riage rolls on in consequence of this rotation ; for if the locomotive were to 

 remain at rest, the wheels could not revolve without sliding on the rails, and 

 overcoming a great amount of shding friction ; but by rolling, the wheels have 

 only the much smaller rolling friction to overcome. The machine, therefore, 

 moves onward, this being the direction in which its motion will experience 

 the least resistance. 



The load which a locomotive is capable of drawing depends, not only upon 

 the force of its steam power, but also upon the weight of the engine, or, in 

 other words, upon the pressure of the driving wheels upon the rails, the fric- 

 tion increasing with the pressure. If we assume that two locomotives have 

 equally strong engines, but that one is heavier than tiio other, a greater 

 •weight will be propelled by the heavier of the two. 



Friction is generally resorted to as the most convenient method cf retard- 

 ing the motion of bodies, and brinpring them to rest. The different modifica- 

 tions of machinery employed for this purpose are termed Brakes. 



