FRICTION AT THE AXLE. 



Ill 



miles per hour, because five times the distance is gone 

 over ; but, measured by a dynamometer or spring-bal- 

 ance, the pressure would be the same. In precisely 

 the same way, the weight of a stone remains the same, 

 whether lifted slowly or quickly by a lever. If the 

 friction of the wheels of a wagon on their axles be 

 equal to ten pounds, driving the horse fast or slowly 

 will not increase or diminish it. But fast driving will 

 require more strength, for the same reason that a man 

 would need more strength to carry a bag of wheat up 

 two flights of stairs than one, in one minute of time. 



FRICTION AT THE AXLE. 



A carriage wheel, or any other wheel revolving on an 

 axle, will run more easily as the axle is made smaller. 

 This is not owing to the rubbing surfaces being less in 

 size, as some mistakenly suppose, for it has just been 

 shown that this makes very Uttle or no difference, pro- 

 vided the pressure is the same ; but it is owing to the 



leverage of the wheel 

 on the friction at the 

 axis ; and the smaller 

 the axle, the greater 

 is this leverage ; for, 

 if the axle, a {Fig- 

 ure 88), be six inches 

 in circumference, and 

 the wheel, 6 c, be ten 

 feet in circumference, 

 then the outer part of 

 the wheel will move 

 twenty times further than the part next the axle. 



