CH. X ROLLING FRICTION 1 53 



pneumatic tires and 120 pounds (228 pds per ton) 

 with iron tires, — about the same results which are 

 obtained now. 



Notwithstanding these successes, the device 

 seems to have been entirely lost sight of until it 

 was re-invented a few years ago. 



The ton here used, as in all the computations 

 which follow, is that of 2000 pounds. 



Morin's experiments, which are especially valu- 

 able, because they were made with large vehicles on 

 the road and not with small models, further showed 

 that the rolling friction increased directly with the 

 weight of the carriage and load, and that for a mven 

 kind of road it could be computed by the simple 



P 

 formula, R = A-, where P is the weight (pressure), 



r the radius of the wheel, and A a constant or coeffi- 

 cient," 1 determined by experiment. 



* A coefficient is a proportion and can be thus exemplified : If A 

 and B are associated in a business of which A owns % and B ?/ , the 

 coefficient of A will be % or 0.25, so that any profits or losses must 

 be multiplied by 0.25 to determine A's share. In the friction of a 

 body sliding on a surface, if the coefficient of friction is 0.05, the 

 weight of the body multiplied by 0.05 expresses the friction, which in 

 this case will be 5 per cent, of the weight or pressure. 



In using the French coefficients from Morin's book, it must be 

 borne in mind that in the formula for the rolling friction, the wheel 

 radius in metres enters as a divisor, and the French coefficient must 

 be multiplied by 3.281, the value of a metre in feet, to obtain a 

 coefficient for use with English measures. For instance, A (French) 

 0.015 ' s th e same as A (English) 0.05. 



In the formula for resistance from axle friction, the multiplier ^ 1S 



