VEHICLE, HARNESS, AND SADDLE 



287 



whick rubs on the axle. The tire rolls not only on the gro'und 

 but also over any obstacle which the surface of the road may 

 present. The width of the tire has already been discussed under 

 " Draft of the Load," Chapter XIII. 



The resilience of the tire is a matter the importance of 

 which has been made more appar- 

 ent by the development of bicy- 

 cling and motoring. The resil- 

 ience is the springing back of the 

 tread behind the point of contact 

 of the wheel with the ground. It 

 may be present either in the tire 

 of the wheel or the surface over 

 which the wheel rolls. The force 

 of the resilience is equivalent 

 to the additional force required 

 to compress tbe tread ahead of 

 the point of contact, as the wheel 

 rolls. 



Bubher tires have the general 

 effect of the wheel travelling over 

 a yielding surface, which in- 

 creases the draft, but if the 

 rubber is sufficiently resilient to 

 restore the force thus expended, 

 the actual draft required may 

 be reduced, since the rubber 

 cushion acts like a spring in ab- 

 sorbing shock and thus prevent- 

 ing waste of power in lifting 

 the load over every little obstacle, 

 then letting it fall, with a pound, 

 on the road again. This saving 

 is greater the higher the speed 

 at which the vehicle is pulled. 



The pneumatic tire, acting on the same principle, very mate- 

 rially reduces the draft of vehicles so equipped, as shown by the 

 experiments of Morin : 



FiQ. 154. — The evolution of the wheel. 



