CH. X PNEUMATIC TIRE I 5 I 



however, the wheel is drawn over an elastic mate- 

 rial, india-rubber, for instance, as in some of Morin's 

 experiments, the material recovers its form behind 

 the wheel, and in so doing restores most of the 

 force expended in compressing it. Notwithstanding, 

 therefore, that the wheel sinks into such an elastic 

 track, hauling over it requires but little more effort 

 than over a hard surface. With carriages this con- 

 dition occurs very nearly, when india-rubber tires 

 are used, which, notwithstanding their constant com- 

 pression in front of the wheel, again give out the 

 force behind the point of contact with the ground.* 

 Such tires seem to increase the draught somewhat, 

 but less than might be expected, which is due to the 

 fact that they diminish the shocks. Experiment 

 shows that, while at a walk, the resistance on hard 

 roads is the same for vehicles with or without springs, 

 it increases with the speed more rapidly for rigid 

 vehicles than for those on springs. This is no doubt 

 due to the fact that the shocks which occur at higher 

 speeds are largely taken up by the springs, and the 

 force thus absorbed is again given out by them, and 

 not all wasted in raising the load and hammering the 

 road, and the same effect is produced by the interpo- 

 sition of the elastic rubber. 



The pneumatic tire, by which the whole load is 

 literally floated on air, has given results at first some- 



* This springing back of the road, or of the tire, has been termed 

 resilience by some of the later experimenters on Draught. 



