WIDTH OP WHEELS. 109 



than on a walk ; but on hard roads it becomes greater 

 as the velocity increases. Thus a carriage on a dry 

 pavement requires one half greater force when the 

 horses are on a trot than on a walk ; but on a muddy 

 road the difference between the two rates of speed is 

 only about one sixth. On a rail-road, where a draught 

 of ten pounds will draw a ton ten miles an hour, the 

 resistance increases so much at a high degree of speed 

 as to require a force of fifty pounds per ton at sixty 

 miles an hour — that is, it would require five times as 

 much actual power to draw a train one hundred miles 

 at the latter rate as at the former ; but as the speed 

 is six times as great, the actual force during a given 

 time would be five times six, or thirty times as great. 



WIDTH OF WHEELS. 



Wheels with wide tire run more easily than narrow 

 tire, on soft roads ; on hard, smooth roads, there is no 

 sensible difference. Wide tire is most advantageous 

 on gravel and new broken-stone roads, both by causing 

 the vehicles to run more easily, and by improving the 

 surface. For the latter reason, the New York turnpike 

 law allows six-inch wheels to pass at half price, and 

 twelve-inch wheels to pass free of toll. Wheels with 

 broad tire on a farm would pass over clods, and not 

 sink between them ; or would only press the surface 

 of new meadows, without cutting the turf. But where 

 the ground becomes muddy, the mud closes on both 

 sides of the rim, and loads the wheels. On clayey 

 soils, narrow tire unfits the roads for broad wheels. 

 For these reasons, broad wheels are decidedly objection- 

 able for clayey or soft soils, and they are chiefly to be 



