WIDTH O'F WHEELS. 87 



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 j^ounds 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 

 t^imes 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 jDass 

 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 tlie 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 

 recommended for broken-stone roads, and gravelly, or dry, 

 sandy localities. They are also much better for the wheels 

 of sowing or drilling machines, which only pass over 

 mellowed surfaces. 



The larger the wheels are made, the more easily they 

 run ; thus a wheel six feet in diameter meets with only 

 half the resistance of a wheel three feet in diameter. 



A flat piece of wood, sliding on one of its broad sur- 



