220 POWER AND THE PLOW 



high wheels and smooth roads contribute to light draft. The 

 harder the road surface, the less do the wheels depress the 

 surface soil. Since the force pulling the load is constantly 

 endeavoring to lift it to the surface, the effect on the wheel is 

 that of constantly rolling up an inclined plane, the gradient 

 of which is determined by the percent, of the radius of the wheel 

 which is below the surface of the ground. This fact largely 

 accounts for the low figure of from 8 to 10 Ibs. of draft per gross 

 ton on railways as compared to 150 Ibs. on ordinary dirt road. 



The width of wheels affects the draft differently under 

 different circumstances. In general the wide tire gives from 

 20 to 120 per cent, less draft than the narrow tire on the same 

 size of wheel. However, when the dust is deep, or when there 

 is a thin coating of mud with a hard surface below it, the 

 narrow tire pulls easier. This is probably because the wheel 

 must sooner or later sink to the hard surface and a narrow 

 tire encounters less resistance. Again, where there is only one 

 wide tired wagon in a community, and this wagon must 

 continually travel in ruts made by narrow tires, the work of 

 filling up these ruts plus that of carrying the load makes the 

 wide-tired wagon pull harder. 



Each rise of one foot in 100 adds 20 Ibs. to the draft of each 

 ton, including the weight of vehicle. On a good macadam road 

 the draft per ton is only about 60 Ibs.; hence, a rise of only 

 52.8 feet to the mile adds a third to the draft. The better the 

 road, the worse is the effect of grade, since a greater load can 

 be hauled on the level, whereas on the hill the action of gravity 

 is independent of the ground friction. It is for this reason that 

 railways spend immense sums in cutting down grades. 



Road surfaces greatly affect the draft. Taking the draft 

 on a plank road as 100, the draft on other surfaces in a certain 

 test was as follows: Macadam road, 152 to 220; gravel road, 

 300 to 318; common dirt road, 300 to 509. The lowest draft 

 on a plank road was 25 Ibs. per ton, and the highest on a dirt 

 road was 224 Ibs. per ton. Other tests have shown up as high 



