Draft of Wagons. 



435 



The arrangement at is provided with a screw and gradu- 

 ated so that the block may be raised or lowered at will, 

 setting it so as to represent the wheel passing over an ob- 

 struction, 3, 4, 5, etc., per cent, of the radius of the wheel. 

 By setting the road-bed inclined as shown in the figure, the 

 draft is first noted and then the thumb screw at D is turned 

 until the wheel rises upon the block and the difference be- 

 tween the two readings of the scale expresses the increased 

 draft due to the obstruction. 



Fid. 210. 



When the obstruction is only four per cent, of the radius 

 of the wheel the draft is increased more than two-fold. 

 That is to say, if a wheel is 48 inches in diameter, an ob- 

 struction of four per cent, would be only .96 of an inch, 

 and yet the draft is made by it more than twice as heavy. 



When the wheel cuts in one inch the draft would not in- 

 crease quite so much because the wheel never rises quite 

 out of the rut, but the difference between the draft on the 

 macadam and dirt road is due mostly to the difference in 

 the yielding, or cutting in of the wheels. 



An experiment conducted by the United States Depart- 

 ment of Agriculture, testing the draft of ordinary wagons 

 on a steel wagon road, showed that a single small horse 



