224 THE HORSE IN SERVICE 



going. If the load is not evenly distributed, whichever wheel or 

 wheels are overloaded cut more deeply into the road-bed, thus 

 increasing the draft as described above. Furthemiore, by the 

 cutting in of one or more wheels the weight of Uie load is shifted 

 in that direction, which increases the degree of the cutting still 

 deeper. 



7. The line of draft. As a general principle the line of draft 

 should be parallel to the direction in which the wheels travel. 

 On an absolutely unyielding surface this plane is parallel to the 

 general surface of the roadway, hence the draw bars on railway 

 cars and locomotives line up parallel with the rails. The ordi- 

 nary wagon, however, is hauled over surfaces more or less yield- 

 ing, consequently, allowing for the depression of the wiieels, 



Fig. 125. — The draft acting in the line A B pulls on a bent lever, BCD, raising the 

 weight which may be considered as concentrated at D. The longer the arm B C and the 

 shorter the arm C D, the more easily will the draft raise the weight of the wheel and for the 

 same sized obstacle the larger wheel will evidently have the advantage. In the figure the 

 lines of draft make the same angle with the radius-arm of the lever. If the lines of draft 

 are parallel, the advantage of the larger wheel is still greater. 



they are really travelling more or less up hill at an angle with 

 the apparent surface of the road. There should, therefore, be 

 a corresponding slant in the line of draft, to avoid pulling down 

 upon or against the incline of the depressed surface. The ad- 

 justment of the line of draft is governed by the length of the 

 tracer, on the one hand, and the height of their attachment at both 

 hame and single-tree, on the other. In making such adjustment 

 it should be borne in mind that, other things being equal, the 

 traction is less the nearer the team is to its load. 



8. Attachment of traces to hames. Whatever adjustment 

 of traces is made for the purpose of giving the proper line of 

 draft, it should not interefere with the angle which this line 



