94 MECHANICS. 



traces would be parallel with its surface, that is, on a level ; 

 but as there is always some friction, the line of draught 

 should be a little rising, so as to tend to lessen the pressure 

 of the wheels on the road. 



Now this upward direction of the draught should 

 always he exactly of such a slope^ that if the same slope 

 were given to the road, the vjagon would just descend hy 

 its weight. The more rough or muddy the road is, the 

 steeper should be this line of drauglit or direction of the 

 traces." On a good common road it would be much less, 

 and on a plank-road but slightly varied from a horizontal 

 direction. On a rail-road the line should be about level. 

 On good sleighing, some of the strength of the team is 

 commonly lost by too steep a line of draught. 



The reason of this rule may be understood by the fol- 

 lowing explanation : Let the obstruction, , in the annexed 

 figure (fig. 100) represent the friction the 

 wheel constantly meets with in rolling 

 over a common road. To overcome this 

 friction, the wheel must rise in the di- 

 rection of the dotted line. Tiierefore, if 

 ^ the force is made to pull in this direction, 

 "-z^^^^T^^^-^ it will act more advantageously than in 

 any other, because this is the course in which the centre of 

 the wheel must move. Now if a downward slope were 

 given to the road at this obstruction, the wheel and the 

 obstruction would both be brought on a level, and the 

 wheel would move with the slightest degree of force. 



It will be understood from the preceding rule that a sled 

 running on bare ground should be drawn by traces bearing 

 upward in a large degree. The same remark will apply to 

 the plow, Avhich slides upon the ground in a similar way, 

 with the pressure of the turning sod as a load. Hence 



* Provided the wheels arc not made smaller for this purpose, increasing 

 their resistance. 



