118 MECHANICS. 



rise in the direction of the dotted line. Therefore, if 

 the force is made to pull in this direction, 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 be brought both 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 ruiming on bare ground should be drawn by traces 

 bearing upward in a large degree. The same remark 

 will apply to the plow, which slides upon the ground 

 in a similar way, with the pressure of the turning sod 

 as a load. Hence the reason that a great saving of 

 strength results from the use of short traces in plowing. 

 An experiment was tried for the purpose of testing this 

 reasoning ; first, with traces of such length that the 

 horses' shoulders were about ten feet from the point of 

 the plow ; and secondly, with the distance increased to 

 about fifteen feet. With the short traces a strength 

 was required equal to 2| cwt., but with the long traces 

 it amounted to 3^ cwt. 



But the draught-traces may be made too short. 

 When this is the case, the plow is necessarily thrown 

 too much upon its point to keep it from flying out of 

 the ground, by which means it works badly in turning 

 the furrow. In addition to this evil, the plowman is 

 compelled to bear down heavily, adding to the friction 

 of the sole on the bottom of the furrow, and greatly in- 

 creasing his labor. 



The line of draught should be so adjustsd that the 

 plow may press equally all along on its sole or bottom, 



