4:30 Farm Mechanics. 



road-bed is set for a 10 per cent, grade and then the load, 

 including the carriage, varied it will be found that the 

 draft on the scale will be always 10 per cent, of the load. 



531. The Mechanical Principle Involved in the Relation of 

 Draft to Grade. Jt is a general truth or principle in over- 

 coming any resistance or in doing work of any kind that 

 the force or power doing the work, when multiplied by the 

 distance through which it moves, is always equal to the re- 

 sistance or work multiplied by the distance through which 

 it is moved. Stated mathematically the equation stands 



Power X Power Distance = Weight X Weight Distance 



or 

 P. X P. D. = W. X W. D. 



Suppose the road-bed in Fig. 209 has a length of 100 

 and the grade is 10 per cent., then if a load of 60 is drawn 

 along the length of the road the power will have passed 

 over a distance of 100, acting parallel with the road-bed, 

 but, leaving friction out of consideration, the work done is 

 to lift the load vertically through a distance of only 10, 

 and since the distance which the weight is raised is only 

 1*0 of that over which the power has acted it is only neces- 

 sary that the power shall be iV of the weight or 



P. X P- D. = W. X W. D. 



P. X 100 = 60 XlO 

 whence 100 P. = 600 

 and Power = 6 Ibs. 



532. The Steepest Grade Admissible. When it is asked 

 what is the steepest grade which should be permitted on a 

 given road there are many factors which must be consid- 

 ered, but the most general rule is to make the grade as small 

 as practicable on roads where Iwrses are expected to carry 

 all they can well handle on good, nearly level roads, and 



