DEFINITIONS AND MECHANICAL PRINCIPLES 13 



If P = Power, Pa = Power arm, W = Weight, and Wa = Weight 

 arm, P X Pa W X Wa. 



If three of these quantities are known, the other is 

 easily calculated. The arm or leverage is always the 

 perpendicular distance between the direction of the force 

 and the fulcrum. 



25. The two-horse evener or doubletree. The two- 

 horse evener is a lever of the second class where the clevis 

 pin for the whiffletree at one end acts as the fulcrum for 



FIG. 2 WAGON EVENER IN OUTLINE. SHOWING THE ADVANTAGE THE 



LEADING HORSE HAS WHEN THE CLEVIS HOLES 



ARE NOT A STRAIGHT LINE 



the power applied by the horse at the other end. The 

 weight is the load at the middle. If the three holes for 

 the attachment of each horse and the load be in a straight 

 line and the arms be of equal length, each horse pulls an 

 equal share of the load even if the evener is not at right 

 angles with the line of draft. But more often the end 

 holes in the evener are placed in a line behind the hole 

 for the center clevis pin. Then if one horse permits his 

 end of the evener to recede, he will have the larger portion 

 of the load to pull because his lever arm has been short- 



