306 ELEMENTS OF AGRICULTURE 



medium weight. Those weighing over 1,700 pounds are 

 heavy draft horses. The heavier horses bring the higher 

 prices. 



When a team cannot pull a load, it is the feet that 

 give way. They slip because there is not friction enough 

 to hold them. This explains why a horse can pull more 

 when a man is on his back. It gives him more weight 

 so that he can stick to the ground. On dirt roads, horses 

 "dig their toes in," so as to help in getting a foothold. 

 On pavements they cannot do this, but the sharp shoes 

 help. This is one reason why such heavy draft horses 

 are desired in cities. One way to make a horse heavier 

 is to have the whiffletree low down, so that the tugs pull 

 down on the horse's back and hold him to the ground. 

 If the doubletree is put under the wagon tongue, the 

 team can pull a heavier load. In his book on "The Horse," 

 Professor Roberts tells of an experiment with a 1,500- 

 pound horse hitched to a post and pulling on a dynamom- 

 eter (a large spring balance). When the whiffletree 

 was fastened six inches from the ground, the horse pulled 

 2,310 pounds; when two feet from the ground, 1,980 

 pounds; when three feet, 1,732 pounds. This is one reason 

 why horses draw a walking plow easier when the tugs 

 are rather short, but here there is another reason in that 

 the short tugs lessen the friction on the bottom of the 

 furrow. 



With a driving horse, the load is light, so the whiffle- 

 tree should be high. Then, in driving, we do not want 

 any additional weight on the horse's feet, because rapid 

 driving is very hard on the feet and legs. Perhaps some 

 of you have seen hansom cabs in cities. These are two- 



