MOTIVE POWER 479 



which an individual horse should exert, the average pulling 

 power is estimated at one-tenth of the weight of the animal. 



Since the horse is attached to the load by means of a 

 collar placed on his shoulders, his pulling has a tendency 

 to raise his shoulders from the ground and pull him over 

 backward on a pivot, which is the point on the ground 

 under his hind feet. The heavier the horse, the more force 

 is, required pulling backward to resist his pull. This con- 

 dition gives rise to the statement that a horse can pull a 

 heavier load with a man on his back than he can otherwise, 

 since the weight of the man tends to hold him to the ground. 

 The additional pull is the same as the extra exertion by the 

 horse, and it tends to wear him out physically and lessen 

 the period of time through which he can exert the pull. 



Balancing team power. In connecting two or more 

 horses to a load, either to a vehicle to be hauled on the 



road or to a machine 



r c - /—J/na/e free p/ns- 



for performing / 



work, the horses 



should be of equal I " *A 



weight and strength 



^; 



Drafr p/n 



Line cf 

 if the most efficient (/raf// ^Erener arm 



team Service is to be Fig. 370. Showing the position of draft pin, 

 t~f A WVi singletree pins, evener arm, and line of draft 



it is necessary to work horses together that are of unequal 

 weight or strength, the inequality in the animals may be 

 overcome by mechanical appliances. The mechanical appli- 

 ance by which a team is attached to a load should be of 

 such a nature that it will not place one horse at a disadvan- 

 tage in relation to the other. The means of attachment is the 

 evener or doubletree, the individual horses being connected with 

 the evener by the singletree or whiffletree. (See Fig. 370.) 

 If the draft pinhole or the singletree pinholes are not bored 

 in proper relation to one another in the evener, one horse may 

 be required to do more than his share of the work. The ob- 

 ject to be attained when two or more horses are working on 



