294 FARM MOTORS 



415. Effect of increase of speed. As stated before, a 

 horse at maximum speed cannot carry any load, and as 

 the speed is increased from the normal draft speed, the 

 load must be decreased. It is stated that the amount of 

 work a horse is capable of doing in a day is constant 

 within certain limits, varying from one to four miles an 

 hour. Assuming this, the following equation holds true : 



2,y 2 X traction at 2*/ 2 miles = miles per hour'X traction. 



416. Effect of the length of working day. Within 

 certain limits the traction a horse is able to exert varies 

 inversely with the number of hours. When the speed re- 

 mains constant the traction may be determined approxi- 

 mately by the following equation, provided the length of 

 day is kept between five and ten hours. 



10 hours X i/io weight of horse = number of hours X traction. 



417. Division of work. It may not be absolutely true 

 that the ability of a horse to do work depends largely 

 upon his weight, nevertheless it is not far from correct. 

 It is not advisable to work horses together when differing 

 much in size, but it is often necessary to do so. When 

 this is done the small horse should be given the ad- 

 vantage. In determining the amount of the entire load 

 each horse should pull when hitched to an evener it may 

 be considered a lever of the second class ; the clevis pin 

 of one horse acting as the fulcrum. From the law of me- 

 chanics (see Art. 24) : 



Power X power arm weight X weight arm. 



Example: Suppose two horses weighing 1,500 and 

 1,200 pounds respectively are to work together on an 

 evener or doubletree 40 inches long. If each is to do a 

 share of the work proportionately to his weight, it will be 

 possible to substitute their combined weight for the total 



