DYNAMICS 153 



If exact information were required it would be necessary 

 to determine the effort, i.e. the draught, and if that datum 

 were known the result would be found directly by multiplying 

 it by the distance. 



The work done by horses when ploughing could also be 

 calculated if all the data were known, but the draught cannot 

 be deduced by any arithmetical process ; it must be directly 

 determined in each case, and it varies according to the nature 

 of the soil and the slope of the land. In ploughing medium 

 loamy soil with a pair of horses on the level, it is estimated 

 that each animal performs, roughly, from 10 to 12 million foot- 

 pounds of work in the day of eight hours. 



For ordinary practical purposes, the capacity of horses for 

 continuous work l may be put at i million foot-pounds per 

 hour per 1000 Ibs. live weight. Higher estimates have been 

 given, but they are generally repudiated by men of practical 

 experience. The engineers' unit of 550 foot-pounds per second 

 is equal to nearly 2 million foot-pounds per hour. It should 

 be remembered in this connection that when the amount of 

 work is excessive, i.e. when the animal becomes fatigued, the 

 output of work for a given amount of energy in the food is 

 markedly diminished. On the other hand, light horses (600 

 or 700 Ibs. live weight) give out more work for a given amount 

 of energy in the food than heavy horses, i.e. those of about 

 double the weight mentioned. 



The terms "light," "ordinary," and "heavy work," commonly 

 used in practice, are extremely vague, and quite incapable of 

 scientific definition ; but so far as can be ascertained they 

 may generally be interpreted as follows : 



FOR HORSES OF 1000 LBS. LIVE WEIGHT. 



Light work means from | to i million foot-pounds per hour 

 Ordinary i ij 

 Heavy i|,, 2 



1 By continuous work is meant the average daily work for a consider- 

 able period. A good horse, when called upon for a spurt, can work at a 

 much higher rate for short periods. This is probably the explanation of 

 the very different estimates of the capacity of horses. 



