PRODUCTION OF WORK, MILK, AND WOOL 105 



is because the work of the heart is increased, the body temperature rises, 

 and much heat is lost by the evaporation of water thru the skin and 

 lungs. This decreases the amount of work which a given quantity of 

 food will produce. Furthermore, in trotting or galloping the rise and 

 fall of the body is much greater than in walking, and therefore the 

 horse has available for onward movement a smaller part of the total 

 energy he expends. 



143. The animal as a machine. The extensive investigations by Zuntz 

 and his associates on men, dogs, and horses show that, aside from small 

 variations due to the nature of the work and other factors, the part 

 of the energy expended which is actually transformed into external work 

 is quite constant for each class. Animals working at a moderate rate of 

 speed converted into external useful work from 28.8 to 36.6 per ct. (on 

 the average about one- third) of the additional energy they expended 

 during the work. The rest of the energy used up takes the form of heat 

 within the body and is lost so far as the production of work is concerned. 

 This does not take into consideration the energy of the feed lost in the 

 feces and urine, nor that expended in the work of digestion and in the 

 ordinary maintenance of the body. 



It is shown in Chapter XVIII that when a horse is working at full 

 capacity during the day, it will convert about 8 per ct. or more of the 

 gross energy of its feed into actual useful external work, such as hauling 

 a load, without counting the energy expended in the work of moving its 

 own body. (458) If credit is also given for this work of locomotion of 

 the body, the percentage efficiency is 15 per ct. or more. 



Compared with these estimates of the efficiency of animals as machines, 

 it was found in recent tests of 65 different farm tractors at the Univer- 

 sity of Nebraska 9 that on the average the tractors converted 8.0 per ct. 

 of the gross energy of the fuel (chiefly kerosene) into work of draft, 

 which did not include the work of locomotion of the tractor itself. In 

 developing belt power, where there was no work of locomotion, the 

 tractors on the average turned 13.4 per ct. of the gross energy of their 

 fuel into the work performed. This may be compared to the efficiency of 

 15 per ct. or more for the horse, when credit is included for the work of 

 moving the body. Thus, as a mere machine, the animal compares favor- 

 ably with the best modern tractors. 



The efficiency of the animal as a machine is especially striking when 

 it is borne in mind that the tractor is supplied with purified fuel (kero- 

 sene or gasoline) from which impurities and waste material have been 

 largely removed by distillation. On the other hand, the horse must 

 secure its energy from crude materials, including hay, of which only 

 about half is actually digestible. Furthermore, he must digest this feed 

 and himself separate out the useful from the waste material. Also, he 

 must transport the fuel he secures from his feed and make all body re- 

 pairs. Last but not least, he must maintain his body during the part of 



"Nebraska Tractor Tests, 1921. 



