PRODUCTION OF WORK, MILK AND WOOL 79 



engine falls below 10 per ct. Gasoline engines range in efficiency 

 from 10 to 27 per ct. Thus, as a mere machine for mechanical work 

 the animal body compares favorably with the best modern engines. 

 In addition to performing external work, the body must prepare and 

 transport its own fuel, store it until needed, make all repairs, and 

 maintain a definite temperature, as well as direct, move, and control 

 itself. When all these functions are considered, the marvelous per- 

 fection of the animal body as a machine becomes apparent. 



By processes still unknown the animal body produces heat, light, 

 electricity, and muscular energy with an efficiency greater than any 

 machine made by man. With animals the fuel is burned at low tem- 

 peratures. The glow worm and fire fly produce light without sensible 

 loss of heat or other energy, and the torpedo fish and electric eel gen- 

 erate electricity by means unknown. Such mysterious and wonderful 

 processes which continuously occur in the animal body baffle scientists 

 and inventors alike. 



Factors influencing energy required for work. — The amount of 

 energy required to produce a given amount of useful work depends 

 upon many factors. Practice in doing a certain kind of work lessens 

 the amount of energy expended. In one instance, training for two 

 weeks decreased by over 20 per ct. the energy used by a man climbing 

 a tower. On the other hand, fatigue greatly increases the energy 

 required to do a given task. This is largely due to the fact that with 

 increasing fatigue the muscles normally used, and which are thus most 

 efficient in performing the given work, are put out of use. Then other 

 less used muscles are called upon to a constantly increasing degree, 

 and these cannot perform the work so economically. Increasing the 

 speed at which the work is done also lessens the efficiency with which 

 it is performed. This 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 feed will produce. 



The part of the expended energy appearing in useful work varies 

 with the build of the animal, the development of its muscles, and the 

 structure of its limbs. For example, a lame horse may use nearly 

 twice as much energy in traveling a certain distance as one with sound 

 legs. An animal which is able to perform one kind of work most 

 economically may have to expend undue energy at other kinds of 

 work. Thus, horses bred for generations to the saddle can carry the 

 rider with a smaller expenditure of energy than those whose breed- 

 ing and form specially fit them for draft purposes. 



Additional discussions of the factors influencing the production of 

 work, as applied to horses, are given in Chapter XVIII. 



