THE CHOICE OF POWER 295 



to ten horsepower. In an emergency each horse may double 

 or triple his power, and sustain it for some time without injury. 

 The steam engine through the expansive force of steam, is more 

 flexible than the gas tractor and the wide range hi point of 

 cutoff gives it admirable overload capacity. In the gas engine 

 the maximum power is developed at the moment of explosion. 

 A considerable momentum, sufficient to carry the tractor over 

 ordinary emergencies, is stored in the flywheel. However, 

 if a sudden obstacle overloads the gas tractor while its speed 

 is reduced, it promptly stops working. In that respect it is 

 perhaps the most advanced of the three, since it tolerates less 

 abuse on the part of the operator. 



As conditions are found on the farm, the horse has probably 

 the widest range of usefulness. His economy in small power 

 units, his great overload capacity, and the fact that he is 

 ordinarily ready at an instant's notice, make him by far the 

 most convenient power for widely diversified work. For oper- 

 ations requiring stationary power or great tractive power, he is 

 seriously handicapped, and yields to both types of tractor. 

 The steam engine is economical of labor only in the largest 

 units, hence is adapted more particularly for enormous 

 volumes of work in the field. Its great weight adapts it to 

 plowing new land and heavy hauling on good roads, rather than 

 for lighter work, though the lack of suitable bridges reduces 

 its usefulness in the latter particular. The gas tractor is eco- 

 nomical of fuel and labor in both large and small units. It is 

 generally lighter than the steam tractor for the same tractive 

 effort and is made in more widely divergent forms. It is 

 cleaner and more convenient to handle, is much quicker to start, 

 and undoubtedly stands second to the animal in general utility. 



In so far as it is possible to generalize in ranking the three 

 types of power for plowing purposes on the foregoing essen- 

 tials, it has been done in the following table. Only the stand- 

 ard representatives of each type are, of course, considered. 

 Aside from the factors, involved in selecting a motor for plow- 



