292 POWER AND THE PLOW 



claims as to the class of labor which can safely be intrusted 

 with it. 



Of the three classes, the horse develops continuously the least 

 power in proportion to his weight, and his bulk is distributed 

 over a wider ground area, making his use in large units cum- 

 bersome. For each horse in the field, 20 to 25 square feet 

 must be allowed, while in a tractor the power of sixty horses 

 may be concentrated in a single unit, requiring less than 5 

 square feet for each horsepower. In the units where horses 

 and tractors compete, the engine is much more easily ma- 

 nipulated. While the bulk of the tractor is concentrated, the 

 actual supporting surface on the ground is sufficient to reduce 

 the pressure per unit to, or only slightly above, the pressure of 

 the horse's foot. 



The commercial success of any type of power depends to 

 considerable extent on its fuel economy. The steam engine 

 at the present time uses a wide range of fuels, taking them 

 just as they come. Its sources of supply are wide, and its 

 most convenient fuel, coal, is by no means exhausted. The 

 gas engine, as used on tractors, requires fuel that has been made 

 fit for use by a refining process, hence the sources of supply 

 are more easily controlled by individuals. The steam engine 

 of the present and the alcohol engine of the future will make 

 use of all the waste materials about the farm. The horse 

 utilizes no waste material, except certain unimproved pas- 

 tures, and for the production of power requires a large part of 

 the most valuable products of the farm. His fuel, however 

 may be raised by every farmer every year, and on many sections 

 of Western prairie, a great portion of his maintenance still 

 comes from wild hay, which can be stored during slack seasons 

 at the mere cost of gathering. 



In utilizing fuel, the average farm horse in any year prob- 

 ably converts less than 2 per cent, of heat supplied into energy 

 delivered. The steam engine in plowing has only the same 

 ratio or a trifle more, while the gas tractor at the same time 



