THE CHOICE OF POWER 



293 



converts from 10 to 15 per cent, of the heat energy in its fuel 

 into useful work. In stationary work the power of the horse 

 is decreased in transmission, while both types of tractor save 

 the enormous loss due to the propulsion of their own weight, 

 the steam engine delivering 4 to 6 per cent, and the gas engine 

 from 20 to 25 per cent, of its fuel energy to the driving belt. 



The local cost and quality of fuels will vary widely, but the 

 following table, showing the heat units which could be 

 bought wholesale for $1.00 under northern Indiana conditions, 

 gives a fair idea of the relative cost of energy in various fuels: 



ONE DOLLAR'S WORTH OF HEAT UNITS IN VARIOUS FUELS 



Low-priced fuel is not necessarily cheap fuel, as its value 

 depends largely upon the heat units it contains. Moreover, 

 some cheap fuels are as cheap as they are, largely because 

 scarcity has led to the general abandonment of their use. 

 Cordwood, for instance, stands second on the list in point of 

 cheap heat units, yet if any considerable dependence were 

 placed upon its use for power, its price would be prohibitive. 

 Some of the lightest petroleum products are in the same 

 category, and, as we have seen, others are tending to become 

 scarce. At the prevailing prices it would seem that even the 

 more abundant petroleum fuels are almost prohibitive in cost 

 for a given quantity of heat units. However, even heat 

 units are not a reliable indication of the value of a fuel for 



