FUELS FOR STEAM TRACTORS 73 



per cent. The heat required to evaporate this must be de- 

 ducted from the 8000 to 8500 B.t.u. which will be contained 

 in a pound of good air-dried wood. A cord of 128 cubic feet of 

 ordinary wood contains 60 to 80 cubic feet of solid wood. 

 When thoroughly air dried, hickory or hard maple weighs about 

 4500 Ibs.; white oak, 3850 Ibs.; birch, red oak, or black oak, 

 3250 Ibs.; poplar, chestnut, or elm, 2350 Ibs.; and average 

 pine, 2000 Ibs. 



Crude oil makes an excellent fuel, being cheap, highly con- 

 centrated, and easily handled. The usual cost is from two to 

 three cents per gallon, which will usually weigh about seven 

 and one half pounds and contain about one and one half times 

 as many heat units per pound as the best steam coal. Many 

 builders of steam tractors furnish attachments for converting 

 coal-burning engines into oil burners. The usual method is 

 to mix oil and steam by forcing them through a jet located in 

 the fire box, the vapor burning readily and producing an abun- 

 dance of heat. Oil is used almost entirely by steam plowing 

 tractors on the Pacific coast, but not extensively elsewhere. 



