72 POWER AND THE PLOW 



contain from 12,000 to 14,500 B.t.u. per pound. The best 

 coal contains from 80 to 90 per cent, of carbon and hydrogen. 



Crude oil and its products are nearly pure carbon and hydro- 

 gen in varying proportions. Wood and straw contain only 

 about 50 per cent, of carbon. A large part of the possible 

 oxidation has already taken place in the latter, hence their 

 heating value is low. Alcohol, as it will be seen later, comes 

 from vegetable materials and has also a lower thermal value 

 than coal and oil. Were it possible for all engines to utilize 

 all fuels with equal efficiency, the steam engine would have 

 a tremendous advantage on account of the cheapness of its 

 fuel. If its energy could all be utilized, one pound of coal, 

 costing two fifths of a cent and having 14,000 B.t.u., would 

 produce about eleven million foot-pounds of work, or ap- 

 proximately the useful work of one horse for one day. 



Straw is used only where it has no value for feeding or 

 bedding purposes, consequently straw-burning engines are 

 commonly found only on the Great Plains. If we disregard 

 the fertilizing value of the straw, the use of it in engines is a 

 source of economy, as the first cost of coal is saved. The labor 

 of keeping the engine supplied with straw is practically the same 

 as the average for hauling coal from the railway station. The 

 use of straw in plowing is not convenient, as a large tender must 

 be carried for rounds of any length. For threshing, however, 

 it is convenient and cheap. Straw contains about 8000 B.t.u. 

 per pound, or a little more than half the heat value of the best 

 grades of coal. It requires somewhat more skill in firing than 

 does coal, owing to the rapidity with which it generates heat. 



Wood is used to a very limited extent in plowing, owing to 

 the scarcity of timber on most of the Great Plains area. Where 

 abundant, however, it can be used at the rate of about two 

 and one fourth pounds of wood for one pound of coal. Air- 

 dried wood will net about 5000 to 6000 B.t.u. per pound. 

 Fresh wood contains from 30 to 50 per cent, of moisture, ac- 

 cording to species, and seldom dries in the air to less than 20 



