418 ENGINEERING ON THE FARM 



to 3 inches, egg. Buckwheat is the cheapest grade and nut 

 the highest priced. Hard coal gives off but little more 

 heat than good soft coal, an£ about one-quarter more than 

 the ordinary soft coal. More heat is obtained from hard 

 coal because there is very little soot. When soot collects 

 on the heating surfaces, it prevents the passage of heat to 

 the water or air of the system. For hard coal a deep 

 fire box gives best results. Hard coal burns slowly and 

 requires less air than soft coal or wood, and the deep fire 

 serves as a check on the air. Soft coal and wood burn 

 best in a thin layer over a larger surface and require 

 more frequent firing. 



Firing. Economical stoking is an art. Hard coal is 

 popular because the average person does not care to fire 

 every half-hour. However, a little admixture of brains 

 with the coal will pay, even with hard coal. In general, add 

 as small a quantity of coal as possible at each firing, and 

 fire often. D.o not entirely cover the bed of live coals, but 

 leave a small hole, where sufficient heat can get through 

 to fire the gases as they distill off from the coal; otherwise 

 they are lost up the chimney. These gases burn clear and 

 hot and form a large part of the coal. Keep the grates 

 clean and clear of clinkers. Use a slice-bar, and prevent 

 a tendency to cake at the bottom of the fire. 



Coke. Coke is formed from a soft coal partly burned 

 without sufficient air for complete combustion. After the 

 smoke and soot have been driven off, the fire is quenched 

 with water. What is left is light and porous and burns 

 well. Coke may be made of lignite or peat. The market 

 offers a petroleum coke, which is not coke at all, but a 

 residue from oil-refining retorts. 



Wood. Wood may be used in sections where it is plenti- 

 ful and can be had without a long freight haul. Two cords 

 of wood equal in heat units, approximately, a ton of good 

 coal. A cord of wood is 8 feet long, 4 feet wide, and 4 

 feet high. 



