BOILERS, FUELS AND FLUES 221 



true in large establishments. The price 

 varies with the quality, distance from the 

 mines, etc. 



The average cost for soft coal to 61 grow- 

 ers, living east of the Mississippi River, for 

 the season of 1911-12, was $2.33 per ton. The 

 average amount used for the season was n.6 

 tons for each 1,000 square feet under glass. 



Underfed Boilers. The term "underfed" 

 is applied to a method of stoking, in which 

 the coal is fed from the bottom instead of 

 the top of the furnace. It is claimed for 

 this system that it insures a more perfect 

 combustion and that cheaper grades of coal 

 may be used. Boilers employing this prin- 

 ciple have not come into very general use in 

 greenhouse heating, probably because they 

 will not handle successfully all grades of 

 coal. 



Self-stoking Boilers. Stoking devices are 

 practical only in large establishments us- 

 ing large boilers. There are several types, 

 some of which work on practically the same 

 principle as the underfed furnaces mentioned 

 above, except that their action is automatic. 

 In other forms the grate bars are arranged 

 in the form of an endless chain, which is 



