Geography of Ohio 181 



fuel must be varied to suit types of coal; a particular coal may have 

 low efficiency in a furnace which will get a much higher amount of 

 heat out of another coal to which it is adapted. The laboratory 

 work now under way by the federal government, as well as by 

 some of the state institutions, will add materially to the impor- 

 tance of the lower grade coals. 



