DRAUGHT IN MARINE BOILERS. 119 



Nevertheless, the following general conclusions may be 

 drawn, to be substantiated or not as fuller information is 

 obtained. 



The conclusions he had formed from a study of these ex- 

 periments are that, for maximum efficiency — 



• The ratio of heating surface to grate requires to be in- 

 creased beyond what is now usual. 



The air should be supplied by artificial means and not by 

 chimne}'- draught, as artificial draught commands a great 

 advantage over natural draught, insomuch that in the 

 former case the air supply can be and is best kept to 

 within 1 lb. in excess of that required for complete com- 

 bustion, ensuring the highest furnace temperature ; whereas 

 in the latter case an excess of as much as 100% is required 

 to give a good chimney draught. 



That air supplied under pressure has a very beneficial 

 effect upon the rate of combustion per sq. ft. of grate. This 

 is generally ascribed to the mechanical action of the air 

 disintegrating the fuel, allowing the distillation of the gases, 

 and the fretting away of the particles of carbon for ad- 

 mixture with the oxygen to be performed more rapidly. 



That forced draught as at present applied cannot be 

 conducive to efficiency, principally on account of the loss 

 by the high temperature of the outgoing gases. 



That any attempt to heat the ingoing air by the out- 

 going gases is more beneficial in the case of natural draught 

 than in that of forced draught. The increase of efficiency in 

 the latter case probably cannot exceed 4%, which cannot be 

 considered commensurate with the complicated and ex- 

 pensive fittings required. 



That with " Natural Draught " the efficiency of the boiler 

 is limited by the loss of heat necessary to maintain the 

 draught (25%), and that although every precaution may 



