DRAUGHT IN MARINE BOILERS. 121 



Now having regard to the fact that neither of the fore- 

 going systems of draught appears to satisfy the conditions 

 necessary to maximum efiBciency, which is most nearly 

 approached by that in use with the D boiler, it appears 

 that the best result would be obtained by some system of 

 artificial draught, whereby on the one hand some portion of 

 the 25% lost in the creation of natural draught, and on 

 the other hand some of the 33% loss consequent on the 

 high temperature of the outgoing gases in the case of forced 

 draught, might be prevented ; and that probably some 

 system of artificial draught suppl^dng the minimum quan- 

 tity of air required for combustion, at a pressure not exceed- 

 ing one inch of water, and heated by the escaping gases, 

 combined with such an extension of heating surface as 

 would ensure that the temperature of the outgoing gases be 

 not much above that of the water in the boiler, would be 

 found to give the best results in economy of consumption. 



It is only left to call attention to the analysis of the out- 

 going gases, and to the fact that the analysis shows that the 

 combustion in ordinary working is much more perfect than 

 perhaps is generally supposed, as in each case the whole of 

 the carbon has been burnt to carbonic acid, a result which 

 few would perhaps have been willing to admit without such 

 evidence. 



