MR. hood's CALCULATIOXS. 



39 



occun'ences. It slioiild be a rule from TvMch vre know of only one excep- 

 tion, that no air should be admited into the furnace, beyond that which is 

 absolutely necessary to cause and to maintain combustion, and even that 

 quantity should be made to enter the furnace at the bottom, and to pass 

 through the volume of fuel undergoing combustion. 



The soUtary exception above alluded to is in the case of coal being used, 

 which emits a more than usual quantity of flame, as some of the Stafford- 

 shire and most of the Scotch coal do. In such cases air may be advan- 

 tageously admitted over the fuel and will aid the more perfect combus- 

 tion of the gaseous matter with which such coal is charged. 



The following table and practical remarks have been laid down by Hood. 

 '^The quantity of coal which is required to be burnt in each particular 

 furnace must determine the area of the bars, and as this has been 

 ascertained experimentally for steam boilers it is merely necessary to 

 reduce it to a standard suitable for a hot water boiler. This data being 

 obtained, there will be no difficulty in determiniug the proper size of furnaces, 

 which is, we think, of much importance. Supposing the ordinary kind 

 of furnace bars to afford about thirty inches of opening for the air in each 

 square foot of surface, measured as the bars are placed in the furnace, and 

 allowing half inch openings between the bars, when the bars themselves 

 are about one inch and a half wide, then the relative proportions bet^ een 

 the area of the bars and the length of the pipe should be as follows — 



Area of Bars, 



4-in. Pipe. 



3-in. Pipe. 



2-in. Pipe 



75 square inches wiU supply 150 feet 



or 200 feet 



or 300 feet 



100 



200 



266 



400 



150 .. 



300 



400 



600 



200 



400 



533 



800 



250 



500 



666 



1000 



300 



600 



800 



1200 



400 



800 



1066 



1600 



500 



1000 



1333 



2000 



Thus suppose there are sis hundred feet of pipe, four inches in 

 diameter, in an apparatus, then the area of bars should be three himdred 

 squai'e inches, so that thirteen inches in breadth and twenty three in 

 length win give the requisite quantity of surface ; but when it is required 

 to obtain the greatest heat in the shortest time, the area of the bars 

 should be increased, so that a larger fire may be produced." 



" In order to make the fire bm'n a long time without attention," a \ery 

 important consideration, and probably also to a greater extent than is 

 usually thought in the economy of fuel, " the furnace shoidd extend beyond 



