STEAM NAVIGATION. 



is fed, the door being open, a stream of cold air rushes in, passing 

 over the burning fuel and lowering the temperature of the flues : 

 this is an evil to be avoided. But, on the other hand, if the fur- 

 naces be fed at distant intervals, each furnace will be unduly 

 heaped with fuel, a great quantity of smoke will be evolved, 

 and the combustion of the fuel will be proportionally imperfect. 

 The process of coking in front of the grate, which would insure a 

 complete combustion of the fuel, has been already described in our 

 tract on the Steam Engme. A frequent supply of coals, however, 

 laid carefully on the front part of the grate, and gradually pushed 

 backwards as each fresh feed is introduced, would require the fire 

 door to be frequently opened, and cold air to be admitted. It 

 would also require greater vigilance on the part of the stokers, 

 than can generally be obtained in the circumstances in which they 

 work. In steam-vessels the furnaces are therefore fed less fre- 

 quently, fuel is introduced in greater quantities, and a less perfect 

 combustion produced. 



When several furnaces are constructed under the same boiler, 

 communicating with the same system of flues, the process of feed- 

 ing, and consequently opening one of them, obstructs the due 

 operation of the others, for the current of cold air which is thus 

 admitted into the flues checks the draught, and diminishes the 

 efficiency of the furnaces in operation. It was formerly the practice 

 in vessels exceeding one hundred horse-power, to place four 

 furnaces under each boiler, communicating with the same system 

 of flues. Such an arrangement was found to be attended with a 

 bad draught in the furnaces, and therefore to require a greater 

 quantity of heating surface to produce the necessary evaporation. 

 This entailed upon the machinery the occupation of more space in 

 the vessel in proportion to its power ; it has therefore been more 

 recently the practice to give a separate system of flues to each 

 pair of furnaces, or, at most, to every three furnaces. When three 

 furnaces communicate with a common flue, two will always be in 

 operation, while the third is being cleared out ; but if the same 

 quantity of fire were divided among two furnaces, then the clearing 

 out of one would throw out of operation half the entire quantity 

 of fire, and during the process the evaporation would be injuriously 

 diminished. 



46. It is found by experience, that the side plates of furnaces are 

 liable to more rapid destruction than their roofs, owing, probably, 

 to a greater liability to deposit. Furnaces, therefore, should not 

 be made narrower than a certain limit. Great depth from front to 

 back is also attended with practical inconvenience, as it renders 

 firing tools of considerable length, and a corresponding extent of 

 stoking room necessary. It is recommended by those who have had 

 H6 



