154 HEATING. 



for large boilers is unnecessary. A very moderate heat, ap- 

 plied on the most economical principle, and the furnace so con- 

 structed as to make the fuel burn for a long time, without much 

 attention, and without much escape of smoke, is the grand 

 desideratum, and which is easily accomplished, with a moderate 

 degree of care and skill in the erection. 



Passing over, then, as unnecessary for our purpose at present, 

 the many ingenious forms which have been given to furnaces, 

 we will proceed to describe the most simple plan, which, in our 

 experience, is the most effectual in the combustion of the fuel, 

 as well as the least expensive in the construction. 



It should be an object of consideration, in building the fur- 

 nace, to confine the generated heat within the cavity of the 

 furnace as much as possible, so that the gases generated by the 

 combustion of fuel may be prevented from passing too rapidly 

 along the flue ; this is more especially requisite with boiler 

 furnaces. The throat of the furnace should be contracted as 

 much as possible. In furnaces where the only entrance for air 

 is by the bars, provision should be made for the entrance of 

 enough but no more than enough for the combustion of 

 the fuel, and the entrance should, in all cases, be regulated by 

 a damper, on the ash-pit door. It should be considered, that 

 the rarity of the heated gases causes them to force their pas- 

 sage through the throat of the furnace, just in the proportion 

 of its size. 



We have already shown that any air entering through the 

 door of the furnace reduces the intensity of the heat, although 

 it is supposed by some that the passage of air over the burning 

 fuel promotes the more perfect combustion of the gaseous pro- 

 ducts of the coal. But even if this be correct, the heat will be 

 reduced, and less heat will be generated in a given time, than 

 if the whole gaseous products escaped by the chimney. 



The kind of fuel to be burnt must, in all cases, determine 

 the width of the bars ; and as a certain open area is necessary 

 for the admission of air to effect combustion, it is desirable that 

 this area should be known. 



Supposing the ordinary kind of furnace bars to afford about 

 thirty inches of opening for air for every square foot of surface, 



