74 ANNUAL OF SCIENTIFIC DISCOVERY. 



black smoke is not so much the want of a draught, as the stoppage and com- 

 bustion of the products of the fuel, nearest to the grate bars, being passed 

 through a layer of fresh fuel and upon the incandescent. By far the greater 

 quantity of smoke is caused by the passing of the heated gases through this 

 layer of cool, fresh fuel, and the better the draught the blacker and more 

 abundant will be the smoke. The means, then, by which the formation of 

 smoke can be prevented, or consumed wjien formed f are draught, adequate 

 dimensions of boiler, and good management. 



Adequate dimensions of the flue or chimney are primarily necessary to pre- 

 vent or consume smoke. Adequate dimensions of the boiler are not necessary 

 to consume smoke, but to consume it economically and consistently with the 

 performance of the work to be done. Draught is the first requisite, for with- 

 out it smoke can neither be prevented from burning nor consumed after form- 

 ation. To many it may appear a very extraordinary thing, that when 

 smoke is burned, less steam is raised in a given time. It seems sound reasoning 

 to say smoke is fuel, and since steam is raised by the burning of fuel, the burn- 

 ing of smoke should raise steam. This is so far correct ; but I have observed 

 that in many cases where smoke is consumed by the admission of air above and 

 not through the fuel, there is not so much coal consumed, and since coal is fuel, 

 it is evident that the burning of smoke may, by decreasing the consumption of 

 fuel, lessen the heat of the furnace, and thereby reduce the quantity of water 

 evaporated in a given time. In a steam boiler furnace, if the air be admitted 

 above the fuel, then it is consumed less quickly, and the rate of evaporation 

 is slower ; and as the most active part of the boiler is that right over the fire, 

 and for a short distance beyond the bridge, it is evident that any reduction of 

 the heat of that fire will have to be balanced by increased heat in the flues, 

 and whether that will or will not be the case, when smoke is burned, will de- 

 pend upon the relative constituent parts of the fuel, the setting of the boiler, 

 and the power of the draught. Nothing can be more perplexing and unsat- 

 isfactory than the comparative results of experiments made in different places, 

 when the construction and setting of the boiler, with the dimensions of the 

 several parts, aro not also given. 



A good draught in a furnace is the main element in its economy. I have 

 known a ton of coal saved in a week by adding to the height of the chimney; 

 the sharper tho draught the greater is the heat, and the greater the heat the 

 more perfect the combustion, and the more perfect the combustion the greater 

 is the economy, and, with good management, the less the smoke thus draught 

 is the prime requisite both for economy and for the consumption of smoke. 



About twelve year's since, a warm controversy was carried on respecting 

 the feeding of furnaces with warm or cold air. Mr. Muir thinks it does not 

 much matter if it be or be not heated before admission into the furnaces ; 

 first, because, though it is heated, the degree is not very high ; and secondly, 

 because it is generally heated by the furnace itself, and in that case heating 

 the air is like robbing Peter to pay Paul. When the air is heated, a larger 

 aperture is required for the admission of the required quantity. When the 

 aperture is of the requisite size, it is so much gained if, before entering, the 

 air could receive heat which would otherwise be 



