All About Engines

88 All About Engines examples. And everyone knows, too, that the higher the chimney the greater the draught, for a fire never burns so well in the topmost room of a house as it does in lower rooms. If the average temperatures inside and outside a chimney are 1200 Fahr, and 50° Fahr., the air inside will weigh only seven-eighths of an equal volume of air outside, and the force tending to drive air up the chimney will be equal to one- eighth of the weight of a column of outside air, equal in volume to that contained in the chimney. Sup- pose the chimney to be 50 feet in height and 1 square foot in area of cross section, the force called into play would be nearly J lb. The first man to use artificial means of increasing the draught was George Stephenson, who, in the locomotives he built for hauling coal at the Killing- worth Colliery in 1815, conceived the idea of turn- ing the exhaust steam into the chimney. This was effective in two ways. Firstly, since water vapour is lighter than the gases, it reduced the weight of the gases in the chimney ; and secondly, as we have seen in the case of the injector, a jet of liquid or gas carries the surrounding gases along with it and creates a reduced pressure in its neighbourhood. The motion of a locomotive itself produces a draught. Air enters the front of the ashpit—is scooped up, as it were—and passes upwards through the firebars, and the draught is greater the higher the speed of the train. But notwithstanding this, all locomotives to this day take advantage of the exhaust steam to increase the fierceness of the fire ; and the draught