416 STEPHENSON'S PATENT 



inside at the bottom where it joins on to the cylinders, and tapers to 2|- inches at 

 the top. 



The waste steam rushes out of the pipe with great force up the chimney, carrying 

 the air with it, and causing a very powerful draught through the tubes and the fire ; 

 a whole cylinder full of steam is let out at each stroke, and the two cylinders deliver 

 their waste steam alternately, so that when the engine is running fast an almost 

 constant current of steam in the chimney is produced, and the interval between the 

 blasts can scarcely be perceived. By this method the fire is not blown, as is usual, 

 by forcing air into it, but by extracting the air from the flues and drawing air 

 through the fire. In the first locomotives no means were used for increasing the 

 draught of the chimney, and their power of generating steam was consequently very 

 limited ; the introduction of the steam blast for urging the fire, and of the tubes for 

 conveying the heated air through the water, are the principal causes of the great 

 power of the present locomotives. 



There is, however, a considerable loss of power attending the use of the blast-pipe, 

 from the obstruction it causes to the egress of the waste steam ; for the waste steam 

 opposes the action of the steam in the cylinders, and should be allowed to escape 

 freely that its pressure might be as small as possible. This causes the greater 

 economy of working in the large stationary and marine engines, where the waste 

 steam is condensed and its opposing pressure is reduced almost to nothing : but in a 

 locomotive engine, on the contrary, its average resistance is not less than 61bs. on the 

 square inch ; and when running very fast, and the issue of waste steam is almost 

 continuous, the whole loss of power amounts to nearly half that of the engine. But 

 the draught must be obtained by an expenditure of some of the power of the engine, 

 whatever means maybe em ployed to produce it; and the plan of producing it by the 

 blast of waste steam is the best, as no power is wasted upon working any machinery 

 for the purpose, and it has the advantage of great simplicity in its application. In 

 some locomotive engines, made by Messrs. Braithwaite and Ericcson, a revolving 

 fan-wheel, worked by the engine, has been used to perform the same operation of 

 drawing the air from the flues ; and in the " Novelty," by the same makers, one of 

 the engines that competed for the prize on the Liverpool and Manchester Railway, 

 air was forced into the fire-box, but that plan was afterwards abandoned from 

 its being found not so advantageous. 



The force of the draught produced by the steam-blast is so great that cinders are 

 drawn through the tubes and even thrown red hot out of the top of the chimney ; 

 sparks are also emitted occasionally, and have sometimes caused accidents. To pre- 

 vent the cinders and sparks from getting out of the chimney, a wire sieve is often 

 fixed on the top of the chimney, but this has a disadvantage in impeding the draught 



