416 
STEPHENSON’S PATENT 
inside at the bottom where it joins on to the cylinders, and tapers to 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 6 lbs. 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 employed 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