LOCOMOTIVE ENGINE. 
451 
the rapidity of the motion, which allows very little time for it to escape, and from the 
use of the blast pipe, which obstructs its passage. The area of the extremity of the blast 
pipe is only five square inches, while that of the steam port is eight square inches, re 
quiring the velocity of the steam in the blast pipe to be considerably greater than in the 
cylinder. The average negative pressure of the waste steam throughout the stroke is 
6 lbs. per square inch when running at the usual rate of 25 to 28 miles an hour; and at 
greater velocities the negative pressure has been found to increase to double that amount 
and even more. The effective pressure of the steam upon the piston at such high 
velocities is considerably below the full pressure of the steam in the boiler; as the 
steam cannot be supplied to follow up the piston so quickly with the full pressure, 
and the regulator has to be only partially opened, so as to throttle the steam and check 
its passage into the cylinders; which diminishes its pressure, as it has still to occupy 
the same space. The negative pressure of the waste steam amounts, for this reason, 
to 30 or 40 per cent, of the positive pressure of the steam upon the piston when the 
engine is running very fast, and the power of the engine is diminished nearly one half. 
For this reason an advantage is obtained by letting out the steam before the end 
of the stroke; and the steam still exerts a very considerable pressure on the piston 
to the end of the stroke, so that the whole power during the stroke is very little 
diminished though the steam begins to be let out before the end, and the resistance 
of this pressure of the waste steam during the next stroke is saved; the lead 
given to the letting out the steam, or the eduction lead, is often made greater than 
the steam lead, to increase this effect. The steam is shut off a little before the end of 
the stroke in consequence of the lead of the slide, and acts expansively for that por 
tion, saving so much of the steam, but diminishing the total power a little; the 
extent of this action is, however, very limited, as the piston is less than a quarter of 
an inch from the end of its stroke when the steam is shut off. In stationary and marine 
condensing engines the steam has usually very little or no lead; but it is shut off 
at two-thirds or three-quarters of the stroke, giving a great amount of expansive 
action; and the eduction has a great deal of lead, the port being nearly full open 
at the commencement of each stroke. 
Feed Pumps.—The feed pumps, K K, (Plates LXXXIX., XC., and XCI.,) are 
fixed by means of flanches to plates which are bolted on to the frame of the engine ; 
they are fixed on the outside of, and a little below the piston rods, and exactly parallel 
to them. Each pump is worked by an arm, G G, fixed on to the piston rod; it has a 
socket at the end, fitted on the piston rod, and fixed by a small pointed screw tapped 
into it and bearing against the piston rod ; the arm is inclined obliquely downwards, so 
as to clear the guides of the piston rod as it is moved backwards and forwards by the