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 Ibs. 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 



