LOCOMOTIVE ENGINE. 
433 
out thence up the chimney. At the end of this fore stroke the position of the slide 
valve is again reversed to admit the steam in front of the piston and impel it to the 
back of the cylinder, or make the back stroke; the waste steam behind the piston 
escaping through the back port and the inside of the slide into the waste ports as be 
fore : and on repeating the forward motion of the slide, another fore stroke is produced. 
The steam is thus made to produce a reciprocating motion of the piston from one end 
of the cylinder to the other; by moving the slide backwards and forwards from one 
extreme position, when one port is opened to the steam, to the other extreme, when 
that port is closed and the other opened. The motion of the slide is in the same 
direction as that of the piston, but precedes it, as it must take place before 
the stroke of the piston ; and it is produced by the machinery, as will be ex 
plained afterwards. The amount of motion, or length of stroke of the piston, is 
18 inches, and it moves to within half an inch of each of the cylinder covers. In 
stationary engines, the action of the steam in the cylinder is exactly similar; but the 
cjdinders are vertical instead of horizontal, and the strokes of the piston are up and 
down instead of fore and back. The horizontal position of the cylinders is disad 
vantageous in causing an unequal wear of the pistons and cylinders, from the weight of 
the pistons and piston rods acting always on one side; and in also producing a strain 
on the piston rod from the wearing of the piston. However, in locomotives, where 
the pistons are very small and light, this unequal wear is quite imperceptible, 
though in large stationary engines, whose pistons are several feet in diameter, the 
action would be very injurious; and many small stationary engines are also made 
with horizontal cylinders, as the arrangement has several advantages in simplifying 
and strengthening the machinery. 
When the waste steam is let out at the end of each stroke, there is also let out the 
steam occupying the space of half an inch at the end of the cylinder beyond 
the stroke of the piston, and the quantity required to fill the ports, which are both lost 
as they are expended without producing any effect. The steam lost at the ends 
of the cylinder cannot be avoided, as some clearance must be allowed for the piston 
to prevent the chance of its striking against the cylinder covers; and also to 
allow space for the escape of water that may accumulate in the cylinder/ either 
from priming or from condensation of steam. To let out this accumulated water, a 
cock, q, (Plate XC.,) fixed in a boss in the centre of each cylinder cover, is opened 
occasionally. A small pipe, r, (Plates XC. and XCII.,) with a cock in it, is also 
fixed into the lower part of the blast pipe, passing through the bottom of the smoke-box 
for the purpose of letting out any water that may accumulate there. The steam 
lost in the ports can be diminished by shortening them, and for this purpose double 
slide valves have been used; the ports were carried directly up from the cylinder at 
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