450
STEPHENSON’S PATENT
for four years, before requiring to be rebored, but the time varies much with the
quality of the metal, it being necessary sometimes much sooner.
A larger passage for the entrance of the steam is required in a locomotive than in
a stationary engine, in proportion to the size of the cylinders; as the piston moves
quicker, and the steam has to be admitted proportionally quicker. The best velocity
for the piston of a steam engine is given by Watt as 220 feet per minute; and the
area of the steam port, so as to admit the steam to move the piston at that velocity
with its full pressure, he gave as one twenty-fifth of the area of the cylinder. In this
locomotive, the velocity of the piston when the engine is running at 20 miles an hour
is 350 feet per minute nearly, and at 40 miles an hour, nearly 700 feet per minute ;
the usual velocity being about 440, or double of the velocity in stationary engines. The
size of the ports is one fourteenth of the cylinder, or rather less than Watt’s proportion,
which would be one twelfth and a half, as the piston moves twice as fast; the steam
ports in some locomotives are made as large as one eleventh, and in others only one
seventeenth of the cylinder, but one fourteenth appears to be a very good pro
portion.
The slide begins to open the steam port a little before the commencement of the
stroke of the piston, so that the steam is shut off from the piston and let on to the
opposite side for the commencement of the next stroke, a little before the end of
each stroke; acting for this interval in opposition to the motion of the piston.
This is called the lead of the slide, and it is made generally about a quarter of an
inch, being produced by fixing the eccentrics a little in advance of the position
at right angles to the cranks. It is found necessary to let the steam on to the opposite
side of the piston before the end of the stroke, in order to bring it up gradually
to a stop, and diminish the violent jerk that is caused by its motion being changed so
very rapidly as five times in a second. The steam, let into the end of the cylinder
before the piston arrives at it, acts as a spring cushion to assist in changing its motion,
and if it were not applied, the piston could not be kept tight upon the piston rod.
A little lead of the slide is also necessary that the steam may be admitted through the
port into the cylinder, and be completely ready to begin the next stroke when the piston
is at the end of the cylinder; but so much is not necessary for this.
The principal advantage gained by giving lead to the slide is in beginning to get rid
of the waste steam before the commencement of the stroke; so that when the piston
commences its stroke there is but little waste steam before it to resist its progress, the
steam beginning to be let out of the cylinder before it has driven the piston to the
end of the stroke. This is a very important point in a locomotive, as the resistance
or negative pressure of the waste steam upon the piston is very considerable; from