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