1873 - Old Forge Coal Mines

56 ANNUAL REPORT OF THE
densing engines it is common to allow the area of each steam port to equal
]-30th, and in high pressure engines l-16th of the square of the cylinder's
diameter.
To enable the eccentric to approximate hand power, the slide valves are
placed at select distances in advance of the piston, and these distances are
termed leads, so that at each return of stroke the steam in the cylinder
may approach to an equal density with that in the steam boiler. Where
high velocities are necessary the lead of the valves is of great advantage
in cutting off the steam before the engine completes its full length of stroke,
but where power is the object it requires the steam to be continued to the
end of the stroke.
Here are illustrated the two extremes of the mechanical power of steam.
The one point is that of running at full speed and carrying no weight ; the
other is that of being loaded until the engine cannot move. The duty then
of the practical mind is to study the useful effect.
The following results are given by engineers of passenger steam packets
and locomotive engines:
When the lead of the valve is inch the velocity=l.
Do do do.' % " do =1.01