er to condensing engines working at the ordinary speed; but the
excess is but small, and is upon the right side. For engines travelling
very fast it gives a good deal more area than the common proportion, which
is too small in nearly every case. In locomotive engines the eduction pipe
passes into the chimney and the force of the issuing steam has the effect
of maintaining a rapid draught through the furnace as before explained. The
orifice of the waste steam pipe, or the blast pipe as it is termed, is much
contracted in some engines with the view of producing a fiercer draught,
and an area of 1/22d of the cylinder is a common proportion; but this is as
much contraction as should be allowed, and is greater than is advisable.

321. _Q._--In engines moving at a high rate of speed, you have stated that
it is important to give the valve lead, or in other words to allow the
steam to escape before the end of the stroke?

_A._--Yes, this is very important, else the piston will have to force out
the steam from the cylinder, and will be much resisted. Near the end of the
stroke the piston begins to travel slowly, and if the steam be then
permitted to escape, very little of the effective stroke is lost, and time
is afforded to the steam, before the motion of the piston is again
accelerated, to make its escape by the port. In some locomotives, from
inattention to this adjustment, and from a contracted area of tube section,
which involved a strong blast, about half the power of the engine has been
lost; but in more recent engines, by using enlarged ports and by giving
sufficient lead, this loss has been greatly diminished.

322. _Q._--What do you call sufficient lead?

_A._--In fast going engines I would call it sufficient lead, when the
eduction port was nearly open at the end of the stroke.

323. _Q._--Can you give any example of the benefit of increasing the lead?

_A._--The early locomotives were made with very little lead, and the
proportions were in fact very much the same as those previously existing in
land engines. About 1832, the benefits of lap upon the valve, which had
been employed by Boulton and Watt more than twenty years before, were
beginning to be pretty generally apprehended; and, in the following year,
this expedient of economy was applied to the steamer Manchester, in the
Clyde, and to some other vessels, with very marked success. Shortly after
this time, lap began to be applied to the valves of locomotives,