g these rules, all the dimensions are to be taken in
inches, and the answers will be found in inches also.

193. _Q._--Is it a benefit or a detriment to open the eduction passage
before the end of the stroke?

_A._--In engines working at a high rate of speed, such as locomotive
engines, it is very important to open the exhaust passage for the escape of
the steam before the end of the stroke, as an injurious amount of back
pressure is thus prevented. In the earlier locomotives a great loss of
effect was produced from inattention to this condition; and when lap was
applied to the valves to enable the steam to be worked expansively, it was
found that a still greater benefit was collaterally obtained by the earlier
escape of the steam from the eduction passages, and which was incidental to
the application of lap to the valves. The average consumption of coke per
mile was reduced by Mr. Woods from 40 lbs. per mile to 15 lbs. per mile,
chiefly by giving a free outlet to the escaping steam.

194. _Q._--To what extent can expansion be carried beneficially by means of
lap upon the valve?

_A._--To about one-third of the stroke; that is, the valve may be made with
so much lap, that the steam will be cut off when two thirds of the stroke
have been performed, leaving the residue to be accomplished by the agency
of the expanding steam; but if more lap be put on than answers to this
amount of expansion, a very distorted action of the valve will be produced,
which may impair the efficiency of the engine. If a further amount of
expansion than this is wanted, it may be accomplished by wire drawing the
steam, or by so contracting the steam passage that the pressure within the
cylinder must decline when the speed of the piston is accelerated, as it is
about the middle of the stroke.

195. _Q._--Will you explain how this result ensues?

_A._--If the valve be so made as to shut off the steam by the time two
thirds of the stroke have been performed, and the steam be at the same time
throttled in the steam pipe, the full pressure of the steam within the
cylinder cannot be maintained except near the beginning of the stroke where
the piston travels slowly; for, as the speed of the piston increases, the
pressure necessarily subsides, until the piston approaches the other end of
the cylinder, where the pressure would rise again but that the operation of
the lap on the valve by this time has had the effect of closing the
communication between