the cylinder and steam pipe, so as to prevent more
steam from entering. By throttling the steam, therefore, in the manner here
indicated, the amount of expansion due to the lap may be doubled, so that
an engine with lap enough upon the valve to cut off the steam at two-thirds
of the stroke, may, by the aid of wire drawing, be virtually rendered
capable of cutting off the steam at one-third of the stroke.

196. _Q._--Is this the usual way of cutting off the steam?

_A._--No; the usual way of cutting off the steam is by means of a separate
valve, termed an expansion valve; but such a device appears to be hardly
necessary in ordinary engines. In the Cornish engines, where the steam is
cut off in some cases at one-twelfth of the stroke, a separate valve for
the admission of steam, other than that which permits its escape, is of
course indispensable; but in common rotative engines, which may realize
expansive efficacy by throttling, a separate expansion valve does not
appear to be required.

197. _Q._--That is, where much expansion is required, an expansion valve is
a proper appendage, but where not much is required, a separate expansion
valve may be dispensed with?

_A._--Precisely so. The wire drawing of the steam causes a loss of part of
its power, and the result will not be quite so advantageous by throttling
as by cutting off. But for moderate amounts of expansion it will suffice,
provided there be lap upon the slide valve.

198. _Q._--Will you explain the structure or configuration of expansion
apparatus of the usual construction?

[Illustration: Fig 34.]

_A._--The structure of expansion apparatus is very various; but all the
kinds operate either on the principle of giving such a motion to the slide
valve as will enable it to cut off the steam, at the desired point, or on
the principle of shutting off the steam by a separate valve in the steam
pipe or valve casing. The first class of apparatus has not been found so
manageable, and is not in extensive use, except in that form known as the
link motion. Of the second class, the most simple probably is the
application of a cam giving motion to the throttle valve, or to a valve of
the same construction, which either accurately fits the steam pipe, or
which comes round to a face, which, however, it is restrained from touching
by a suitable construction of the cam. A kind of expansion valve, often
employed in marine engines of low speed, is the kind used in the Cornis