_A._--In a high pressure engine the steam, after having pushed the piston
to the end of the stroke, escapes into the atmosphere, and the impelling
force is therefore that due to the difference between the pressure of the
steam and the pressure of the atmosphere. In the condensing engine the
steam, after having pressed the piston to the end of the stroke, passes
into the condenser, in which a vacuum is maintained, and the impelling
force is that due to the difference between the pressure of the steam above
the piston, and the pressure of the vacuum beneath it, which is nothing;
or, in other words, you have then the whole pressure of the steam urging
the piston, consisting of the pressure shown by the safety-valve on the
boiler, and the pressure of the atmosphere besides.

3. _Q._--In what way would you class the various kinds of condensing
engines?

_A._--Into single acting, rotative, and rotatory engines. Single acting
engines are engines without a crank, such as are used for pumping water.
Rotative engines are engines provided with a crank, by means of which a
rotative motion is produced; and in this important class stand marine and
mill engines, and all engines, indeed, in which the rectilinear motion of
the piston is changed into a circular motion. In rotatory engines the steam
acts at once in the production of circular motion, either upon a revolving
piston or otherwise, but without the use of any intermediate mechanism,
such as the crank, for deriving a circular from a rectilinear motion.
Rotatory engines have not hitherto been very successful, so that only the
single acting or pumping engine, and the double acting or rotative engine
can be said to be in actual use. For some purposes, such, for example, as
forcing air into furnaces for smelting iron, double acting engines are
employed, which are nevertheless unfurnished with a crank; but engines of
this kind are not sufficiently numerous to justify their classification as