ssure 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
a distinct species, and, in general, those engines may be considered to be
single acting, by which no rotatory motion is imparted.

4. _Q._--Is not the circular motion derived from a cylinder engine very
irregular, in consequence of the unequal leverage of the crank at the
different parts of its revolution?

_A._--No; rotative engines are generally provided with a fly-wheel to
correct such irregularities by its momentum; but where two engines with
their respective cranks set at right angles are employed, the irregularity
of one engine corrects that of the other with sufficient exactitude for
many purposes. In the case of marine and locomotive engines, a fly-wheel is
not employed; but for cotton spinning, and other purposes requiring great
regularity of motion, its use with common engines is indispensable, though
it is not impossible to supersede the necessity by new contrivances.

5. _Q._--You implied that there is some other difference between single
acting and double acting engines, than that which lies in the use or
exclusion of the crank?

_A._--Yes; single acting engines act only in one way by the force of the
steam, and are returned by a counter-weight; whereas double acting engines
are urged by the steam in both direct