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 directions. Engines, as I have already said,
are sometimes made double acting, though unprovided with a crank; and there
would be no difficulty in so arranging the valves of all ordinary pumping
engines, as to admit of this action; for the pumps might be contrived to
raise water both by the upward and downward stroke, as indeed in some mines
is already done. But engines without a crank are almost always made single
acting, perhaps from the effect of custom, as much as from any other
reason, and are usually spoken of as such, though it is necessary to know