After successively examining all the different systems of engines now
in existence, and finding none which, in a plain form, was capable of
fulfilling the conditions imposed, Mr. E.D. Farcot decided to study
out one for himself. Almost from the very beginning of his researches
in this direction, he adopted the Woolf system, which is one that
permits of great variation in the expansion, and one in which the
steam under full pressure acts only upon the small piston. There are
many types of this engine in use, all of which present marked defects.
In one of them, the large cylinder is arranged directly over the small
one so as to have but a single rod for the two pistons; and the two
cylinders have then one bottom in common, which is furnished with a
stuffing-box in which the rod moves. With this arrangement we have but
a single connecting rod and a single crank for the shaft; but, the
stuffing-box not being accessible so that it can be kept in a clean
state, there occur after a time both leakages of steam and entrances
of air.

Mr. Farcot has further simplified this last named type by suppressing
the intermediate partition, and consequently the stuffing-box. The
engine thus becomes direct acting, that is to say, the steam acts
first upon the lower surface of the small piston during its ascent,
and afterward expands in the large cylinder and exerts its pressure
upon the upper surface of the large piston during its descent.
Moreover, the expansion may be begun in the small cylinder, thanks to
the use of a slide plate distributing valve, devised by the elder
Farcot and slightly modified by the son.

As the volume comprised between the two pistons varies with the
position of the latter, annoying counter-pressures might result