G. This shaft has for its bearings two supports, b, attached to the
reservoir, and carries the driving pulleys and a fly wheel. The beam, F,
having to give motion to the piston in describing an arc of a circle at
the extremity attached to the connecting rod, must, for that reason,
have a fixed point of oscillation, or one that we must consider as such
for the instant. Now, such point is selected on a piece, H, having the
shape of the letter C, and which plays an important part in the working
of the pump. This piece is really a two-armed lever, having its center
of oscillation in two brackets, c, at the base of the reservoir. Fig. 17
shows the relation of the beam, F, and lever, H. The upper extremity of
this latter is forked, and embraces the beam, F, whose external surfaces
are provided with two slots, d, in which to move slides, e, attached to
studs, f, which are perfectly stationary on the extremities of the forks
of the lever, H. One of the slots is shown in section on the line 1--2
in Fig. 20, and on the line 3--4 in Fig. 21.

Things thus arranged, if we suppose the piece, H, absolutely stationary,
it is clear that, as the oscillation of the beam, F, is effected on the
studs, f, as centers, the piston of the pump will perform an invariable
travel whose extent will be dependent upon its position between such
point of oscillation and the point of articulation of the connecting
rod, G. But we must observe that even according to such a hypothesis,
the point, f, would not be entirely stationary, because the point
of articulation, a, upon the piston rod being obliged to follow an
invariably straight line, the slots, d, will have to undergo an
alternate sliding motion on the slides, e, save, be it understood,
when the latter are brought to coincide exactly with the center of
articulation, a. Now we shall, in fact, see that the point, f, can move
forward in following the slots, d, and that it may even reach the point
of articulation, a, of the beam, F, on the rod, E, that is to say,