473
KINEMATICS OF MACHINERY.
pressure-organ. In other words, piston-pumps with valves are fluid ratchet-trains.
The view which this proposition affords us of the different systems of pump construction appears to me to be extremely instructive, and greatly to simplify the whole matter. It is interesting to notice that both free ratchet-gear and piston and valve pumps were known and had attained some degree of com- pleteness before the introduction of the steam-engine ; both click and ratchet pawls and lifting valves are force-closed arrangements,
FIG. 352.
FIG. 353.
and so came earlier in the natural course of machine development. We can observe also a distinct tendency making itself felt among modern engineers to supersede the force-closed motion of the valves by a constrained motion- as in pumps with slide valves, etc.*
- Cf., for instance, Hanel's suction valves in Scholl's Fiihrer des Maschinisten
8th Ed. p. 419.
