VALVES AND SPRINGS. 483
f Pipes, Pistons and plungers,
6. Partners of pressure-organs. Steam-cylindars and pumpbarrels
I and chambers,
Stuffing-boxes, V Valves.
Tension-springs,
r, . Pressure-springs,
c. Springs. ^ v r . *
Bending-springs,
Twisting-springs.
We have here another doubtful point, whether, namely, the valves should be included under II. &, or whether they should be placed in Class III. along with the click-trains formed from rigid elements, to which, as we have seen, they completely correspond. They fall along with pistons and stuffing-boxes (which also strictly speaking belong to click- and ratchet-trains) so naturally, however, and have been so often treated along with them, that the arrange- ment adopted will be on the whole the most convenient. The case is one of those in which logical completeness must be sacrificed to considerations of expediency.
Springs are obviously in their right place among the constructive elements in II. c above. The calculations connected with them fall to a very great extent, however, into the studies of elasticity and the strength of materials. Whether they be treated there or along with the constructive elements must depend upon the cir- cumstances of each particular case.
We may conclude our list of constructive elements with the few which are more or less nearly complete chains, but which almost always occur in machinery as what may be called elementary groups of parts, and which for that reason may be conveniently treated along with the parts more strictly included under the name of constructive elements.
III. Trains.
Click-gear in its simplest forms,
Brakes,
Moveable couplings and clutches.
I r 2
