474 KINEMATICS OF MACHINERY.
which remarkable omission we probably owe to the fact that the pressure-organ machines have been almost entirely left without kinematic treatment of any kind. I have elsewhere" 4 attempted a classification of valves according to their constructive characteristics which may be of service to us so far as it goes. It is as follows :
1. Valves which slide, including
a. Cocks and disc-valves,
b. Slide-valves ;
2. Valves which lift, including
(a). Clacks, hinged-valves, (&). Direct lift-valves.
I gave as the essential difference between the two classes that the fluid pressure upon the sliding-valves had no tendency either to open or to close them, while in the lifting- valves it did both, according to the direction in which it acted. The latter, therefore, can be used as self-acting valves, while the former cannot.
There is a good deal to be said for this classification, which does reach to some extent below the surface. It is, however, by no means exhaustive. It is founded on an examination of its subject from without and not from within, and so fails when it is carried to extreme cases ; in reference, for instance, to those lifting valves which are completely balanced, and which therefore do not possess the property above named as that characteristic of lifting-valves in general. The division also stands so far upon the same ground as those of the old descriptive school that it does not fully explain its own definitions, and especially that it gives no indication of the position of the valves among kinematic arrangements. Now that we have familiarised ourselves with kinematic ideas by a series of analytical exercises, it is possible to give a definition which really goes to the root of the matter. It is this : Valves arid their connections form the click-trains, and under certain circumstances the brakes, of the pressure-organs.
Among these valve-trains also both free and fast clicks exist. The self-acting lifting valves are free clicks, that is, they permit motion past them in one direction and not in the other. The sliding-valves and the balanced-valves above alluded to are fast
- Constructionslehre fur den Maschinenbau, p. 846, et seq. ; Constructeur, 3rd
Ed. p. 583.
