VALVES. 473
125 -
Pipes, Steam- and Pump-cylinders, Pistons, and Stuffing-boxes.
Pipes are, as we have already seen in 41, the indispensable partner-elements of pressure- organ s ; the connections between them serve to form the links of the kinematic chain in which they occur. In the cylinders of steam-engines and pumps we have the vessel V~ containing the pressure-organ ; they are therefore single elements, paired with their pistons or plungers F"+. Piston-rods and stuffing-boxes are partly paired with pressure-organs and partly occur as simple sliding-pairs PiP-. In the tubes therefore we have necessary, and in the four other constructive elements most familiar, forms of pieces which are used as links or as single ele- ments in chains containing pressure-organs. They include, essen- tially, the chambers of rotary engines and pumps, the channels or races of water-wheels, the housing of turbines, and so on.
126 - Valves.
Valves appear to be the most difficult of all the constructive elements to define kinematically. Their forms are so extremely numerous and varied that they seem to correspond more or less completely with a great number of different cases, without belong- ing entirely to any one of them. There are clacks, lifting-valves, piston-valves, tapered, cylindric and flat cocks, the slide-valves of steam-engines, lifting and sliding equilibrium- valves, automatic valves and those which are not self-acting ; there is the throttle-valve, the shutters and sluices for water-wheels and turbines, and many others. All these are valves ; they serve, that is, to divide the capa- city of a vessel containing a pressure-organ in some required manner. They do this in so many ways however, that is, they differ so greatly kinematically, that it appears at first as if it would be impossible to treat them all kinematically as one class. So far as I know, indeed, no attempt has been hitherto made to do this,
