346 KINEMATICS OF MACHINERY.
engines and pumps. The link taking the form of the chamber is the frame d ; its upper part is made the guiding prism for the block c, its lower part being formed as a hollow cylinder, the steam- or pump- cylinder as the case may be. In this moves a tightly-fitting piston or plunger, which is simply an extension downwards of the block c. The feathered arrows show here (as in the following figures) the direction of motion of the in-coming and out-going fluid when the crank a is moving in the direction indicated by the plain arrow. If the machine be a pump the valves almost always act automatically, the pressure of the fluid itself opening and closing them. If the machine be an engine, on the other hand, a special kinematic chain, the valve gear is required for working them. There are a few pumps also in which a similar chain or gearing is employed. It must not be forgotten that the kinematic condition for the chamber is that it must be prismatic; the choice of a circular cylinder for its form is merely accidental, and made for the sake of convenience. It has none the less furnished the name, steam-cylinder, pump- cylinder, etc., by which the chamber is commonly known. In some cases where the chambers of ventilating machines have been made of wood, exactly the same considerations of convenience have led to the adoption of a square instead of a circular cross section. The position of the chain relatively to the horizon does not affect it in any way. Fig. 1 therefore serves equally for horizontal engines and pumps, or for those which are inclined, or inverted, or in any other position ; whatever practical difference there may be between these various " systems," kinematically they are identical. One other point requires to be noticed, what is known as the " double- action " of the piston. It is necessary for our purposes to understand clearly what this means. It is that in each period, that is here in each revolution of the crank, the piston is paired with the fluid not only on one but on both sides ; and from this it follows that in one whole period the chamber must always be twice filled with fluid. With plunger pumps or single-acting engines this is not the case ; the distinction made between them in practice is therefore borne out kinematically.
We have placed below our figure the contracted special formula of the mechanism, from which it will be seen that it contains nothing beyond the four links which we already know, if we dis- regard the valve-gear and the fluid organ. The block c appears as
