THE RECEPTOR. 497
be able to include, in a single definite conception, the whole kinematic relations between the tool and the work-piece.
' 132. The Receptor.
There has been less variety in the common conception of the receptor than in that of the tool, on account of the limited number of bodies which seem suitable for fulfilling the function assigned to it. These bodies are water, wind, steam and some other gases, weights, springs and living agents. By the receptor of any com- plete machine has hitherto been generally understood that part to which one or other of these bodies directly imparts the energy by which the machine accomplishes its work. It is important that we should acquaint ourselves with the characteristics of the various ways in which this transference of energy is effected in the cases of the bodies mentioned.
Taking first water-wheels and turbines, we find the receptor at once in their buckets. Oar earlier investigations ( 43) have already shown us that the wheel is not used by itself, but that its buckets are kinematically paired with the water and this again with its channel or pipe. The receptor is here, therefore, un- questionably a link in the kinematic chain. In the various forms of hydraulic engines we note exactly the same thing. Here also the water, paired with the piston, enclosed in the cylinder, guided by the valves, forms a link in the kinematic chain; the whole mechanism is one which we have already examined ( 126) and found to be a ratchet-train. It is, however, impossible to say certainly whether the piston is the receptor, or the cylinder, or both, or, indeed, whether the valve gear does not also form a part of it along with both.
The wind is utilized as a source of energy under force-closure of the driving organ, in such a way that a kinematic pairing, in this case a higher screw-pairing, occurs between the wind and the sails of the wheel.
Steam and other gases working expansively are commonly used in piston machines, and occasionally in machines arranged some- what like turbines, always, however,- in such a way that they are K K K