502 KINEMATICS OF MACHINERY.
133.
Kinematic Nature of the Complete Machine.
We have found that the " tool," which has so often been con- sidered an essential member of every machine, occurs only in one half of existing machines. We have seen, too, that the receptor, which has also been considered essential to every machine, is in very many cases quite indeterminate. The prospect that the third member, the communicator, should prove to be essential becomes therefore very small. There are very numerous cases in which it cannot be distinctly identified, although, in some instances, there are large groups of parts which are obviously employed for no other purpose than the transmission of motion. But every link of the kinematic train transmits a greater or less effort from one point of the machine to another ; every link may be looked upon as a communicator between the driving force and the resistance ; and in most instances it is impossible to say where the function of transmission begins and where it ends, so that we must conclude that the communicator also, as a special subdivision of every machine, must be given up. All three, receptor, communicator, and tool, may exist and may be clearly recognisable in one and the same machine ; they are not, however, essential organs of machines in general, they must be reckoned among their accidental members only, for which we shall shortly find another classification.
The fundamental idea to which our investigations have led us, an. idea which we have found to be the foundation of, and hidden by, many subsidiary ones, is this : the complete machine is a closed kinematic chain. The driving body and the body on which work is done are equally links or elements in the chain. The laws governing the motion of the motor or driver are the same as those according to which the work-piece is driven and the tool, where it exists, performs its function ; they are simply those laws under which the relative motions of any other links or elements take place.
Only one difference appears which tends to impair the sim- plicity of this conclusion ; it is the difference between the form- changing and the place-changing machines. This last remaining distinction deserves somewhat closer examination.
