508 KINEMATICS OF MACHINERY.
We have still to consider those direct-actors which are driven by animal power. We saw above that in these machines the body of the man or animal combined with the mechanism in a kinematic chaining sometimes of great complexity. The special complication, however, lies always in the organic part of the chain, the links of which receive the necessary constraint by the action of forces commanded by the will. If we bear in mind that in the example given the grindstone worked by the foot and equally in the hand-pump, in the tread-wheel, the horse-gin, etc., the mechanism driven by muscular energy forms in itself a closed kinematic chain, we see that the relation of the organic driving parts to the inorganic machine is precisely that of the prime-mover to the direct-actor driven by it. The man or animal is to be regarded as a prime-mover, of which the parts hands, arms, feet move so as to drive in the required manner the given artificial machine. The locomotive has often enough been called a steam-horse, we may reverse the comparison and call the gin-horse, Fig, 359, the locomotive of the machine which it drives. Its direct work is simply that of moving against a cer- tain resistance. A man working in a tread-wheel, or clambering Borgnis' endless ladder is in exactly the same position, his work is that of continually raising the weight of his own body. The assistance given by the living agents to the process is purely physical in each case, and not intellectual ; it is not in the least degree necessary that they should know the object of the machine in order to do their work. This work is precisely that which would be performed by an inorganic prime-mover in driving the same mechanism.
So far, therefore, as machines driven by muscular power are themselves closed kinematic chains, they may be regarded as complete machines, and do not in themselves differ from machines driven by any other than muscular force.
This brings us to another important question, which certainly has a right to a place in any complete treatment of the theory of machines, although it has never yet found one. It is the question of the share taken by living agents, and especially by men, in the executive portion of the machine's action. If the application of animal power to machines be considered at all in the study of
