pulley, or throwing the wheel work "out of gear"; while the converse movement, which restores that continuity, is followed by the renewed action of the machine, which goes on until the continuity is again broken. Thus he will be led to regard its maintenance as essential to the working of the machine; but nothing that he has yet learned explains to him why it is essential. He has only got at the material collocation which his educated vision enables him to recognize; and, for anything he knows to the contrary, the change in that collocation may be in itself adequate to determine the result.
But let him lay hold of the band which stretches between the main shaft and the axis of one machine, or attempt to stay with his hand the rotation of the train of wheels which connects it with another—he then at once becomes conscious, through his "force-sense," of the power which the band or the wheelwork is the instrument of conveying; and as he finds that the "pull" upon his hand is just the same whether the machine is in motion or not, provided that the band or wheel remains in mechanical connection with the main shaft, he comes to the conviction that the source of the power is in the shaft, and that, so far from any one of the machines having an inherent power of movement, its motion entirely depends upon the force supplied to it from the shaft. And when, under the guidance of this conception, he again examines the working of the several kinds of machine, he finds that, while the power is the same for all, the diversity in their respective products is traceable to the diversity in their construction—that is, to the material collocations through which the one moving force exerts itself in action.
But, having thus acquired the notion of moving power, and having satisfied himself of the derivation of the force that gives motion to each of the entire aggregate of machines, from one main shaft, our inquirer finds himself again posed. Has this shaft itself an inherent power of motion; or does it derive that power from any ulterior source? He sees the shaft apparently terminate in the two end-walls of the building; and, finding no evidence of its connection with anything else, he may feel himself drawn toward the conclusion that it moves of itself—that is, by the "potency" of its own material constitution. But, before adopting this rationale, he sees all the machines stop at once, and finds that the shaft also has ceased to revolve. Here is a new and startling phenomenon. After pondering on it for an hour, and carefully looking out for an explanation, he sees the shaft and its connected machines resume their motion, and yet is certain that no agency visible to him has had any concern in that renewal. By continued watching, he finds this suspension and renewal to be periodical, so that he can frame a law that shall express them in terms of time. Thus he might give a complete phenomenal account of the action of the shaft which should be perfectly consistent with the assumption of its "inherent potency," and which might be sufficiently satisfactory to his mind to justify him in believing that there is no more to be learned
