Suppose once more that, across the road from an engine-house, there was another building, where all sorts of machines--lathes, planers, drills, etc.--were running, but that the source of the power for all this was out of sight, and that one could see no connection between this and the engine on the other side of the street. Would one need to suppose there was anything mysterious between the two--a force, a fluid, an immaterial something? This question is put on the supposition that one should not be aware of the shaft that might be between the two buildings, and that it was not obvious on simple inspection how the machines got their motions from the engine. No one would be puzzled because he did not know just what the intervening mechanism might be. If the boiler were in the one building, and the engine in the other with the machines, he could see nothing moving between them, even if the steam-pipes were of glass. If matter of any kind were moving, he could not see it there. He would say there _must_ be something moving, or pressure could not be transferred from one place to the other. Substitute for the furnace and boiler a galvanic battery or a dynamo; for the machines of the shop, one or more motors with suitable wire connections. When the dynamo goes the motors go; when the dynamo stops the motors stop; nothing can be seen to be turning or moving in any way between them. Is there any necessity for assuming a mysterious agency, or a force of a _nature_ different from the visible ones at the two ends of the line? Is it not certain that the question is, How does the motion get from one to the other, whether there be a wire or not? If there be a wire, it is plain that there is motion in it, for it is heated its whole length, and heat is known to be a mode of motion, and every molecule which is thus heated must have had some antecedent motions. Whether it be defined or not, and whether it be called by one name or another, are quite immaterial, if one is concerned only with the _nature_ of the action, whether it be matter or ether, or motion or abracadabra. Once more: suppose we have a series of active machines. (Fig. 11.) An arc lamp, radiating light-waves, gets its energy from the wire which is heated, which in turn gets its energy from the electric current; that from a dynamo, the dynamo from a steam-engine; that from a furnace and the chemical actions going on in it. Let us call the chemical actions A, the furnace B, the engine C, the dynamo D, the electric lamp E, the ether waves F. (Fig. 12.) [Illustration: FIG. 11.]