at equal intervals, and will deduce from this the length of the wave; and, if he knows the duration of the vibrations of the diapason, he will obtain, by these data, the velocity of the sound. We operate in the same way with our electrical vibrations. The conductor in which the vibrations are made fills the part of the diapason. The circuit, interrupted at a certain point, takes the place of the resonator, and may be called the electric resonator. We remark that sparks fly out at some points in the chamber, and quiet prevails in others. We notice that the spots inactive, electrically, follow in a regular order. We deduce from this, that the propagation is not instantaneous; and we can even measure the length of the wave. We are asked whether the waves are longitudinal or transversal. Let us place our metallic wire in two different positions in the same place in the room. It indicates an electrical excitation the first time, but not the second. Nothing more is needed to decide the question. The waves are transversal. If we are asked to give the velocity of propagation, we have only to multiply the length of wave which we have just measured by the duration of the vibration, which we can calculate. We find the velocity like that of light. If the correctness of this calculation is doubted, we have another resource. The velocity of electric waves in metallic wires is enormous, and quite equal to their velocity in the air. Further than this, it was directly measured a considerable time ago; for the problem was easily studied on wires kilometres long. We therefore have a purely experimental valuation of this velocity, and, although the result is only approximate, it does not contradict the one we have just got.
These experiments are all very simple at the bottom, and yet they have most important consequences. They overthrow every theory that assumes that electrical forces traverse space instantaneously, and mark the triumph of Maxwell's system. It is no longer a simple thread of union between two orders of distinct phenomena. While his theory of light seemed at first to be probable, it is now hard not to regard it as true. But it may be that in approaching this end we shall be able to dispense with the support of the theory. Our experiments took place very near that neutral zone which, according to it, unites the domains of light and electricity. Only one step remains to be taken to land in this domain of optics, which is well known to us. It will not be superfluous. There are many friends of Nature interested in the problem of light who are capable of comprehending simple experiments, but to whom Maxwell's theory is still unintelligible. Moreover, the scientific method requires us to avoid roundabout ways when it is possible to follow a direct one. If, then, we succeed in producing phenomena like those of light by means of
