186 THE POPULAR SCIENCE MONTHLY. 



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 sec- 

 ond. 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 re- 

 source. The velocity of electric waves in metallic wires is enor- 

 mous, 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 there- 

 fore 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 instanta- 

 neously, 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 prob- 

 able, 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 sup- 

 port 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 ex- 

 periments, 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 suc- 

 ceed in producing phenomena like those of light by means of 



