It is certain that Reis did not in any of his writings explicitly name an undulatory current: but it is equally certain that, whether he mentioned it or not, he both used one and intended to use one. He did not concern himself as to the precise manner in which the current fluctuated provided only he attained the end in view--namely, that the vibrations of the armature of the receiver should be similar to those of the transmitter. This he did lay down with great clearness and emphasis as his guiding principle; and he cared not about the intermediate question as to how the current did the work. He told the world that the electromagnet at the receiving end must be magnetised and demagnetised correspondingly with the vibrations imparted by the air to the tympanum of his transmitter, in order that the armature might be set into vibrations similar to those of the speaker’s voice. If the tympanum of the transmitter vibrated or oscillated or undulated--the terms are synonymous--so must the armature of the receiver. Graham Bell has told us precisely the same thing: “The current traversing the coils of the electromagnet _E_ occasions an increase and diminution in its intensity” [that is to say, magnetises and demagnetises it], “and the armature _A_^1 is thrown into vibration” ... “and thus imparts to the air at _n_^1 a facsimile copy of the motion of the air that acted upon the membrane _n_.” Bell agrees then absolutely in every detail with what Reis said on this point. That sound-waves should be transmitted by a Telephone requires indeed a process of several stages. (1.) The sound-waves must strike upon the tympanum of the transmitter and make it undulate, or, oscillate, or vibrate--whichever term you please--in a corresponding manner. (2.) The undulating tympanum must act upon the circuit, and either itself induce undulating or vibrating currents (Bell’s plan, by magnetic induction), or else throw a current already flowing there, into undulations, or vibrations, or oscillations (Reis’s plan, by varying contact-resistance), but in either case these undulations of the current must correspond to the original undulations of the air-waves. (3.) The undulating, or vibrating, or oscillating current must run round the coils of the electromagnet and cause its magnetic force to undulate, or oscillate, or vibrate by demagnetising it and then magnetising it, but this also must be in a manner corresponding to the original undulations. (4.) Further, the armature of the receiver must be set into undulations, or vibrations, or oscillations corresponding to those of the force of the electromagnet, and therefore to the undulations of the current that is magnetising and demagnetising it, and therefore identically corresponding with the original undulations of the sound-waves. (5.) The armature must communicate its vibrations to the air and to the ear of the listener. Of these successive stages Reis explicitly told the world that his instrument was to do the