An articulating telephone is not necessary in order to obtain simultaneous transmission: the musical telephones devised by MM. Petrina, Gray, Froment, &c., are quite sufficient, and a brief explanation of their principle will make this intelligible. Suppose that there are seven electro-magnetic vibrators at the two corresponding stations, which are tuned with the same tuning-fork on the different notes of the scale, and suppose that a key-board, resembling the Morse telegraph key, is arranged so that, by lowering the keys, electric reaction takes place on each vibrator: it is easy to see that these vibrators may be made to react in the same way on the corresponding vibrators of the opposite station; but they must be tuned on the same note, and the sounds emitted will continue while the keys are lowered. By keeping them down for a shorter or longer time, the long or short sounds which constitute the elements of telegraphic language in the Morse system may be obtained, and consequently an audible transmission becomes possible. Let us now suppose that a telegraphist accustomed to this mode of transmission is placed before each of the vibrators, and that they transmit different messages at the same moment in this way: the telegraphic wire will be instantaneously traversed by seven currents, broken and massed upon each other, and they might be expected to produce a medley of confused sounds on the vibrators at the receiving station; but since they each harmonise with the corresponding vibrator, they have no sensible influence except on those for which they are intended. The dominant sound may be made still more distinct by applying a Helmholtz resonator to each vibrator,[18] that is, an acoustic instrument which will only vibrate under the influence of the note to which it is tuned. In this way it is possible to select the transmitted sounds, and only to allow each _employé_ to hear that which is intended for him. Consequently, however confused the sounds may be on the receiving vibrators, the person to whom _do_ is assigned will only receive _do_ sounds, the person to whom _sol_ is assigned will only receive _sol_ sounds, so that correspondence may be carried on as well as if they had each a special wire.