M. Lacour enumerates the ways in which this system may be applied as follows: ‘If the keys in connection with the sending tuning-forks are placed side by side, and are lowered in succession, or two or three together, it will be enough to play on the keys as on a musical instrument, in order that the air may be heard at the receiving station, or the signals transmitted simultaneously may each belong to a different message. This system will therefore allow the furthest station on a line to communicate with one or several intermediate stations, and _vice versa_, without disturbing the communication at other stations. In this way two stations can exchange signals, unperceived by the rest. The power of sending many signals at once affords a good means of improving the autographic telegraph. In the instruments now in use, such as those of Caselli and D’Arlincourt, there is only one tracing stylus, and this stylus must pass over the whole surface of the telegram in order to obtain a copy of it, but with the telephone a certain number of styli may be placed side by side in the form of a comb, and this comb need only be drawn in a certain direction to pass over the surface of the telegram. In this way a more faithful copy will be obtained in a shorter time.’ M. Lacour also observes that his system possesses a merit already pointed out by Mr. Varley, namely, that the instruments permit the passage of ordinary currents without revealing their presence, whence it follows that the accidental currents which often disturb telegraphic transmissions will have no effect on these systems. M. Lacour began without applying an electro-magnetic system to his instrument in order to maintain the movement of the tuning-fork, but he soon saw that this accessory was indispensable, and he made the tuning-forks themselves electro-magnetic. It also occurred to him to convert the transmitted currents into pulsatory currents by inserting an induction coil in the circuit, which was also done by Mr. Elisha Gray. Finally, in order to obtain the immediate action of the tuning-forks and the immediate cessation of their action, he constructed them so as to reduce their inertia as much as possible. This was effected by inserting the two branches of the tuning-fork in the same coil and by lengthening its handle, and turning it back so that it might pass through a second coil, dividing into two branches and embracing the two vibrating branches, but without touching them. When a current traverses both coils, it produces, in the kind of horseshoe magnet formed by the two systems, opposite polarities which provoke a double reaction in the vibrating branches--a reaction by repulsion exerted by the two branches in virtue of the same polarity, and a reaction by attraction by the other two branches in virtue of their opposite polarities; and this double action is repeated by the movements of a contact breaker applied to one of the vibrating branches of the tuning-fork.