When the vibration of the plate D tends to the right, and the electric contact takes place at S′, the current of the battery P′, after traversing the primary helix, reaches the electro-magnets M M′, which give it an impulse in the contrary direction. When, on the other hand, it tends to the left, the current is sent through the second primary circuit, which will be balanced by the first. Consequently there will be a series of reversed currents in the induced circuit corresponding to the key A′, which will alternately charge and discharge the condenser C, thus sending into the line a corresponding series of electric undulations which will react on the telephone placed at the end of the line; and as the duration of the transmitted currents will vary with the time that the key A′ is lowered, a correspondence in the Morse code may be obtained in the telephone, while another correspondence is exchanged with the key A and the ordinary Morse receivers. In order to render the vibratory signals visible, Mr. Varley proposes to use a fine steel wire, stretched through a helix and facing a narrow slit, to reproduce the vibrations. A light, which is intercepted by the wire, is placed behind the slit. As soon as a current passes, the wire vibrates and the light appears. A lens is placed so as to magnify the image of the luminous slit, and project it on a white screen while the wire is in vibration. VARIOUS USES OF THE TELEPHONE. _Its domestic application._--We have seen that telephones may be used with advantage in public and private offices: they can be set up at a much less expense than acoustic tubes, and in cases where the latter would never be employed. With the aid of the calls we have described, they offer the same advantages, and the connection between the instruments is more easily concealed. The difference of price in establishing them is in the ratio of one to seven. For this purpose electro-magnetic telephones are evidently the best, since they require no battery and are always ready to work. They are already in use in many Government offices, and it is probable that they will soon be combined with electric bells for the service of hotels and of large public and private establishments: they may even be used in private houses for giving orders to servants and porters, who may thus save visitors from the fatigue of a useless ascent of several storeys. In factories, telephones will certainly soon replace the telegraphic communication which has already become general. They may not only be used for ordinary messages, but to call for help in case of fire, and they will become an integral part of several systems already established for this purpose. In countries which have free telegraphic communication, the telephone has already replaced in great measure the private telegraph instruments which have hitherto been in use; and if the same privilege is extended to France, no other mode of correspondence will be used.