It is possible that, with this system, the disturbing influence on the magnetic needle of the mass of iron in iron-plated vessels might be almost destroyed, and a more exact orientation than that of the compass might be obtained. The same process may make it possible to estimate and measure the variations of terrestrial magnetism. M. de Parville has not himself tried to apply this system; but Mr. Blake’s experiments, of which we spoke in an early part of this work, make it probable that it might be done with advantage. _Application to industry._--One of the earliest and most important applications of the telephone is that which was first made to the service of mines in England and America in the autumn of 1877. The great length of the galleries is well known, and had already involved the use of the electric telegraph for transmitting orders; but the miners did not understand how to work these instruments, and the service was ill performed. Thanks to the telephone, through which the first corner can send and receive a message, there is no longer any difficulty in the communication between the galleries and the surface of the mine. The ventilation of mines can also be regulated by the aid of telephones. If one of these instruments is placed near a wheel kept in motion by the air which passes through the ventilating shaft, and another is placed in the inspector’s office he can ascertain by the sound if the ventilation is duly carried on, and if the machine works regularly. _Application to scientific research._--M. d’Arsonval’s experiments, which we have already mentioned, show that the telephone can be used as an extremely sensitive galvanoscope; but since it can only produce sounds under the influence of broken currents, the circuit on which the experiment is made must be divided at rather close intervals. It has been seen that it is not even necessary to insert the telephone in the circuit: it may be influenced, when at a distance, either immediately or by the induction of the broken current on a circuit placed parallel to the first, and the force of these effects may be increased by the reaction of a core of iron, round which the inducing circuit is wound. The drawback to this system is that the direction of the current is not ascertained, so that it cannot be used as a measuring instrument; but, on the other hand, it is so sensitive, so easy to arrange, and so inexpensive, that it might be of the greatest use as a galvanoscope.