_Friction Telephones._--Mr. E. Gray has quite recently applied the principle of producing sounds by the friction of animal tissues to the construction of a speaking telephone which may be heard through a whole room, like the singing condenser. He obtains this result by means of clockwork, which causes the rotation of the metallic disk of which we have spoken (p. 23), and on which a piece of skin is so arranged as to produce friction. A carbon or liquid telephone is placed at the sending station, in such a way as to react on an induction coil, as in the systems of Edison, Navez, or Pollard, and speech is reproduced on the rotating disk, and is audible, as we have said, without the necessity of approaching the ear to the instrument. The best arrangement of the metallic disk on which the animal tissue rubs is that of a cylindrical box, of which the outer lid is made of a thin sheet of zinc with a highly polished, slightly oxidised surface; for the agent of friction, glove-leather slightly moistened with acidulated water may be used, or a sinew of an ox, or skin taken from the ear or tail of a pig. MODIFICATIONS INTRODUCED IN THE CONSTRUCTION OF THE BELL TELEPHONES. The modifications which we have been considering relate to the principle of the instrument; those which we have now to consider are only modifications in the form and arrangement of the different organs which form the Bell telephone itself, and which have been designed with the object of increasing the intensity and distinctness of the sounds produced.