So perfect is the modern telephone that the writer remembers on one occasion hearing the door-bell ring in a house more than a hundred miles away, with which he was at the moment in telephonic connection, though three exchanges were in the circuit.
SUBMARINE TELEPHONY.
Though telegraphic messages are transmitted easily through thousands of miles of cable,16 submarine telephony is at present restricted to comparatively short distances. When a current passes through a cable, electricity of opposite polarity induced on the outside of the cable damps the vibration in the conductor. In the Atlantic cable, strong currents of electricity are poured periodically into one end, and though much enfeebled when they reach the other they are sufficiently strong to work a very delicate "mirror galvanometer" (invented by Lord Kelvin), which moves a reflected ray up and down a screen, the direction of the movements indicating a dot or a