very loud noise outside. A musician will note the slightest discord occurring in a concert in which there are a great number of instruments being played, and in which there is a great volume of sound reaching the ear, while other sounds may be unheard by him. The man who taps the wheels of your railroad car is able to detect the slightest difference in tone, and is thus informed that there is a crack or flaw in the wheel. One who handles large quantities of coin will have his attention drawn to the slightest difference in the “ring” of a piece of gold or silver, that informs him that there is something wrong with the coin. A train engineer will distinguish the strange whir of something wrong with the train behind him, amidst all the thundering! rattle and roar in which it is merged. The foreman in a machine shop in the same manner detects the little strange noise that informs him that something is amiss, and he rings off the power at once. Telegraphers are able to detect the almost imperceptible differences in the sound of their instruments that inform them that a new operator is on the wire; or just who is sending
