It is a fact that the mind will hear the faintest sounds from things in which is centered interest and attention, while at the same time ignoring things in which there is no interest and to which the attention is not turned. A sleeping mother will awaken at the slightest whimper from her babe, while the rumbling of a heavy wagon on the street, or even the discharge of a gun in the neighborhood may not be noticed by her. An engineer will detect the slightest difference in the whir or hum of his engine, while failing to notice a 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 the message; and, in some cases, the mood or temper of the person transmitting it. Trainmen and steamboat men recognize the differences between every engine or boat on their line, or river, as the case may be. A skilled physician will detect the faint sounds denoting a respiratory trouble or a "heart murmur" in the patients. And yet these very people who are able to detect the faint differences in sound, above mentioned, are often known as "poor hearers" in other things. Why? Simply because they hear only that in which they are interested, and to which their attention has been directed. That is the whole secret, and in it is also to be found the secret of training of the ear-perception. It is all a matter of interest and attention--the details depend upon these principles.