THE SENSE OF HEARING.
The Perception of descrip tion that has been given of the very compli cated structure of the ear will enable one to perceive how sounds are conveyed into the depths of the ear, but it is extremely difficult to understand how that apparatus enables its tc have knowledge of all the multitude of various sounds of which we are daily conscious.
We must go back again to the physical world to get light on the difficulty. It is a well known fact that if one takes two tuning-forks, such as are used for experiment in a natural philosophy laboratory, both tuned to sound exactly the same note, and if the forks are placed at some distance from one another, and aa of fbarn nmrla 4n annnrl thn nfhar not being touched, in an instant both will be heard sounding. The sound of the one fork has set the other agoing. This, it must be noticed, happens only if both utter a note of the same pitch. If the two forks sound different notes, the one may be set humming loudly and long, the other remains dumb. But if the two are tuned to the same note, the one is no sooner set sounding than the other is heard humming also. This is calved sympathetic vibration, but there is no mystery about the occurrence. The one fork produces waves exactly similar to those the other would produce. When, therefore, one is sounded, the waves pass out and reach the other fork, against which they strike. Wave after wave hitting on the fork, each wave exactly suiting the swing the fork makes when in motion, gradually sets it into vibration. It is just like a small boy setting in motion a swing on which a heavy boy is sitting. If he tried to make it give a big swing with one push, he might push with all his might and accomplish nothing. If he be a wise small boy, however, he does not attempt this. He gives the swing an ordinary push, and it swings slightly. He waits till it has moved forward, then back wards, and, just as it is about to sway for wards again, he gives it another push, which, added to its own impulse, increases its move ment. So he goes on till, in a short time, he
has it in full swing. But if he does not time his pushes properly he will speedily stop the swing. It is the same with the tuning-fork. Each wave from its neighbour reaches it at the proper time and speedily sets it in full vibration. If uow the second fork have one limb loaded, say with a piece of wax, the note of the fork is flattened. It is no longer in tune with the first one, and can no longer be set in sympathetic vibration with it, because its swing is no longer in time with the waves of its neighbour. One fork, therefore, can be set in vibration only by another sounding exactly the same note; and the fork will recognize its own note and hum in harmony with it, though the sound reach it from a considerable dis tance.
Suppose, then, that, on a table at one end of a room, one had a set of tuning-forks, each tuned to a particular note. Let a number of musicians proceed to play on their instruments at the other end of the room. No matter how complex the body of sound they produce, each tuning-fork will pick out infallibly its own note from the mass of sound, if its own note happens to have been produced; and if the players suddenly cease, the tuning-forks will be heard sounding. By finding which of them are vibrating, one can tell what notes were produced by the players. Suppose one could have a set of tuning-forks so numerous that there was one for every note that could be sounded, it is evident one would have here an apparatus for analysing the complex sound, that is, splitting it up into its elements. One may try a simple experiment of this kind with a piano. Let the damping pedal be lifted from the wires, and let someone sing loudly in the room, and then suddenly stop. Some of the pianoforte wires will be heard sounding. They have picked out of the song their own notes, and have been set in sympathetic vibra tion by them.