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SOUND AND MUSIC.

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CHAPTER I.

ON SOUND IN GENERAL AND THE MODE OF ITS TRANS

MISSION.

1. IN listening to a sound, all that we are immediately conscious of is a peculiar sensation. This sensation obviously depends on the action of our organs of hearing; for if we close our ears the sensation is greatly weakened, or, if originally but feeble, altogether extinguished. Persons whose auditory apparatus is malformed, or has been destroyed by disease, may be totally unconscious of any sound, even during a thunderstorm or the discharge of artillery. It would be entirely in accordance with the mode of action of our other senses if what we feel as Sound is represented, externally to ourselves, by a state of things very different to the sensation we experience. Analogy, then, indicates

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that some purely mechanical phenomena external to the ear will prove to be turned into the sensation we call Sound by a process carried on within that organ and the brain with which it is in direct communication. This mechanical agency, whatever may be its nature, is usually set going at a distance from the ear, and, to reach it, must traverse the intervening space. In doing so it can pass through solid and liquid as well as gaseous bodies. For instance, if one end of a felled tree is gently scratched with the point of a penknife, the sound is distinctly audible to a listener whose ear is pressed against the other end of the tree. When a couple of pebbles are knocked together under water, the sound of the blow reaches the ear after first passing through the intervening liquid. That Sound travels through the air is a matter of universal experience, and needs no illustration. In every case accessible to common observation where Sound passes from one point of space to another, it necessarily traverses matter, either in a solid, liquid or gaseous form. We may hence conjecture that the presence of a material medium of some kind is indispensable to the transmission of Sound. This important point can be readily brought to the test of experiment, as follows. Let a bell kept ringing by clockwork be placed under the receiver of an air-pump, and the air gradually

exhausted. Provided that suitable precautions are taken to prevent communication of Sound to the external air through the body of the receiver, the bell will appear to ring more and more feebly as the exhaustion proceeds, until at last it altogether ceases to be heard. While the air is being readmitted, the sound of the bell will gradually recover its original loudness. This experiment shows that Sound cannot travel in vacuo, but requires for its transmission a material medium of some kind. The air of the atmosphere is, in the vast majority of cases, the medium which conveys to the ear the mechanical impulse which that wonderful organ translates, as it were, into the language of Sound.

2. Having ascertained that a material medium acts in every case as the carrier of Sound, we have next to examine in what manner it performs this function. The roughest observations suffice to put us on the right track in this enquiry by pointing to a connexion between Sound and Motion. The passage through the air of sounds of very great intensity is accompanied by effects which prove the atmosphere to be in a state of violent commotion. The explosion of a powder-magazine is capable of shattering the windows of houses at several miles' distance. In the case of sounds of only ordinary loudness the accompanying air-motion manifests itself in no such

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