SOUND AND MUSIC.

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

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

unmistakable way: a little attention will, however, usually detect a certain amount of movement on the part of the sound-producing apparatus, which is probably capable of being communicated to the surrounding air. Thus, a sounding pianoforte-string can be both seen and felt to be in motion: the movements of a finger-glass stroked on the rim by a wet finger can be recognised by observing the thrills which play on the surface of the water it contains: sand strewed on a horizontal drum-head is thrown off when the drum is beaten. These considerations raise a presumption that Sound is invariably associated with agitation of the conveying medium—that it is impossible to produce a sound without at the same time setting the medium in motion. If this should prove to be the case, there would be ground for the further conjecture that motion of a material medium constitutes the mechanical impulse which, falling on the ear, excites within it the sensation we call Sound. Let us try to form an idea of the kind of motion. which the conditions of the case require.

3. We may conveniently begin by determining the rate at which Sound travels. This varies, indeed, with the nature of the conveying medium. It will suffice, however, for our present purpose to ascertain its velocity in air, the medium through which the vast majority of sounds reach our ears. As long as

we confine our attention to sounds originating at but small distances from us, their passage through the intervening space appears instantaneous. If, however, a gun is fired at a considerable distance, the flash is seen before the report is heard-a proof that an appreciable interval of time is occupied by the transmission of the sound. The occurrence of an echo, in a position where we can measure the distance passed over by the sound in travelling from the position where it is produced to that where it rebounds, gives us the means of measuring the velocity of Sound; since we can, by direct observation, ascertain how long a time is spent on the out-andhome journey. The following easy experiment gives a near approximation to the actual velocity of Sound -in fact a much closer one than the rough nature of the observation would have led one to expect. In the North cloister of Trinity College, Cambridge, there is an unusually distinct echo from the wall at its eastern extremity. Standing near the opposite end of the cloister, I clapped my hands rhythmically at a rate such that the strokes and echoes were heard alternately at equal intervals of time. A friend at my side, watch in hand, counted the number of strokes and echoes. The result was that there were 76 in half a minute, i.e. 38 strokes and 38 echoes. A little consideration will show that the sound