sion 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 the communication of Sound through the base of the receiver itself, the bell will appear to ring more and more feebly as the exhaustion proceeds, until, at last, it altogether ceases to be heard. On re-admitting the air, the sound of the bell will gradually recover its original loudness. It results from this experiment 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, in every case, acts 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 connection 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. Sounds of moderate loudness, such as the rattle of carriage-wheels, the stamping of feet, the clapping of hands, are produced by movements of solid bodies. which cannot take place without setting up a very perceptible agitation of the air. In the case of weaker sounds, the accompanying air-motion cannot, it is true, be ordinarily thus recognized; but, even here, a little attention will usually detect a certain amount of movement on the part of the soundproducing 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 recognized 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. It will be convenient to 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, in such a manner that the strokes and echoes succeeded each other 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 traversed the cloister and returned to the point of its origination regularly once in each interval between a stroke and its echo. Since each such interval was exactly equal to that between an echo and the following stroke, the whole movement of Sound took place in alternate equal intervals, i.e. in half the observed time, or fifteen seconds. Accordingly the sound travelled to and fro in the cloister 38 times in 15 seconds. The length thus traversed, I found to be 419 feet. The velocity of Sound per

second thus comes out equal to

38 × 419

15

or 10611

feet and a fraction. Sound, then, travels through the air at the rate of upwards of 1000 feet in a second, which is more than 600 miles an hour, or about 15 times the speed of an express-train. In solid and

This is about 50 feet below its real value under the circumstances of my observation. See Tyndall's Sound, p. 24.

liquid bodies its velocity is still greater, attaining, in the case of steel-wire, a speed of from 15,000 to 17,000 feet in a second1, or, roughly speaking, about 200 times that of an express-train.

4. Though the Sound-impulse thus advances with a steady and very high velocity, the medium by which it is transmitted clearly does not share such a motion. Solid conductors of Sound remain, on the whole, at rest during its passage, and a slight yielding of their separate parts is all that their constitution generally admits of. In fluids, or in the air, a rapid forward motion is equally out of the question. The movement of the particles composing the Sound-conveying medium will be found to be of a kind examples of which are constantly presenting themselves, but without attracting an amount of attention at all commensurate with their interest and importance.

5. An observer who looks down upon the sea from a moderate elevation, on a day when the wind, after blowing strongly, has suddenly dropped, sees long lines of waves advancing towards the shore at a uniform pace and at equal distances from each other. The effect, to the eye, is that of a vast army marching up in column, or of a ploughed field moving along