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

traversed the cloister and returned to the point of its origination regularly once in each interval of time elapsing between the delivery of a stroke and the perception of its echo. Since each such interval was exactly equal to that between the perception of an echo and the delivery of 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 by pacing to be about 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 1,000 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 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 second3, or, roughly speaking, about 200 times that of an express train.

4. Though the Sound-impulse advances with

1 This is about 50 feet below its exact value under the circumstances of my observation. See Tyndall's Sound, Third edition, p. 23.

2 Tyndall's Sound, p. 38.

a steady and 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 continuous 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 horizontally in a direction perpendicular to the lines of its ridges and hollows. The actual motion of the water is, however, very different from its apparent motion, as may be ascertained by noticing the behaviour of a cork or other body floating on the surface of the sea, and therefore sharing its movement. The floating body does not

advance with the waves, but rides over their crests and sinks into their troughs as though it were a buoy at anchor. Hence, while the waves travel steadily forward horizontally, each of the fluid particles which compose them describes over and over again a fixed orbit of its own.

Thus, when we say that the waves advance horizontally, we mean, not that the masses of water of which they at any given instant consist advance, but that these masses, by virtue of the separate motions of their individual particles, successively arrange themselves in the same relative positions, so that the curved shapes of the surface, which we call waves, are horizontally transmitted without their materials sharing in the progress. The accompanying figure will show how this happens.

Α

A

Fig. 1.

-B

--B'

Let the full curved line AB represent a section of a part of the sea-surface at any given instant made by a vertical plane through the direction of wave-motion, and suppose that during, say, the next ensuing second of time, the separate fluid particles rearrange themselves by virtue of their