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

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 second, 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.

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

respective orbital motions in such a manner that, at the end of that second, they constitute a curved line identical in shape and size with AB, and only differing from it in horizontal position. Let the dotted line A'B' represent the curve thus formed. As the two outlines AB and A'B' are exactly alike, the joint effect produced by the separate particlemovements on the eye of a spectator is just what it would have been had we pushed the curve AB along horizontally until it came to occupy the position A'B'.

In order further to illustrate this point, let us suppose that a hundred men are standing in a line and that the first ten are ordered to kneel down: a spectator who is too far off to distinguish individuals will merely see a broken line like that in the figure below.

Now, suppose that after one second the eleventh man is ordered to kneel and the first to stand; after two seconds the twelfth man to kneel and the second to stand; and so on. There will then continue to be a row of ten kneeling men, but during each second it will be shifted one place along the line. The distant observer will therefore see a depression steadily advancing along the line. The

state of things presented to his eye after twenty, sixty and ninety seconds, respectively, is shown in Fig. 2.

Fig. 2.

There is here no horizontal motion on the part of the men composing the line, but their vertical motions give rise, in the way explained, to the horizontal transference of the depression along the line. The reader should observe that for no two consecutive seconds does the kneeling row consist of exactly the same men, while in such positions as those shown in the figures, which are separated by ten or more seconds of time, the men who form it are totally different.

6. Let us now return to the sea-waves and examine more closely the elements of which they consist.

Fig. 3 represents a vertical section of one complete wave.