The voices mix with their echoes into a chaos of noise, out of which no intelligible utterance can emerge. The echoes of a room are materially damped by its furniture. The presence of an audience may also render intelligible speech possible, where, without an audience, the definition of the direct voice is destroyed by its echoes. On the 16th of May, 1865, having to lecture in the Senate House of the University of Cambridge, I first made some experiments as to the loudness of voice necessary to fill the room, and was dismayed to find that a friend placed at a distant part of the hall could not follow me because of the echoes. The assembled audience, however, so quenched the sonorous waves, that the echoes were practically absent, and my voice was plainly heard in all parts of the Senate House.

Sounds are also reflected from the clouds. When the sky is clear, the report of a cannon on an open plain is short and sharp; while a cloud is sufficient to produce an echo like the rolling of distant thunder. A feeble echo also occurs when sound passes from one mass of air to another of different density. Humboldt relates that from a certain position on the plains of Antures, the sound of the great falls of the Orinoco resembles the beating of a surf upon a rocky shore, being much louder by night than by day. This is not due to the greater stillness of the night, for the hum of insects and the roar of beasts rendered the night much noisier than the day. Humboldt thus explained the observation :-Between him and the falls lay a vast grassy plain, with multitudes of bare rocks protruding from it. When exposed to the sun, these rocks assumed a temperature far higher than that of the adjacent grass; over each of them therefore rose a column of heated air, less dense than that which surrounded it. Thus by day the sound had to pass through an atmosphere which frequently changed its density; the partial

EFFECT OF A NON-HOMOGENEOUS ATMOSPHERE. 19

echoes at the limiting surfaces of rare and dense air were incessant, and the sound was consequently enfeebled. At night those differences of temperature ceased to exist, and the sound-waves, travelling through a homogeneous atmosphere, reached the ear undiminished by reflection. The case has its parallel in light, which is also reflected at the limiting surfaces of different optical media; so that a mixture of different media, each of itself transparent, may intercept light. Thus by the mixture of air and water foam is produced, which in moderate thicknesses is impervious to light, in consequence of repeated reflection. All colourless transparent substances, when reduced to powder, are white and opaque for the same reason.

Thunder-peals are unable to penetrate the air to a distance commensurate with their intensity, because of the non-homogeneous character of the atmosphere which accompanies thunder-storms. From the same cause battles have raged, and been lost, within a short distance of the reserves of the defeated army, while they were waiting for the sound of artillery to call them to the scene of action. Falling snow has been often referred to as offering a great hindrance to the passage of sound, but I imagine it to be less obstructive than is usually supposed. Sound appears to make its way freely between the falling flakes. On the 29th of December, 1859, I traced a line across the Mer de Glace of Chamouni, at an elevation of nearly V. seven thousand feet above the sea.

The glacier there is half a mile wide, and during the setting out of the of line snow fell heavily. I have never seen the atmo

ed

sphere in England so thickly laden. Still I was able to

see through the storm quite across the glacier, and also

a to make my voice heard. When close to the opposite side, one of the assistants chanced to impede my view. I called out to him to stand aside, and he did so immediately. At the end of the line the men shouted

nous

sommes finis,' and I distinctly heard them through the half-mile of falling snow.

Sound'

Sir John Herschel, in his excellent article in the Encyclopædia Metropolitana, has collected with others the following instances of echoes:-An echo in Woodstock Park repeats seventeen syllables by day, and twenty by night; one on the banks of the Lago del Lupo, above the fall of Terni, repeats fifteen. The tick of a watch may be heard from one end of the abbey church of St. Albans to the other. In Gloucester Cathedral, a gallery of an octagonal form conveys a whisper seventy-five feet across the nave. In the whispering gallery of St. Paul's, the faintest sound is conveyed from one side to the other of the dome, but is not heard at any intermediate point. At Carisbrook Castle, in the Isle of Wight, is a well 210 feet deep, and 12 wide. The interior is lined by smooth masonry; when a pin is dropped into the well, it is distinctly heard to strike the water. I may add that shouting or coughing into this well produces a resonant ring of some duration.*

I have now to point out another important analogy between sound and light, which has been established by M. Sondhauss. I ignite our electric lamp, and place in front of it this fine large lens; the lens compels the rays of light that fall upon it to deviate from their direct and divergent course, and to form this convergent cone behind it. refraction of the luminous beam is a consequence of the retardation suffered by the light in passing through the glass. Sound may be similarly refracted by causing it to pass through a lens which retards its motion. Such a

This

* Placing himself close to the upper part of the wall of the London Colosseum, a circular building 130 feet in diameter, Mr. Wheatstone found a word pronounced to be repeated a great many times. A single exclamation appeared like a peal of laughter, while the tearing of a piece of paper was like the patter of hail.

† Poggendorff's Annalen, vol. lxxxv. p. 378; Philosoph. Mag. vol. v. p. 73.

REFRACTION OF SOUND BY LENSES.

21

lens is formed when we fill a thin balloon with some gas heavier than air. Here, for example, is a collodion balloon B, fig. 8, filled with carbonic acid gas, the envelope being so thin as to yield readily to the pulses which strike against

FIG. 8.

B

w

I

FIG. 9.

-b

it, transmitting them to the gas inside.* I now hang up my watch, w, close to the lens, beyond which, and at a distance of four or five feet from the lens, I place my ear assisted by the glass funnel ƒƒ. By moving my head about, I soon discover a position in which the ticking is a particularly loud. This, in fact, is the focus of the lens. If I move my ear from this focus the intensity of the sound falls; me if when my ear is at the focus the balloon be removed, the ticks are enfeebled; on replacing the balloon their force

αρ

0"

is restored. The lens, in fact, enables me to hear the ticks distinctly when they are perfectly inaudible to the unaided ear.

How a sound-wave is thus converged may be comprehended by reference to fig. 9. Let m n be a section of the sound-lens, and a b a portion of a sonorous wave approaching

* Thin india-rubber balloons also form excellent sound-lenses,

it from a distance. The middle point, o, of the wave first touches the lens, and is first retarded by it. By the time the ends a and b, still moving through air, reach the balloon, the middle point o, pursuing its way through the heavier gas within, will have only reached o'. The wave is therefore broken at o', and the direction of motion being at right angles to the face of the wave, the two halves will now encroach upon each other. The convergence of the two halves of the wave is augmented on quitting the lens. For when o' has reached o", the two ends a and b will have pushed forward to a greater distance, say to a' and b'. Soon afterwards the two halves of the wave will cross each other, or, in other words, come to a focus, the air at the focus being agitated by the sum of the motions of the

When a long sea-roller meets an isolated rock in its passage, it rises against the rock, and embraces it all round. Facts of this nature caused Newton to reject the undulatory theory of light. He contended that if light were a product of wave motion we could have no shadows, because the waves of light would propagate themselves round opaque bodies as a wave of water round a rock. It has been proved since his time that the waves of light do bend round opaque bodies; but with that we have nothing now to do. A sound-wave certainly bends thus round an obstacle, though as it diffuses itself in the air at the back of the obstacle it is enfeebled in power, the obstacle thus producing a partial shadow of the sound. Anybody who has ever heard a railway train approach through cuttings and along embankments, will have noticed great variations in the intensity of the sound. The interposition of a hill in the Alps suffices to diminish materially

For the sake of simplicity, I have shown the wave broken at o' and its two halves straight. The surface of the wave, however, is really a curve, with its concavity turned in the direction of its propagation.