such a flame was discovered by Professor Le Conte in the United States, who had the sagacity to seize upon the most essential features of the phenomenon. A similar observation was subsequently made by Prof. Barrett, while assistant in the physical laboratory of the Royal Institution; and both he and myself, my present assistant Mr. Cottrell, and Mr. Philip Barry, have succeeded in pushing such flames to an extraordinary degree of sensitiveness. The following brief description of a sensitive flame 24 inches high, issuing from the single orifice of a steatite-burner, is taken from my forthcoming "Lectures on Sound: ""The slightest tap on a distant anvil causes it to fall to 7 inches. When a bunch of keys is shaken, the flame is violently agitated, and emits a loud roar. The dropping of a sixpence into a hand, already containing coin, knocks the flame down. The creaking of boots sets it in violent commotion. The crumpling or tearing of a bit of paper, or the rustle of a silk dress, does the same. Responsive to every tick of a watch held near it, it falls and explodes. The winding up of the watch produces tumult. From a distance of 30 yards we may chirrup to this flame, and cause it to fall and roar. Repeating a passage from the 'Faerie Queene,' the flame sifts and selects the manifold sounds of my voice, noticing some by a slight nod, others by a deeper bow, while to others it responds by violent agitation."

We are now prepared to understand a drawing and description of the apparatus first employed in the demonstration of aerial reflection. I take both drawing and description substantially from an account of the apparatus given by a writer in Nature, February 5, 1874:

"A tunnel tt' (Fig. 1), 2 inches square, 4 feet 8 inches long, open at both ends, and having a glass front, runs through the box, a b c d. The spaces above and below are divided into cells opening into the tunnel by transverse orifices exactly corresponding vertically. Each alternate cell of the upper series-the first, third, fifth, etc.-communicates by a bent tube (e e e) with a common upper reservoir (g), its counterpart cell in the lower series having a free outlet into the air. In like manner the second, fourth, sixth, etc., of the lower series of cells are connected by bent tubes (n n n) with the lower reservoir (i), each having its direct passage into the air through the cell immediately above it. The gas-distributors (g and i) are filled from both ends at the same time, the upper with carbonic-acid gas, the lower with coal-gas, by branches from their respective supply-pipes (f and h). A well-padded box (P) open to the end of the tunnel forms a little cavern, whence the sound-waves are sent forth by an electric bell (dotted in the figure). A few feet from the other end of the tunnel, and in a direct line with it, is a sensitive fiame (k), provided with a funnel as sound-collector, and guarded from chance currents by a shade.

"The bell was set ringing. The flame, with quick response to

the tubes above. That which was a homogeneous medium, had now fifty limiting surfaces, from each of which a portion of the sound was thrown back. In a few moments these successive reflections became so effective that no sound having sufficient power to affect the flame

each blow of the hammer, emitted a sort of musical roar, shortening and lengthening as the successive sound-pulses reached it. The gases were then admitted. Twenty-five flat jets of coal-gas ascended from the tubes below, and twenty-five cascades of carbonic acid fell from

FIG. 1.-APPARATUS FOR SHOWING THE INFLUENCE OF A NON-HOMOGENEOUS ATMOSPHERE ON THE TRANSMISSION OF SOUND.

could pierce the clear, optically-transparent, but acoustically-opaque atmosphere in the tunnel. So long as the gases continued to flow, the flame remained perfectly tranquil. When the supply was cut off, the gases rapidly diffused into the air. The atmosphere of the tunnel became again homogeneous, and therefore acoustically transparent, and the flame responded to each sound-pulse as before."

Not only do gases of different densities act thus upon sound, but atmospheric air in layers of different temperatures does the same. Across a tunnel resembling t t', Fig. 1, sixty-six platinum wires were stretched, all of them being in metallic connection. The bell, in its padded box, was placed at one end of the tunnel, and the sensitive. flame k, near its flaring point, at the other. When the bell rang, the flame flared. A current from a strong voltaic battery being sent through the platinum wires, they became heated: layers of warm air rose from them through the tunnel, and immediately the agitation of the flame was stilled. On stopping the current, the agitation recommenced. In this experiment the platinum wires had not reached a red heat. Employing half the number and the same battery, they were raised to a red heat, the action in this case upon the sound-waves being also energetic. Employing one-third of the number of wires, and the same strength of battery, the wires were raised to a white heat. Here, also, the flame was immediately rendered tranquil by the stoppage of the sound.

But not only do gases of different densities, and air of different temperatures, act thus upon sound, but air saturated in different degrees with the vapors of volatile liquids can be shown by experiment to produce the same effect. Into the path pursued by the carbonic acid in our first experiment, a flask, which I have frequently employed to charge air with vapor, was introduced. Through a volatile liquid, partially filling the flask, air was forced into the tunnel t t', which was thus divided into spaces of air saturated with the vapor, and other spaces in the ordinary condition. The action of such a medium upon the sound-waves issuing from the bell is very energetic, instantly reducing the violently-agitated flame to stillness and steadiness. The removal of the heterogeneous medium restores the noisy flaring of the flame.

A few illustrations of the action of non-homogeneous atmospheres produced by the saturation of layers of air with the vapors of volatile liquids may follow here.

Bisulphide of Carbon.-Flame very sensitive, and noisily responsive to the sound. The action of the non-homogeneous atmosphere was prompt and strong, stilling the agitated flame.

Chloroform.-Flame still very sensitive; action similar to the last. Iodide of Methyl.-Action prompt and energetic.

Amylene. Very fine action; a short and violently-agitated flame was immediately rendered tall and quiescent.

Sulphuric Ether.-Action prompt and energetic.

The vapor of water at ordinary temperatures is so small in quantity, and so attenuated, that it requires special precautions to bring out its action. But with such precautions it was found competent to reduce to quiescence the sensitive flame.

As the skill and knowledge of the experimenter augment, he is often able to simplify his experimental combinations. Thus, in the present instance, by the suitable arrangement of the source of sound and the sensitive flame, it was found that not only twenty-five layers, but three or four layers of coal-gas and carbonic acid, sufficed to still the agitated flame. Nay, with improved manipulation the action of a single layer of either gas was rendered perfectly sensible. So also as regards heated layers of air, not only were sixty-six or twenty-two heated platinum wires found sufficient, but the heated air from two or three candleflames, or even from a single flame, or a heated poker, was found perfectly competent to stop the flame's agitation. The same remark applies to vapors. Three or four layers of air saturated with the vapor of a volatile liquid stilled the flame; and, by improved manipulation, the action of a single saturated layer could be rendered sensible. In all these cases, moreover, a small, high-pitched reed might be substituted for the bell.

ATMOSPHERE.

In the experiments at the South Foreland, not only was it proved that the acoustic clouds stopped the sound, but in the proper position the sounds which had been refused transmission were received by reflection. I wished very much to render this echoed sound evident experimentally, and stated to my assistant that we ought to be able to accomplish this. Mr. Cottrell met my desire by the following beautiful experiment, which has been thus described before the Royal Society: "A vibrating reed B (Fig. 2) was placed so as to send sound-waves

through a tin tube, 38 inches long, and 14 inch diameter, in the direction BA, the action of the sound being rendered manifest by its causing a sensitive flame placed at F" to become violently agitated.

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The invisible heated layer immediately above the luminous por. tion of an ignited coal-gas flame, issuing from an ordinary bat's-wing burner, was allowed to stream upward across the end A of the tin tube. A portion of the sound issuing from the tube was reflected at the limiting surfaces of the heated layer, the part transmitted being now only competent to slightly agitate the sensitive flame at F".

"The heated layer was then placed at such an angle that the reflected portion of the sound was sent through a second tin tube, A F (of the same dimensions as B A). Its action was rendered visible by causing a second sensitive flame placed at the end of the tube F to become violently affected. This echo continued active so long as the heated layer intervened; but upon its withdrawal the sensitive flame placed at F", receiving the whole of the direct pulse, became again violently agitated, and at the same moment the sensitive flame at F, ceasing to be affected by the echo, resumed its former tranquillity.

"Exactly the same action takes place when the luminous portion of a gas-flame is made the reflecting layer, but in the experiments above described the invisible layer above the flame only was used. By proper adjustment of the pressure of gas, the flame at F' can be rendered so moderately sensitive to the direct sound-wave that the portion transmitted through the reflecting layer shall be incompetent to affect the flame. Then by the introduction and withdrawal of the bat's-wing flame the two sensitive flames can be rendered alternately quiescent and strongly agitated.

"An illustration is here afforded of the perfect analogy between light and sound; for if a beam of light be projected from B to F', and a plate of glass be introduced at A in the exact position of the reflecting layer of gas, the beam will be divided, one portion being reflected in the direction A F, and the other portion transmitted through the glass toward F', exactly as the sound-wave is divided into a reflected and transmitted portion by the layer of heated gas or flame."

Thus far, therefore, we have placed our subject in the firm grasp of experiment; nor shall we find this test failing us further on.-Contemporary Review.

VOL. VI.-36