# Popular Science Monthly/Volume 6/March 1875/The Atmosphere in Relation to Fog-Signaling I

 THE ATMOSPHERE IN RELATION TO FOG-SIGNALING.
By JOHN TYNDALL, LL. D., F. R. S.
INTRODUCTION.

THE cloud produced by the puff of a locomotive can quench the rays of the noonday sun; it is not therefore surprising that in dense fogs our most powerful coast-lights, including even the electric light, should become useless to the mariner.

Disastrous shipwrecks are the consequence. During the last ten years no less than 273 vessels have been reported as totally lost on our own coasts in fog or thick weather. The loss, I believe, has been far greater on the American seaboard, where trade is more eager and fogs more frequent than they are here. No wonder, then, that earnest efforts should be made to find a substitute for light in sound-signals, powerful enough to give warning and guidance to mariners while still at a safe distance from the shore.

Such signals have been established to some extent upon our own coasts, and to a still greater extent along the coasts of Canada and the United States. But the evidence as to their value and performance is of the most conflicting character, and no investigation sufficiently thorough to clear up the uncertainty has hitherto been made. In fact, while the velocity of sound has formed the subject of refined and repeated experiment by the ablest philosophers, the publication of Dr. Derham's celebrated paper in the "Philosophical Transactions" for 1708 marks the latest systematic inquiry into the causes which affect the intensity of sound in the atmosphere.

Jointly with the Elder Brethren of the Trinity House, and as their scientific adviser, I have recently had the honor of conducting an inquiry designed to fill the blank here indicated.

One or two brief references will suffice to show the state of the question when this investigation began. "Derham," says Sir John Herschel, "found that fogs and falling rain, but more especially snow, tend powerfully to obstruct the propagation of sound, and that the same, effect was produced by a coating of fresh-fallen snow on the ground, though when glazed and hardened at the surface by freezing it had no such influence."[1]

In a very clear and able letter addressed to the President of the Board of Trade in 1863,[2] Dr. Robinson, of Armagh, thus summarizes our knowledge of fog-signals:

"Nearly all that is known about fog-signals is to be found in the 'Report on Lights and Beacons;' and of it much is little better than conjecture. Its substance is as follows:
"'Light is scarcely available for this purpose. Blue lights are used in the Hoogly; but it is not stated at what distance they are visible in fog; their glare may be seen farther than their flame.[3] It might, however, be desirable to ascertain how far the electric light or its flash can be traced.[4]
"'Sound is the only known means really effective; but about it testimonies are conflicting, and there is scarcely one fact relating to its use as a signal which can be considered as established. Even the most important of all, the distance at which it ceases to be heard, is undecided.
"'Up to the present time all signal-sounds have been made in air, though this medium has grave disadvantages: its own currents interfere with the sound-waves, so that a gun or bell which is heard several miles down the wind is inaudible more than a few furlongs up it. A still greater evil is that it is least effective when most needed; for fog is a powerful damper of sound.'"

Dr. Robinson here expresses the universally prevalent opinion, and he then assigns the theoretic cause. Fog, he says, "is a mixture of air and globules of water, and, at each of the innumerable surfaces where these two touch, a portion of the vibration is reflected and lost.[5]... Snow produces a similar effect, and one still more injurious."

Reflection being thus considered to take place at the surfaces of the suspended particles, it followed that the greater the number of particles, or, in other words, the denser the fog, the more injurious would be its action upon sound. Hence optic transparency came to be considered a measure of acoustic transparency. On this point Dr. Robinson, in the letter referred to, expresses himself thus: "At the outset, it is obvious that, to make experiments comparable, we must have some measure of the fog's power of stopping sound, without attending to which the most anomalous results may be expected. It seems probable that this will bear some simple relation to its opacity to light, and that the distance at which a given object, as a flag or pole, disappears, may be taken as the measure.... Still, clear air" was regarded in this letter as the best vehicle of sound, the alleged action of fogs, rain, and snow, being ascribed to their rendering the atmosphere "a discontinuous medium."

Prior to this investigation the views here enunciated were those universally entertained. That sound is unable to penetrate fogs was taken to be "a matter of common observation." The bells and horns of ships were affirmed "not to be heard so far in fogs as in clear weather." In the fogs of London, the noise of the carriage-wheels was reported to be so much diminished that "they seem to be at a distance when really close by." My knowledge does not inform me of the existence of any other source for these opinions regarding the deadening power of fog than the paper of Derham, published one hundred and sixty-seven years ago. In consequence of their a priori probability, his conclusions seem to have been transmitted unquestioned from generation to generation of scientific men.

Instruments and Observations.—On the 19th of May, 1873, this inquiry began. The South Foreland, near Dover, was chosen as the signal-station, steam-power having been already established there to work two powerful magneto-electric lights. The observations were mostly made afloat, one of the yachts of the Trinity Corporation being usually employed for this purpose. Two stations had been established, one at the top, the other at the bottom, of the South-Foreland Cliff; and, at each, trumpets, air-whistles, and steam whistles of great size, were mounted. The whistles first employed were of English manufacture. To these were afterward added a large United States whistle, also a Canadian whistle, of great reputed power.

On the 8th of October another instrument, which has played a specially important part in these observations, was introduced. This was a steam-siren, constructed and patented by Mr. Brown, of New York, and introduced by Prof. Henry into the light-house system of the United States. As an example of international courtesy worthy of imitation, I refer with pleasure to the fact that, when informed by Major Elliott, of the United States Army, that our experiments had begun, the Light-house Board at Washington, of their own spontaneous kindness, forwarded to us for trial a very noble instrument of this description, which was immediately mounted at the South Foreland.

The principle of the siren is easily understood. A musical sound is produced when the tympanic membrane is struck periodically with sufficient rapidity. The production of these tympanic shocks by puffs of air was first realized by Dr. Robinson, and his device was the first and simplest form of the siren. A stopcock was so constructed that it opened and shut the passage of a pipe 720 times in a second. Air from the wind-chest of an organ being allowed to pass along the pipe during the rotation of the cock, a musical sound was most smoothly uttered. A great step was made in the construction of the instrument by Cagniard de la Tour, who gave it its present name. He employed a box with a perforated lid, and above the lid a similarly perforated disk capable of rotation. The perforations were oblique, so that when wind was driven through the lid, it so impinged upon the apertures of the disk as to set it in motion. No separate mechanism was therefore required to turn the disk. When the perforations of lid and disk coincided, a puff escaped; when they did not coincide, the current of air was cut off. In this way impulses were imparted to the air, and sound-waves generated. The siren has been greatly improved by Dove, and specially so by Helmholtz. Even in its small form, it can produce sounds of great intensity.

In the steam-siren, as in the ordinary one, a fixed disk and a rotating disk are employed, but radial slits are used instead of circular apertures. One disk is fixed vertically across the throat of a conical trumpet 1612 feet long, 5 inches in diameter where the disk crosses it, and gradually opening out till at the other extremity it reaches a diameter of two feet three inches. Behind the fixed disk is the rotating one, which is driven by separate mechanism. The trumpet is mounted on a boiler. In our experiments, steam of 70 lbs. pressure was for the most part employed. Just as in the ordinary siren, when the radial slits of the two disks coincide, and then only, a strong puff of steam escapes. Sound-waves of great intensity are thus sent through the air, the pitch of the note depending on the velocity of rotation.

To the siren, trumpets, and whistles, were added three guns—an 18-pounder, a 512-inch howitzer, and a 13-inch mortar. In our summer experiments all three were fired; but the howitzer having shown itself superior to the other guns, it was chosen in our autumn experiments, as not only a fair but a favorable representative of this form of signal. The charges fired were for the most part those now employed at Holyhead, Lundy Island, and the Kish light-vessel—namely, 3 lbs. of powder. Gongs and bells were not included in this inquiry, because previous observations had clearly proved their inferiority to the trumpets and whistles.

On the 19th of May the instruments tested were:

On the top of the cliff:

1. Two brass trumpets or horns, 11 feet 2 inches long, 2 inches in diameter at the mouthpiece, and opening out at the other end to a diameter of 2212 inches. They were provided with vibrating steel reeds 9 inches long, 2 inches wide, and 14 inch thick, and were sounded by air of 18 lbs. pressure.

2. A whistle shaped like that of a locomotive, 6 inches in diameter, also sounded by air of 18 lbs. pressure.

3. A steam-whistle, 12 inches in diameter, attached to a boiler, and sounded by steam of 64 lbs. pressure.

At the bottom of the cliff:

4. Two trumpets or horns, of the same size and arrangement as those above, and sounded by air of the same pressure. They were mounted vertically on the reservoir of compressed air; but within about two feet of their extremities they were bent at a right angle, so as to present their mouths to the sea.

5. A 6-inch air-whistle, similar to the one above, and sounded by the same means.

The upper instruments were 235 feet above high-water mark, the lower ones 40 feet. A vertical distance of 195 feet, therefore, separated the instruments. A shaft, provided with a series of twelve ladders, led from the one to the other.

Comparative experiments made at the outset gave a slight advantage to the upper instruments. They, therefore, were for the most part employed throughout the subsequent inquiry.

Our first experiments were a preliminary discipline rather than an organized effort at discovery. On May 19th the maximum distance reached by the sound was about 312 miles.[6] The wind, however, was high and the sea rough, so that local noises interfered to some extent with our appreciation of the sound.

Mariners express the strength of the wind by a series of numbers extending from 0 ${\displaystyle =}$ calm to 12 ${\displaystyle =}$ a hurricane, a little practice in common producing a remarkable unanimity between different observers as regards the force of the wind. Its force on May 19th was 6, and it blew at right angles to the direction of the sound.

The same instruments on May 20th covered a greater range of sound; but not much greater, though the disturbance due to local noises was absent. At four miles' distance in the axes of the horns they were barely heard, the air at the time being calm, the sea smooth, and all other circumstances exactly those which have been hitherto regarded as most favorable to the transmission of sound. We crept a little farther away, and by stretched attention managed to hear at intervals, at a distance of six miles, the faintest hum of the horns. A little farther on we again halted; but though local noises were absent, and though we listened intently, we heard nothing.

This position, clearly beyond the range of whistles and trumpets, was expressly chosen with the view of making what might be considered a decisive comparative experiment between horns and guns as instruments for fog-signaling. The distinct report of the twelve o'clock gun fired at Dover on the 19th suggested this comparison, and through the prompt courtesy of General Sir A. Horsford we were enabled to carry it out. At half-past twelve precisely the puff of an 18-pounder, with a three-lb. charge, was seen at Dover Castle, which was about a mile farther off than the South Foreland. Thirty-six seconds afterward the loud report of the gun was heard, its complete superiority over the trumpets being thus, to all appearance, demonstrated.

We clinched this observation by steaming out to a distance of 8 12miles, where the report of a second gun was well heard by all of us. At a distance of 10 miles the report of a third gun was heard by some, and at 9.7 miles the report of a fourth gun was heard by all.

The result seemed perfectly decisive. Applying the law of inverse squares, the sound of the gun at a distance of 6 miles from the Foreland must have had more than two and a half times the intensity of the sound of the trumpets. It would hardly have been rash under the circumstances to have reported without qualification the superiority of the gun as a fog-signal. No single experiment is, to my knowledge, on record to prove that a sound once predominant would not be always predominant, or that the atmosphere on different days would show preferences to different sounds. On many subsequent occasions, however, the sound of the horns proved distinctly superior to that of the gun. This selective power of the atmosphere revealed itself more strikingly in our autumn experiments than in our summer ones; and it was sometimes illustrated within a few hours of the same day; of two sounds, for example, one might have the greatest range at 10 a. m., and the other the greatest range at 2 p. m.

In the experiments on May 19th and 20th the superiority of the trumpets over the whistles was decided; and, indeed, with few exceptions, this superiority was maintained throughout the inquiry. But there were exceptions. On June 2d, for example, the whistles rose in several instances to full equality with, and on rare occasions subsequently even surpassed, the horns. The sounds were varied from clay to day, and various shiftings of the horns and reeds were resorted to, with a view of bringing out their maximum power. On the date last mentioned a single horn was sounded, two were sounded, and three were sounded, together; but the utmost range of the loudest sound, even with the paddles stopped, did not exceed 6 miles. With the view of concentrating their power, the axes of the horns had been pointed in the same direction, and, unless stated to the contrary, this in all subsequent experiments was the case.

On June 3d the three guns already referred to were permanently mounted at the South Foreland. They were ably served by gunners from Dover Castle.

On the same day dense clouds quite covered the firmament, some of them particularly black and threatening, but a marked advance was observed in the transmissive power of the air. At a distance of 6 miles the horn-sounds were not quite quenched by the paddle-noises; at 8 miles the whistles were heard, and the horns better heard; while at 9 miles, with the paddles stopped, the horn-sounds alone were fairly audible. During the clay's observations a remarkable and instructive phenomenon was observed. Over us rapidly passed a torrential shower of rain, which, according to Derham, is a potent damper of sound. We could, however, notice no subsidence of intensity as the shower passed. It is even probable that, had our minds been free from bias, we should have noticed an augmentation of the sound, such as occurred with the greatest distinctness on various subsequent occasions during violent rain.

The influence of "beats" was tried on June 3d, by throwing the horns slightly out of unison; but, though the beats rendered the sound characteristic, they did not seem to augment the range. At a distance from the station curious fluctuations of intensity were noticed. Not only did the different blasts vary in strength, but sudden swellings and fallings off, even of the same blast, were observed. This was not due to any variation on the part of the instruments, but purely to the changes of the medium traversed by the sound. What these changes were shall be indicated subsequently.

The range of our best horns on June 10th was 834 miles. The guns at this distance were very feeble. That the loudness of the sound depends on the shape of the gun was proved by the fact that thus far the howitzer, with a three-lb. charge, proved more effective than the other guns.

On June 25th a gradual improvement in the transmissive power of the, air was observed from morning to evening; but at the last the maximum range was only moderate. The fluctuations in the strength of the sound were remarkable, sometimes sinking to inaudibility and then rising to loudness. A similar effect, due to a similar cause, is often noticed with church-bells. The acoustic transparency of the air was still further augmented on the 26th: at a distance of 914 miles from the station the whistles and horns were plainly heard against a wind with a force of four; white on the 25th, with a favoring wind, the maximum range was only 612 miles. Plainly, therefore, something else than the wind must be influential in determining the range of the sound.

On Tuesday, July 1st, observations were made on the decay of the sound at various angular distances from the axis of the horn. As might be expected, the sound in the axis was loudest, the decay being gradual on both sides. In the case of the gun, however, the direction of pointing has very little influence.

The day was acoustically clear; at a distance of 10 miles the horn yielded a plain sound, while the American whistle seemed to surpass the horn. Dense haze at this time quite hid the Foreland. At 1012 miles occasional blasts of the horn came to us, but, after a time, all sound ceased to be audible; it seemed as if the air, after having been exceedingly transparent, had become gradually more opaque to the sound.

At 4.45 p. m. we took the master of the Varne light-ship on board the Irene. He and his company had heard the sound at intervals during the day, although he was dead to windward and distant 1234 miles from the source of sound.

Here a word of reflection on our observations may be fitly introduced. It is, as already shown, an opinion entertained in high quarters that the waves of sound are reflected at the limiting surfaces of the minute particles which constitute haze and fog, the alleged waste of sound in fog being thus explained. If, however, this be an efficient practical cause of the stoppage of sound, and if clear, calm air be, as alleged, the best vehicle, it would be impossible to understand how to-day, in a thick haze, the sound reached a distance of 1234 miles, while on May 20th, in a calm and hazeless atmosphere, the maximum range was only from 5 to 6 miles. Such facts foreshadow a revolution in our notions regarding the action of haze and fogs upon sound.

An interval of 12 hours sufficed to change in a surprising degree the acoustic transparency of the air. On the 1st of July the sound had a range of nearly 13 miles; on the 2d the range did not exceed 4 miles.

Contradictory Results.—Thus far the investigation proceeded with hardly a gleam of a principle to connect the inconstant results. The distance reached by the sound on the 19th of May was 312 miles; on the 20th it was 512 miles; on the 2d of June, 6 miles; on the 3d, more than 9 miles; on the 10th it was also 9 miles; on the 25th it fell to 612 miles; on the 26th it rose again to more than 914 miles; on the 1st of July, as we have just seen, it reached 1234, whereas on the 2d the range shrunk to 4 miles. None of the meteorological agents observed could be singled out as the cause of these fluctuations. The wind exerts an acknowledged power over sound, but it could not account for these phenomena. On the 25th of June, for example, when the range was only 612 miles, the wind was favorable; on the 26th, when the range exceeded 914 miles, it was opposed to the sound. Nor could the varying optical clearness of the atmosphere be invoked as an explanation; for, on July 1st, when the range was 1234 miles, a thick haze hid the white cliffs of the Foreland, while on many other days, when the acoustic range was not half so great, the atmosphere was optically clear. Up to July 3d all remained enigmatical; but on this date observations were made which seemed to me to displace surmise and perplexity by the clearer light of physical demonstration.

Solution of Contradictions.—On July 3d we first steamed to a point 2.9 miles southwest-by-west of the signal station. No sounds, not even the guns, were heard at this distance. At two miles they were equally inaudible. But this being a position at which the sounds, though strong in the axis of the horn, invariably subsided, we steamed to the exact bearing from which our observations had been made on July 1st. At 2.15 p. m., and at a distance of 334 miles from the station, with calm, clear air and a smooth sea, the horns and whistle (American) were sounded, but they were inaudible. Surprised at this result, I signaled for the guns. They were all fired, but, though the smoke seemed at hand, no sound whatever reached us. On July 1st, in this bearing, the observed range of both horns and guns was 1012 miles, while, on the bearing of the Varne light-vessel, it was nearly 13 miles. We steamed in to 3 miles, paused, and listened with all attention; but neither horn nor whistle was heard. The guns were again signaled for; five of them were fired in succession, but not one of them was heard. We steamed on in the same bearing to 2 miles, and had the guns fired point-blank at us. The howitzer and the mortar, with 3-lb. charges, yielded a feeble thud, while the 18-pounder was wholly unheard. Applying the law of inverse squares, it follows that, with the air and sea, according to accepted notions, in a far worse condition, the sound at two miles' distance on July 1st must have had more than forty times the intensity which it possessed at the same distance at 3 p. m. on the 3d.

"On smooth water," says Sir John Herschel, "sound is propagated with remarkable clearness and strength." Here was the condition; still, with the Foreland so close to us, the sea so smooth, and the air so transparent, it was difficult to realize that the guns had been fired, or the trumpets blown at all. What could be the reason? Had the sound been converted by internal friction into heat, or had it been wasted in partial reflections at the limiting surfaces of non-homogeneous masses of air? I ventured, two or three years ago, to say something regarding the function of the imagination in science, and, notwithstanding the care then taken, to define and illustrate its real province, some persons, among whom were one or two able men, deemed me loose and illogical. They misunderstood me. The faculty to which I referred was that power of visualizing processes in space, and the relations of space itself, which must be possessed by all great physicists and geometers. Looking, for example, at two pieces of polished steel, we have not a sense, or the rudiment of a sense, to distinguish the inner condition of the one from that of the other. And yet they may differ materially, for one may be a magnet, the other not. What enabled Ampère to surround the atoms of such a magnet with channels in which electric currents ceaselessly run, and to deduce from these pictured currents all the phenomena of ordinary magnetism? What enabled Faraday to visualize his lines of force, and make his mental picture a guide to discoveries which have rendered his name immortal? Assuredly it was the disciplined imagination. Figure the observers on the deck of the Irene, with the invisible air stretching between them and the South Foreland, knowing that it contained something which stifled the sound, but not knowing what that something is. Their senses are not of the least use to them; nor could all the philosophical instruments in the world render them any assistance. They could not, in fact, take a single step toward the solution without the formation of a mental image; in other words, without the exercise of the imagination.

Sulphur in homogeneous crystals is exceedingly transparent to radiant heat, whereas the ordinary brimstone of commerce is highly impervious to it—the reason being that the brimstone does not possess the molecular continuity of the crystal, but is a mere aggregate of minute grains not in perfect optical contact with each other. Where this is the case, a portion of the heat is always reflected on entering and on quitting a grain; hence, when the grains are minute and numerous, this reflection is so often repeated that the heat is entirely wasted before it can plunge to any depth into the substance. The same remark applies to snow, foam, clouds, and common salt, indeed to all transparent substances in powder; they are all impervious to light, not through the immediate absorption or extinction of the light, but through repeated internal reflection.

Humboldt, in his observations at the Falls of the Orinoco, is known to have applied these principles to sound. He found the noise of the falls far louder by night than by day, though in that region the night is far noisier than the day. The plain between him and the falls consisted of spaces of grass and rock intermingled. In the heat of the day he found the temperature of the rock to be considerably higher than that of the grass. Over every heated rock, he concluded, rose a column of air rarefied by the heat; its place being supplied by the descent of heavier air. He ascribed the deadening of the sound to the reflections which it endured at the limiting surfaces of the rarer and denser air. This philosophical explanation made it generally known that a non-homogeneous atmosphere is unfavorable to the transmission of sound.

But what, on July 3d, not with the variously-heated plain of Antures, but with a calm sea as a basis for the atmosphere, could so destroy its homogeneity as to enable it to quench in so short a distance so vast a body of sound? My course of thought at the time was thus determined. As I stood upon the deck of the Irene, pondering the question, I became conscious of the exceeding power of the sun beating against my back and heating the objects near me. Beams of equal power were falling on the sea, and must have produced copious evaporation. That the vapor generated should so rise and mingle with the air as to form an absolutely homogeneous medium was in the highest degree improbable. It would be sure, I thought, to rise in invisible streams, breaking through the superincumbent air, now at one point, now at another, thus rendering the air flocculent with wreaths and striæ, charged in different degrees with the buoyant vapor. At the limiting surfaces of these spaces, though invisible, we should have the conditions necessary to the production of partial echoes and the consequent waste of sound. Ascending and descending air-currents, of different temperatures, as far as they existed, would also contribute to the effect.

Curiously enough, the conditions necessary for the testing of this explanation immediately set in. At 3.15 p. m., a solitary cloud threw itself athwart the sun, and shaded the entire space between us and the South Foreland. The heating of the water, and the production of vapor, were suddenly checked by the interposition of this screen; hence the probability of suddenly-improved transmission. To test this inference, the steamer was immediately turned and urged back to our last position of inaudibility. The sounds, as I expected, were distinctly though faintly heard. This was at 3 miles' distance. At 334 miles the guns were fired, both point-blank and elevated. The faintest pop was all that we heard; but we did hear a pop, whereas we had previously heard nothing, either here or three-quarters of a mile nearer. We steamed out to 414 miles, where the sounds were for a moment faintly heard; but they fell away as we waited, and though the greatest quietness reigned on board, and though the sea was without a ripple, we could hear nothing. We could plainly see the steam puffs which announced the beginning and the end of a series of trumpet-blasts, but the blasts themselves were quite inaudible.

It was now 4 p. m., and my intention at first was to halt at this distance, which was beyond the sound-range, but not far beyond it, and see whether the lowering of the sun would not restore the power of the atmosphere to transmit the sound. But, after waiting a little, the anchoring of a boat was suggested, so as to liberate the steamer for other work; and, though loath to lose the anticipated revival of the sounds myself, I agreed to this arrangement. Two men were placed in the boat and requested to give all attention, so as to hear the sound if possible. With perfect stillness around them, they heard nothing. They were then instructed to hoist a signal if they should hear the sounds, and to keep it hoisted as long as the sounds continued.

At 4.45 we quitted them and steamed toward the South Sand Head light-ship. Precisely fifteen minutes after we had separated from them the flag was hoisted: the sound had at length succeeded in piercing the body of air between the boat and the shore.

We continued our journey to the light-ship, went on board, heard the report of the lightsmen, and returned to our anchored boat. We then learned that when the flag was hoisted the horn-sounds were heard, that they were succeeded after a little time by the whistle-sounds, and that both increased in intensity as the evening advanced. On our arrival, of course we heard the sounds ourselves.

We pushed the test further by steaming farther out. At 534 miles, we halted and heard the sounds; at 6 miles we heard them distinctly, but so feebly that we thought we had reached the limit of the sound-range; but while we waited the sounds rose in power. We steamed to the Varne buoy, which is 734 miles from the signal-station, and heard the sounds there better than at 6 miles' distance. We continued our course outward to 10 miles, halted there for a brief interval, but heard nothing.

Steaming, however, on to the Varne light-ship, which is situated at the other end of the Varne shoal, we hailed the master, and were informed by him that up to 5 p. m. nothing had been heard, but that at that hour the sounds began to be audible. He described one of them as "very gross, resembling the bellowing of a bull," which very accurately characterizes the sound of the large American steam-whistle. At the Varne light-ship, therefore, the sounds had been heard toward the close of the day, though it is 1234 miles from the signal-station. I think it probable that, at a point 2 miles from the Foreland, the sound at 5 p. m. possessed fifty times the intensity which it possessed at 2 p. m. To such undreamt-of fluctuations is the atmosphere liable. On our return to Dover Bay, at 10 p. m., we heard the sounds, not only distinct but loud, where nothing could be heard in the morning.

Remarkable Instances of Acoustic Opacity.—In his excellent lecture entitled "Wirkungen aus der Feme," Dove has collected some striking cases of the interception of sound. The Duke of Argyll has also favored me with some highly-interesting illustrations. But nothing of this description that I have read equals in point of interest the following account of the battle of Gain's Farm, for which I am indebted to the Rector of the University of Virginia:

 "Lynchburg, Va., March 19, 1874.⁠
"Sir: I have just read with great interest your lecture of January 16th, on the acoustic transparency and opacity of the atmosphere. The remarkable observations you mention induce me to state to you a fact which I have occasionally mentioned, but always, where I am not well known, with the apprehension that my veracity would be questioned. It made a strong impression on me at the time, but was an insoluble mystery until your discourse gave me a possible solution.
"On the afternoon of June 28, 1862, I rode, in company with General G. W. Randolph, then Secretary of War of the Confederate States, to Price's house, about nine miles from Richmond; the evening before General Lee had begun his attack on McClellan's army, by crossing the Chickahominy about four miles above Price's, and driving in McClellan's right wing. The battle of Gain's Farm was fought the afternoon to which I refer. The valley of the Chickahominy is about one and a half mile wide from hill-top to hill-top. Price's is on one hilltop, that nearest to Richmond; Gain's Farm, just opposite, is on the other, reaching back in a plateau to Cold Harbor.
"Looking across the valley I saw a good deal of the battle, Lee's right resting in the valley, the Federal left wing the same. My line of vision was nearly in the line of the lines of battle. I saw the advance of the Confederates, their repulse two or three times, and in the gray of the evening the final retreat of the Federal forces.
"I distinctly saw the musket-fire of both lines, the smoke, the individual discharges, the flash of the guns. I saw batteries of artillery on both sides come into action and fire rapidly. Several field-batteries on each side were plainly in sight. Many more were hid by the timber which bounded the range of vision.
"Yet, looking for nearly two hours, from about 5 to 7 p. m. on a midsummer afternoon, at a battle in which at least 50,000 men were actually engaged, and doubtless at least 100 pieces of field-artillery, through an atmosphere optically as limpid as possible, not a single sound of the battle was audible to General Randolph and myself. I remarked it to him at the time as astonishing.
"Between me and the battle was the deep, broad valley of the Chickahominy, partly a swamp, shaded by the declining sun, by the hills and forest in the west (my side). Part of the valley on each side of the swamp was cleared; some in cultivation, some not. Here were conditions capable of providing several belts of air, varying in the amount of watery vapor (and probably in temperature), arranged like lamina? at right angles to the acoustic waves as they came from the battle-field to me.

"Respectfully, your obedient servant,R. G. H. Kean.

"Prof. John Tyndall."

I learn from a subsequent letter that daring the battle the air was still.—J. T.

Echoes from Invisible Acoustic Clouds.—But both the argument and the phenomena have a complementary side, which we have now to consider. A stratum of air less than three miles thick on a calm clay has been proved competent to stifle both the cannonade and the horn-sounds employed at the South Foreland; while, according to the foregoing explanation, this result was due to the reflection of the sound from invisible acoustic clouds which filled the atmosphere on a day of perfect optical transparency. But, granting this, it is incredible that so great a body of sound could utterly disappear in so short a distance without rendering some account of itself. Supposing, then, instead of placing ourselves behind the acoustic cloud, we were to place ourselves in front of it, might we not, in accordance with the law of conservation, expect to receive by reflection the sound which had failed to reach us by transmission? The case would then be strictly analogous to the reflection of light from an ordinary cloud to an observer between it and the sun.
My first care in the early part of the day in question was to assure myself that our inability to hear the sound did not arise from any derangement of the instruments on shore. Accompanied by the private secretary of the Deputy Master of the Trinity House, at 1 p. m. I was rowed to the shore, and landed at the base of the South Foreland Cliff. The body of air which had already shown such extraordinary power to intercept the sound, and which manifested this power still more impressively later in the day, was now in front of us. On it the sonorous waves impinged, and from it they were sent back with astonishing intensity. The instruments, hidden from view, were on the summit of a cliff 235 feet above us, the sea was smooth and clear of ships, the atmosphere was without a cloud, and there was no object in sight which could possibly produce the observed effect. From the perfectly transparent air the echoes came, at first with a strength apparently little less than that of the direct sound, and then dying gradually and continuously away. A remark made by my talented companion in his note-book at the time shows how the phenomenon affected him: "Beyond saying that the echoes seemed to come from the expanse of ocean, it did not appear possible to indicate any more definite point of reflection." Indeed, no such point was to be seen; the echoes reached us, as if by magic, from the invisible acoustic clouds with which the optically transparent atmosphere was tilled. The existence of such clouds in all weathers, whether optically cloudy or serene, is one of the most important points established by this inquiry.
Here, in my opinion, we have the key to many of the mysteries and discrepancies of evidence which beset this question. The foregoing observations show that there is no need to doubt either the veracity
or the capability of the conflicting witnesses, for the variations of the atmosphere are more than sufficient to account for theirs. The mistake, indeed, hitherto has been, not in reporting incorrectly, but in neglecting the monotonous operation of repeating the observations during a sufficient time. I shall have occasion to remark subsequently on the mischief likely to arise from giving instruction to mariners founded on observations of this incomplete character.

It required, however, long pondering and repeated observation before this conclusion took firm root in my mind; for it was opposed to the results of great observers, and to the statements of celebrated writers. In science, as elsewhere, a mind of any depth, which accepts a doctrine undoubtingly, discards it unwillingly. The question of aerial echoes has an historic interest. While cloud-echoes have been accepted as demonstrated by observation, it has been hitherto held as established that audible echoes never occur in optically clear air. We owe this opinion to the admirable report of Arago on the experiments made to determine the velocity of sound at Montlhéry and Villejuif in 1822.[7] Arago's account of the phenomenon observed by him and his colleagues is as follows: "Before ending this note we will only add that the shots fired at Montlhéry were accompanied by a rumbling like that of thunder, which lasted from 20 to 25 seconds. Nothing of this kind occurred at Villejuif. Once we heard two distinct reports, a second apart, of the Montlhéry cannon. In two other cases the report of the same gun was followed by a prolonged rumbling. These phenomena never occurred without clouds. Under a clear sky the sounds were single and instantaneous. May we not, therefore, conclude that the multiple reports of the Montlhéry gun heard at Villejuif were echoes from the clouds, and may we not accept this fact as favorable to the explanation given by certain physicists of the rolling of thunder?"

I think both the fact and the inference need reconsideration. For our observations prove to demonstration that air of perfect visual transparency is competent to produce echoes of great intensity and long duration. The subject is worthy of additional illustration. On the 8th of October, as already stated, the siren was established at the South Foreland. I visited the station on that day, and listened to its echoes. They were far more powerful than those of the horn. Like the others, they were perfectly continuous, and faded, as if into distance, gradually away. The direct sound seemed rendered complex and multitudinous by its echoes, which resembled a band of trumpeters first responding close at hand, and then retreating rapidly toward the coast of France. The siren-echoes on that day had 11 seconds', those of the horn 8 seconds' duration.

In the case of the siren, moreover, the reënforcement of the direct sound by its echo was distinct. About a second after the commencement of the siren-blast, the echo struck in as a new sound. This first echo, therefore, must have been flung back by a body of air not more than 600 or 700 feet in thickness. The few detached clouds visible at the time were many miles away, and could clearly have had nothing to do with the effect.

On the 10th of October, I was again at the Foreland, listening to the echoes, with results similar to those just described. On the 15th I had an opportunity of remarking something new concerning them at Dungeness, where a horn, similar to, though not so powerful as, those at the South Foreland, has been mounted. It rotates automatically through an arc of 210°, halting at four different points on the arc and emitting a blast of 6 seconds' duration, these blasts being separated from each other by intervals of silence of 20 seconds.

The new point observed was this: As the horn rotated the echoes were always returned along the line in which the axis of the horn pointed. Standing either behind or in front of the light-house tower, or closing the eyes so as to exclude all knowledge of the position of the horn, the direction of its axis when it sounded could always be inferred from the direction in which the aërial echoes reached the shore. Not only, therefore, is knowledge of direction given by a sound, but it may also be given by the aerial echoes of the sound.

On the 17th of October, at about 5 p. m., the air being perfectly free from clouds, we rowed toward the Foreland, landed, and passed over the sea-weed to the base of the cliff. As I reached the base, the position of the Galatea was such that an echo of astonishing intensity was sent back from her side; it came as if from an independent source of sound established on board the steamer. This echo ceased suddenly, leaving the aërial echoes to die gradually into silence.

At the base of the cliff a series of concurrent observations made the duration of the aerial siren-echoes from 13 to 14 seconds.

Lying on the shingle under a projecting roof of chalk, the somewhat enfeebled diffracted sound reached me, and I was able to hear with great distinctness, about a second after the starting of the siren-blast, the echoes striking in and reënforcing the direct sound. The first rush of echoed sound was very powerful, and it came, as usual, from a stratum of air 600 or 700 feet in thickness. On again testing the duration of the echoes, it was found to be from 14 to 15 seconds. The perfect clearness of the afternoon caused me to choose it for the examination of the echoes. It was worth remarking that this was our day of longest echoes, and it was also our day of greatest acoustic transparency, this association suggesting that the duration of the echo is a measure of the atmospheric depths from which it comes. On no day, it is to be remembered, was the atmosphere free from invisible acoustic clouds; and on this day, and when their presence did not prevent the direct sound from reaching to a distance of 15 or 16 nautical miles, they were able to send us echoes of 15 seconds' duration.

On various occasions, when fully three miles from the shore, the Foreland bearing north, we have had the distinct echoes of the siren sent back to us from the cloudless southern air.

To sum up this question of aërial echoes. The siren sounded three blasts a minute, each of 5 seconds' duration. From the number of days and the number of hours per day during which the instrument was in action, we can infer the number of blasts. They reached nearly 20,000. The blasts of the horns exceeded this number, while hundreds of shots were fired from the guns. Whatever might be the state of the weather, cloudy or serene, stormy or calm, the aerial echoes, though varying in strength and duration from day to day, were never absent; and on many days, "under a perfectly clear sky," they reached, in the case of the siren, an astonishing intensity. It is to these air-echoes and not to cloud-echoes, that the rolling; of thunder is to be ascribed.

Experimental Demonstration of Aërial Reflection.—Thus far we have dealt in inference merely, for the interception of sound through aërial reflection has never been experimentally demonstrated; and, indeed, according to Arago's observation, which has hitherto held undisputed possession of the scientific field, it does not sensibly exist. But the strength of science consists in verification, and I was anxious to submit the question of aërial reflection to an experimental test. As in most similar cases, it was not the simplest combinations that were first adopted. Two gases of different densities were to be chosen, and I chose carbonic acid and coal gas. With the aid of my skillful assistant, Mr. John Cottrell, a tunnel was formed, across which five-and-twenty layers of carbonic acid were permitted to fall, and five-and-twenty alternate layers of coal-gas to rise. Sound was sent through this tunnel, making fifty passages from medium to medium in its course. These, I thought, would waste in aerial echoes a sensible portion of sound.

To indicate this waste an objective test was found in a gas-flame brought to the verge of flaring. The action of sonorous vibrations on 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 t t' (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 flame (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 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.

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 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 candle-flames, 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.

Fig. 2.—Apparatus for illustrating the Reflection of Sound in a Non-homogeneous 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 134 inch diameter, in the direction B A, the action of the sound being rendered manifest by its causing a sensitive flame placed at F' to become violently agitated.

"The invisible heated layer immediately above the luminous portion 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.

1. "Essay on Sound," par. 21.
2. "Report of the British Association" for 1863, p. 105.
3. A very sagacious remark, as observation proves.
4. Powerful electric lights have been since established, and found ineffectual.
5. This is also Sir John Herschel's way of regarding the subject, "Essay on Sound," par. 38.
6. In all cases nautical miles are meant.
7. Sir John Herschel gives the following account of Arago's observation: "The rolling of thunder has been attributed to echoes among the clouds; and, if it is considered that a cloud is a collection of particles of water, however minute, in a liquid state, and therefore each individually capable of reflecting sound, there is no reason why very loud sounds should not be reverberated confusedly (like bright lights) from a cloud. And that such is the case has been ascertained by direct observation on the sound of cannon. Messrs. Arago, Matthieu, and Prony, in their experiments on the velocity of sound, observed that under a perfectly clear sky the explosions of their guns were always single and sharp; whereas, when the sky was overcast, and even when a cloud came in sight over any considerable part of the horizon, they were frequently accompanied by a long-continued roll like thunder."—("Essay on Sound," par. 38.) The distant clouds would imply a long interval between sound and echo, but nothing of the kind is reported.