Popular Science Monthly/Volume 13/July 1878/Recent Experiments on Fog-Signals
By Professor JOHN TYNDALL, F. R. S.
OUR most intense coast-lights, including the six-wick lamp, the Wigham gaslight, and the electric light, being intended to aid the mariner in heavy weather, may be regarded, in a certain sense, as fog-signals. But fog, when thick, is intractable to light; the sun cannot penetrate it, much less any terrestrial source of illumination. Hence the necessity of employing sound-signals in dense fogs. Bells, gongs, horns, guns, and sirens, have been used for this purpose; but it is mainly, if not wholly, explosive signals that I have now to submit to the notice of the Society. During the long, laborious, and, I venture to think, memorable series of observations conducted under the auspices of the Elder Brethren of the Trinity House at the South Foreland in 1872 and 1873, it was proved that a short 52-inch howitzer, firing 3 pounds of powder, yielded a louder report than a long 18-pounder firing the same charge. Here was a hint to be acted on by the Elder Brethren. The effectiveness of the sound depended on the shape of the gun, and as it could not be assumed that in the howitzer we had hit accidentally upon the best possible shape, arrangements were made with the War Office for the construction of a gun specially calculated to produce the loudest sound attainable from the combustion of 3 pounds of powder. To prevent the unnecessary landward waste of the sound, the gun was furnished with a parabolic muzzle, intended to project the sound over the sea, where it was most needed. The construction of this gun was based on a searching series of experiments executed at Woolwich with small models, provided with muzzles of various kinds. The gun was constructed on the principle of the revolver, its various chambers being loaded and brought in rapid succession into the firing position. The performance of the gun proved the correctness of the principles on which its construction was based.
It had been a widely-spread opinion among artillerists, that a bronze gun emits a specially loud report. I doubted from the outset whether this would help us; and in a letter dated April 22, 1874, ventured to express myself thus: "The report of a gun, as affecting an observer close at hand, is made up of two factors—the sound due to the shock of the air by the violently expanding gas, and the sound derived from the vibrations of the gun, which, to some extent, rings like a bell. This latter, I apprehend, will disappear at considerable distances." The result of subsequent trial, as reported by General Campbell, is,
Fig. 1.—Breech-loading Fog-Signal Gun, with Bell-Mouth, proposed by Major Maitland, R.A., Assistant Superintendent.
that "the sonorous qualities of bronze are greatly superior to those of cast-iron at short distances, but that the advantage lies with the baser metal at long ranges."
Coincident with these early trials of guns at Woolwich, gun-cotton was thought of as a probably effective sound-producer. Theoretic considerations caused me to fix my attention persistently on this substance; for the remarkable experiments of Mr. Abel, whereby its rapidity of combustion and violently explosive energy are demonstrated, seemed to single it out as a substance eminently calculated to fulfill the conditions necessary to the production of an intense wave of sound. What those conditions are we shall now more particularly inquire, calling to our aid a brief but very remarkable paper, published by Prof. Stokes in the Philosophical Magazine for 1868.
The explosive force of gunpowder is known to depend on the sudden conversion of a solid body into an intensely heated gas. The work which the artillerist requires the expanding gas to perform is the displacement of the projectile. Such, however, is not the work that we want our gunpowder to perform. We wish it to transmute its energy not into the mere mechanical translation of the shot, but into vibratory motion. We want pulses to be formed which shall propagate themselves to vast distances through the atmosphere, and this requires a certain choice and management of the explosive material.
A sound-pulse consists essentially of two parts—a condensation and a rarefaction. Now, air is a very mobile fluid, and, if the shock imparted to it lack due promptness, the pulse is not produced. Consider the case of a common clock-pendulum, which oscillates to and fro, and which therefore might be expected to generate corresponding pulses in the air. When, for example, the bob moves to the right, the air to the right of it might be supposed to be condensed, while a partial vacuum might be supposed to follow the bob. As a matter of fact, we have nothing of this kind. The air-particles in front of the bob retreat so rapidly, and those behind it close so rapidly in, that no sound-pulse is formed. A tuning-fork which executes 256 complete vibrations in a second, if struck gently on a pad and held in free air, emits a scarcely audible note. It behaves to some extent like the pendulum-bob. This feebleness is due to the prompt "reciprocating flow" of the air between the incipient condensations and rarefactions, whereby the formation of sound-pulses is forestalled. Stokes, however, has taught us that this flow may be intercepted by placing the edge of a card in close proximity to one of the corners of the fork. An immediate augmentation of the sound of the fork is the consequence.
The more rapid the shock imparted to the air, the greater is the fractional part of the energy of the shock converted into wave-motion. And, as different kinds of gunpowder vary considerably in their rapidity of combustion, it may be expected that they will also vary as producers of sound. This theoretic inference is completely verified by experiment. In a series of preliminary trials conducted at Woolwich on the 4th of June, 1875, the sound-producing powers of four different kinds of powder were determined. In the order of size of grain they bear the names respectively of Fine-grain (F. G.), Large-grain (L. G.), Rifle Large-grain (R. L. G.), and Pebble-powder (P.), (Fig. 2). The charge in each case
amounted to 4½ pounds.; four 24 pound howitzers being employed to fire the respective charges. There were eleven observers, all of whom, without a single dissentient, pronounced the sound of the fine-grain powder loudest of all. In the opinion of seven of the eleven the large-grain powder came next; seven also of the eleven placed the rifle large-grain third on the list; while they were again unanimous in pronouncing the pebble-powder the worst sound-producer. These differences are entirely due to differences in the rapidity of combustion. All who have witnessed the performance of the 80-ton gun at Woolwich must have been surprised at the mildness of its thunder. To avoid the strain resulting from quick combustion, the powder employed is composed of lumps far larger than those of the pebble-powder above referred to. In the long tube of the gun these lumps of solid matter gradually resolve themselves into gas, which on issuing from the muzzle imparts a kind of push to the air, instead of the sharp shock necessary to form the condensation of an intensely sonorous wave.
These are some of the physical reasons why gun-cotton might be regarded as a promising fog-signal. Firing it as we have been taught to do by Mr. Abel, its explosion is more rapid than that of gunpowder. In its case the air-particles, alert as they are, will not, it might be presumed, be able to slip from places of condensation to places of rarefaction with a rapidity sufficient to forestall the formation of the wave. On a priori grounds, then, we are entitled to infer the effectiveness of gun-cotton, while in a great number of comparative experiments, stretching from 1874 to the present time, this inference has been verified in the most conclusive manner.
On the 22d of February, 1875, a number of small guns, cast specially for the purpose—some with plain, some with conical, and some with parabolic muzzles, firing 4 ounces of fine-grain powder—were pitted against 4 ounces of gun-cotton, detonated both in the open and in the focus of a parabolic reflector. The sound produced by the gun-cotton, reënforced by the reflector, was unanimously pronounced loudest of all. With equal unanimity, the gun-cotton detonated in free air was placed second in intensity. Though the same charge was used throughout, the guns differed not among themselves, but none of them came up to the gun-cotton, either with or without the reflector. A second series, observed from a different distance on the same day, confirmed to the letter the foregoing result.
As a practical point, however, the comparative cost of gun-cotton and gunpowder has to be taken into account, though considerations of cost ought not to be stretched too far in cases involving the safety of human life. In the earlier experiments, where quantities of equal price were pitted against each other, the results were somewhat fluctuating. Indeed, the perfect manipulation of the gun-cotton required some preliminary discipline—promptness, certainty, and effectiveness of firing, augmenting as experience increased. As 1 pound of guncotton costs as much as 3 pounds of gunpowder, these quantities were compared together on the 22d of February. The guns employed to discharge the gunpowder were a 12-pound brass howitzer, a 24-pound cast-iron howitzer, and the long 18-pounder used at the South Foreland. The result recorded is, that the 24-pound howitzer, firing 3 pounds of gunpowder, had slight advantage over 1 pound of gun-cotton detonated in the open; while the 12-pound howitzer and the 18-pounder were both beaten by the gun-cotton. On the 2d of May, on the other hand, the gun-cotton is reported as having been beaten by all the guns.
Meanwhile, the parabolic muzzle gun (Fig. 3), expressly intended for fog-signaling, was pushed rapidly forward, and on the 22d and 23d of March, 1876, its power was tested at Shoeburyness. Pitted against it were a 16-pounder, a 52-inch howitzer, 12 pound of gun-cotton detonated in the focus of a reflector, and 12 pound of gun-cotton detonated
Fig. 3.—Gun-Cotton Slab (12-lb) detonated in the Focus of a Cast-iron Reflector.
in free air. On this occasion, nineteen different series of experiments were made, when the new experimental gun, firing a 3-pound charge, demonstrated its superiority over all guns previously employed to fire the same charge. As regards the comparative merits of the gun-cotton fired in the open, and the gunpowder fired from the new gun, the mean values of their sounds were found to be the same. Fired in the focus of the reflector, the gun-cotton clearly dominated over all the other sound-producers.
The whole of the observations here referred to were embraced by an angle of about 70°, of which 50° lay on the one side and 20° on the other side of the line of fire. The shots were heard by eleven observers on board the Galatea, which took up positions varying from 2 miles to 132 miles from the firing-point. In all these observations, the reënforcing action of the reflector, and of the parabolic muzzle of the gun, came into play. But the reënforcement of the sound in one direction implies its withdrawal from some other direction, and accordingly it was found that at a distance of 54 miles from the firing-point, and on a line including nearly an angle of 90° with the line of fire, the gun-cotton in the open beat the new gun; while behind the station, at distances of 82 miles and 132 miles respectively, the gun-cotton in the open beat both the gun and the gun-cotton in the reflector. This result is rendered more important by the fact that the sound reached the Mucking Light, a distance of 132 miles, against a light wind which was blowing at the time.
Most, if not all, of our ordinary sound-producers send forth waves which are not of uniform intensity throughout. A trumpet is loudest in the direction of its axis. The same is true of a gun. A bell, with its mouth pointed upward or downward, sends forth waves far denser in the horizontal plane passing through the bell than at an angular distance of 90° from that plane. The oldest bell-hangers must have been aware of the fact that the sides of the bell, and not its mouth, emitted the strongest sound, their practice being determined by this knowledge. Our slabs of gun-cotton also emit waves of different densities in different parts. It has occurred in the experiments at Shoeburyness that when the broad side of a slab was turned toward the suspending wire of a second slab six feet distant, the wire was cut by the explosion, while, when the edge of the slab was turned to the wire, this never occurred. To the circumstance that the broad sides of the slabs faced the sea is probably to be ascribed the remarkable fact observed on the 23d of March, that in two directions, not far removed from the line of fire, the gun-cotton detonated in the open had a slight advantage over the new gun.
Theoretic considerations rendered it probable that the shape and size of the exploding mass would affect the constitution of the wave of sound. I did not think large rectangular slabs the most favorable shape, and accordingly proposed cutting a large slab into fragments of different sizes, and pitting them against each other. The differences between the sounds were by no means so great as the differences in the quantities of explosive material might lead one to expect. The mean values of eighteen series of observations made on board the Galatea, at distances varying from 14 mile to 4.8 miles, were as follows:
These charges were cut from a slab of dry gun-cotton about 14 inch thick; they were squares and rectangles of the following dimensions: 4 ounces, 2 inches by 2 inches; 6 ounces, 2 inches by 3 inches; 9 ounces, 3 inches by 3 inches; 12 ounces, 2 inches by 6 inches.
The numbers under the respective weights express the recorded value of the sounds. They must be simply taken as a ready means of expressing the approximate relative intensity of the sounds as estimated by the ear. When we find a 9-ounce charge marked 4, and a 12-ounce charge marked 4.03, the two sounds may be regarded as practically equal in intensity, an addition of 30 per cent, in the larger charges producing no sensible difference in the sound. Were the sounds estimated by some physical means, instead of by the ear, the values of the sound would not, in my opinion, show a greater advance with the increase of material than that indicated by the foregoing numbers. Subsequent experiments rendered still more certain the effectiveness, as well as the economy, of small charges of gun-cotton.
It is an obvious corollary from the foregoing experiments that on our "nesses" and promontories, where the land is clasped on both sides for a considerable distance by the sea—where, therefore, the sound has to propagate itself rearward as well as forward—the use of the parabolic gun, or of the parabolic reflector, might be a disadvantage rather than an advantage. Here gun-cotton, exploded in the open, forms a most appropriate source of sound. This remark is especially applicable to such lightships as are intended to spread the sound all round them as from central foci. As a signal in rock-lighthouses, where neither siren, steam whistle, nor gun, could be mounted, and as a handy fleet-signal, dispensing with the lumber of special signal-guns, the gun-cotton will prove invaluable. But in most of these cases we have the drawback that local damage may be done by the explosion. The lantern of the rock-lighthouse might suffer from concussion near at hand, and though mechanical arrangements might be devised, both in the case of the lighthouse and of the ship's deck, to place the firing-point of the gun-cotton at a safe distance, no such arrangement could compete, as regards simplicity and effectiveness, with the expedient of a gun-cotton rocket. Had such a means of signaling existed at the Bishop's Rock Lighthouse, the ill fated Schiller might have been warned of her approach to danger ten, or it may be twenty, miles before she reached the rock which wrecked her. Had the fleet possessed such a signal, instead of the ubiquitous but ineffectual whistle, the Iron Duke and Vanguard need never have come into collision.
It was the necessity of providing a suitable signal for rock lighthouses, and of clearing obstacles which cast an acoustic shadow, that suggested the idea of the gun-cotton rocket to Sir Richard Collinson, Deputy Master of the Trinity House. That idea was to place a disk or short cylinder of the gun-cotton in the head of a rocket, the ascensional force of which should be employed to 2 ounces of gun-cotton. They ascended to a great height, and exploded with a very loud report in the air. On the 27th of July, the rockets were tried at Shoeburyness, the most noteworthy result on this occasion being the hearing of the rockets at the Mouse Lighthouse, 82 miles east by south, and at the Chapman Lighthouse, 82 miles west by north; that is to say, at opposite sides of the firing-point. It is worthy of remark that, in the case of the Chapman Lighthouse, land and trees intervened between the firing-point and the place of observation. "This," as General Younghusband justly remarked at the time, "may prove to be a valuable consideration if it should be found necessary to place a signal-station in a position whence the sea could not be freely observed." Indeed, the clearing of such obstacles was one of the objects which the inventor of the rocket had in view.the disk to an elevation of 1,000 feet or thereabouts, where, by the ignition of a fuse associated with a detonator, the gun-cotton should be fired, sending its sound in all directions vertically and obliquely down upon earth and sea. The first attempt to realize this idea was made on the 18th of July, 1876, at the firework manufactory of the Messrs Brock, at Nunhead. Eight rockets were then fired, four being charged with 5 ounces and four with 7
On the 13th of December, 1876, and again on the 8th of March, 1877, comparative experiments on firing at high and low elevations were executed. The gun-cotton near the ground consisted of 2-pound disks suspended from an horizontal iron bar about 42 feet above the ground. The rockets carried the same quantity of gun-cotton in their heads, and the height to which they attained, as determined by a theodolite, was from 800 to 900 feet.
The latter day was cold, with occasional squalls of snow and hail, the direction of the sound being at right angles to that of the wind. Five series of observations were made on board the Vestal, at distances varying from 3 to 6 miles. The mean value of the explosions in the air exceeded that of the explosions near the ground by a small but sensible quantity. At Windmill Hill, Gravesend, however, which was nearly to leeward, and 52 miles from the firing-point, in nineteen cases out of twenty-four the disk fired near the ground was loudest, while in the remaining five the rocket had the advantage. Toward the close of the day the atmosphere became very serene. A few distant cumuli sailed near the horizon, but the zenith and a vast angular space all round it were absolutely free from cloud. From the deck of the Galatea a rocket was discharged, which reached a great elevation, and exploded with a loud report. Following this solid nucleus of sound was a continuous train of echoes, which retreated to a continually greater distance, dying gradually off into silence after seven seconds' duration. These echoes were of the same character as those so frequently noticed at the South Foreland in 1872-'73, and called by me "aërial echoes."
On the 23d of March the experiments were resumed, the most noteworthy results of that day's observations being that the sounds were heard at Tillingham, 10 miles to the northeast; at West Mersea, 154 miles to the northeast by east; at Brightlingsea, 172 miles to the northeast; and at Clacton Wash, 202 miles to the northeast by 2 east. The wind was blowing at the time from the southeast. Some of these sounds were produced by rockets, some by a 24-pound howitzer, and some by an 8-inch maroon.
In December, 1876, Mr. Gardiner, the managing director of the Cotton-Powder Company, had proposed a trial of this material against the gun-cotton. The density of the cotton, he urged, was only 1.03, while that of the powder was 1.70. A greater quantity of explosive material being thus compressed into the same volume, Mr. Gardiner thought that a greater sonorous effect must be produced by the powder. At the instance of Mr. Mackie, who had been in communication previously with the Deputy Master of the Trinity House and myself, a committee of the Elder Brethren visited the cotton-powder manufactory, on the banks of the Swale, near Faversham, on the 16th of June, 1877. The weights of cotton-powder employed were 2 ounces, 8 ounces, 1 pound, and 2 pounds, in the form of rockets and of signals fired a few feet above the ground. The experiments throughout were arranged and conducted by Mr. Mackie. Our desire on this occasion was to get as near to windward as possible, but the Swale and other obstacles limited our distance to 12 mile. We stood here east-southeast from the firing-point, while the wind blew fresh from the northeast.
The cotton-powder yielded a very effective report. The rockets in general had a slight advantage over the same quantities of material fired near the ground. The loudness of the sound was by no means proportional to the quantity of the material exploded, 8 ounces yielding very nearly as loud a report as 1 pound. The "aërial echoes," which invariably followed the explosion of the rockets, were loud and long continued, shading off, as in all previous cases, by imperceptible gradations, into silence.
On the 17th of October, 1877, another series of experiments with howitzers and rockets was carried out at Shoeburyness. The charge of the howitzer was 3 pounds of L. G. powder. The charges of the rockets were 12 ounces, 8 ounces, 4 ounces, and 2 ounces, of gun-cotton respectively. The gun and the four rockets constituted a series, and eight series were fired during the afternoon of the 17th. The observations were made from the Vestal and the Galatea, positions being assumed which permitted the sound to reach the observers with the wind, against the wind, and across the wind. The distance of the Galatea varied from 3 to 7 miles, that of the Vestal, which was more restricted in her movements, being 2 to 3 miles. Briefly summed up, the result is that the howitzer, firing a 3-pound charge, which it will be remembered was our best gun at the South Foreland, was beaten by the 12-ounce rocket, by the 8-ounce rocket, and by the 4-ounce rocket. The 2-ounce rocket alone fell behind the howitzer.
It is worth while recording the distances to which some of the sounds were heard on the day now referred to:
The day was cloudy, with occasional showers of drizzling rain; the wind about northwest by north all day; at times squally, rising to a force of 6 and 7 and sometimes dropping to a force of 2 or 3. The station at Leigh excepted, all these places were to leeward of Shoeburyness. At four other stations to leeward, varying in distance from 152 to 242 miles, nothing was heard, while at eleven stations to windward, varying from 8 to 26 miles, the sounds were also inaudible. It was found, indeed, that the sounds proceeding against the wind did not penetrate much beyond 3 miles.
On the following day, viz., the 18th of October, we proceeded to Dungeness with the view of making a series of strict comparative experiments with gun-cotton and cotton-powder. Rockets containing 8 ounces, 4 ounces, and 2 ounces of gun-cotton had been prepared at the Royal Arsenal; while others, containing a similar quantity of cotton-powder, had been supplied by the Cotton-Powder Company at Faversham. With these were compared the ordinary 18-pounder gun, which happened to be mounted at Dungeness, firing the usual charge of 3 pounds of powder.
From these experiments it appeared that the gun-cotton and cotton powder were practically equal as producers of sound.
The effectiveness of small charges was also illustrated in a very striking manner, only a single unit separating the numerical value of the 8-ounce rocket from that of the 2-ounce rocket. The former was recorded as 6.9 and the latter as 5.9, the value of the 4-ounce charge being intermediate between them. These results were recorded by a number of very practised observers on board the Galatea. They were completely borne out by the observations of the coast-guard, who marked the value of the 8-ounce rocket 6.1, and that of the 2-ounce rocket 5.2. The 18-pounder gun fell far behind all the rockets, a result probably to be in part ascribed to the imperfection of the powder. The performance of the siren was, on the whole, less satisfactory than that of the rockets. The instrument was worked, not by steam of 70 pounds pressure, as at the South Foreland, but by compressed air, beginning with 40 pounds and ending with 30 pounds pressure. The trumpet was pointed to windward, and in the axis of the instrument the sound was about as effective as that of the 8-ounce rocket. But in a direction at right angles to the axis, and still more in the rear of this direction, the siren fell very sensibly behind even the 2-ounce rocket.
These are the principal comparative trials made between the gun-cotton rocket and other-fog signals; but they are not the only ones. On the 2d of August, 1877, for example, experiments were made at Lundy Island, with the following results: At 2 miles distant from the firing-point, with land intervening, the 18-pounder, firing a 3-pound charge, was quite unheard. Both the 4-ounce rocket and the 8-ounce rocket, however, reached an elevation which commanded the acoustic shadow, and yielded loud reports. When both were in view, the rockets were still superior to the gun. On the 6th of August, at St. Ann's, the 4 ounce and 8-ounce rockets proved superior to the siren. On the Shambles Light-vessel, when a pressure of 13 pounds was employed to sound the siren, the rockets proved greatly superior to that instrument. Proceeding along the sea-margin at Flamborough Head, Mr. Edwards states that at a distance of 14 mile, with the 18-pounder gun hidden behind the cliffs, its report was quite unheard, while the 4-ounce rocket, rising to an elevation which brought it clearly into view, yielded a powerful sound in the face of an opposing wind.
On the evening of February 9, 1877, a remarkable series of experiments was made by Mr. Prentice, at Stowmarket, with the gun-cotton rocket. From the report with which he has kindly furnished me I extract the following particulars. The first column in the annexed statement contains the name of the place of observation, the second its distance from the firing-point, and the third the result observed:
In the great majority of these cases, the direction of the sound inclosed a large angle with the direction of the wind. In some cases, indeed, the two directions were at right angles to each other. It is needless to dwell for a moment on the advantage of a signal commanding ranges such as these.
The explosion of substances in the air, after having been carried to a considerable elevation by rockets, is a familiar performance. In 1873 the Board of Trade actually proposed a light-and-sound rocket as a signal of distress, which proposal was subsequently realized, but in a form too elaborate and expensive for practical use. The idea of the gun-cotton rocket with a view to signaling in fogs is, I believe, wholly due to the Deputy Master of the Trinity House. Thanks to the skillful aid given by the authorities of Woolwich, by Mr. Prentice and Mr. Brock that idea is now an accomplished fact; a signal of great power, handiness, and economy, being thus placed at the service of our mariners. Not only may the rocket be applied in association with lighthouses and light-ships, but in the navy also it may be turned to important account. Soon after the loss of the Vanguard, I ventured to urge upon an eminent naval officer the desirability of having an organized code of fog-signals for the fleet. He shook his head doubtingly, and referred to the difficulty of finding room for signal-guns. The gun-cotton rocket completely surmounts this difficulty. It is manipulated with ease and rapidity, while its discharges may be so grouped and combined as to give a most important extension to the voice of the admiral in command. It is needless to add that at any point upon our coasts, or upon any other coast, where its establishment might be desirable, a fog-signal station might be extemporized without difficulty.
I have referred more than once to the train of echoes which accompanied the explosion of gun-cotton in free air, speaking of them as similar in all respects to those which were described for the first time in my report on fog-signals, addressed to the Corporation of Trinity House in 1874. To these echoes I attached a fundamental significance. There was no visible reflecting surface from which they could come. On some days, with hardly a cloud in the air, and hardly a ripple on the sea, they reached us with magical intensity. They came directly from the body of the air in front of the great trumpet which produced them. The trumpet-blasts were five seconds in duration, but long before the blast had ceased the echoes struck in, adding their strength to the primitive note of the trumpet. After the blast had ended the echoes continued, retreating farther and farther from the point of observation, and finally dying away at great distances. The echoes were perfectly continuous as long as the sea was clear of ships, "tapering" by imperceptible gradations into absolute silence. But when a ship happened to throw itself athwart the course of the sound, the echo from the broadside of the vessel was returned as a shock which rudely interrupted the continuity of the dying atmospheric music.
These echoes have been ascribed to reflection from the crests of the sea-waves. But this hypothesis is negatived by the fact that the echoes were produced in great intensity and duration when no waves existed—when the sea, in fact, was of glassy smoothness. It has been also shown that the direction of the echoes depended not on that of waves, real or assumed, but on the direction of the axis of the trumpet. Causing that axis to traverse an arc of 210°, and the trumpet to sound at various points of the arc, the echoes were always, at all events in calm weather, returned from that portion of the atmosphere toward which the trumpet was directed. They could not, under the circumstances, come from the glassy sea; while both their variation of direction and their perfectly continuous fall into silence are irreconcilable with the notion that they came from fixed objects on the land. They came from that portion of the atmosphere into which the trumpet poured its maximum sound, and fell in intensity as the direct sound penetrated to greater atmospheric distances.
The day on which our latest observations were made was particularly fine. Before reaching Dungeness, the smoothness of the sea and the serenity of the air caused me to test the echoing power of the atmosphere. A single ship lay about half a mile distant between us and the land. The result of the proposed experiment was clearly foreseen. It was this: The rocket being sent up, it exploded at a great height; the echoes retreated in their usual fashion, becoming less and less intense as the distance of the surfaces of reflection from the observers increased. About five seconds after the explosion, a single loud shock was sent back to us from the side of the vessel lying between us and the land. Obliterated for a moment by this more intense echo, the aërial reverberation continued its retreat, dying away into silence in two or three seconds afterward.
I have referred to the firing of an 8-ounce rocket from the deck of the Galatea, on March 8, 1877, stating the duration of its echoes to be seven seconds. Mr. Prentice, who was present at the time, assured me that, in his experiments with rockets, similar echoes had been frequently heard of more than twice this duration. The ranges of his sounds alone would render this result in the highest degree probable.
To attempt to interpret an experiment which I have not had an opportunity of repeating is an operation of some risk; and it is not without a consciousness of this that I refer here to a result considered adverse to the notion of aërial echoes. When the trumpet of a siren is pointed toward the zenith, it is alleged that when the siren is sounded no echo is returned. Now, the reflecting surfaces which give rise to these echoes are for the most part due to differences of temperature between sea and air. If, through any cause, the air above be chilled, we have descending streams—if the air below be warmed, we have ascending streams as the initial cause of atmospheric flocculence. A sound proceeding vertically does not cross the streams, nor impinge upon the reflecting surfaces, as does a sound proceeding horizontally across them. Aërial echoes, therefore, will not accompany the vertical sound as they accompany the horizontal one. The experiment, as I interpret it, is not opposed to the theory of aerial echoes which I have ventured to enunciate. But, as I have indicated, not only to see, but to vary such an experiment, is a necessary prelude to grasping its full significance.
In a paper published in the "Philosophical Transactions" for 1876, Prof. Osborne Reynolds refers to these echoes in the following terms: "Without attempting to explain the reverberations and echoes which have been observed, I will merely call attention to the fact that in no case have I heard any attending the reports of the rockets, although they seem to have been invariable with the guns and pistols. These facts suggest that the echoes are in some way connected with the direction given to the sound. They are caused by the voice, trumpets, and the siren, all of which give direction to the sound; but I am not aware that they have ever been observed in the case of a sound which has no direction of greatest intensity."
The reference to the voice and other references cause me to think that, in speaking of echoes, Prof. Osborne Reynolds and myself are dealing with different phenomena. Be that as it may, the foregoing observations render it perfectly certain that the condition as to direction here laid down is not necessary to the production of the echoes.
There is not a feature connected with the aerial echoes which cannot be brought out by experiments in the laboratory. I have recently made the following experiment: A rectangle, 22 inches by 12, is crossed by 23 brass tubes, each having a slit along it from which gas can issue. In this way, 23 low, flat flames are obtained. A sounding reed, fixed in a short tube, is placed at one end of the rectangle, and a sensitive flame at some distance beyond the other end. When the reed sounds, the flame in front of it is violently agitated, and roars boisterously. Turning on the gas, and lighting it as it issues from the slits, the air above the flames becomes so heterogeneous that the sensitive flame is instantly stilled by the aërial reflection, rising from a height of 6 inches to a height of 18 inches. Here we have the acoustic opacity of the air in front of the South Foreland strikingly imitated. Turning off the gas, and removing the sensitive flame to some distance behind the reed, it burns there tranquilly, though the reed may be sounding. Again lighting the gas as it issues from the brass tubes, the sound reflected from the heterogeneous air throws the sensitive flame into violent agitation. Here we have imitated the aërial echoes heard when standing behind the siren-trumpets at the South Foreland. The experiment is extremely simple, and in the highest degree impressive.