THE ACOUSTIC TRANSPARENCY AND
OPACITY OF THE ATMOSPHERE*

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

A disastrous loss of life and property is the consequence. During the last ten years, for example, the number of total wrecks on the coasts of the United Kingdom, which were reported to have been caused by fog and thick weather, amounted, I am informed, to 273 vessels.

Of late years various efforts have been made, both on our own coasts and on the American seaboard, where trade is more eager and fogs more frequent than they are here, to furnish warning and guidance to ships by means of sound signals of great power established along the coast. Regarding the performance of such signals, the most conflicting evidence exists; and no investigation has been hitherto instituted sufficiently exhaustive to remove the uncertainty.

The problem has occupied for some time the attention of the Elder Brethren of the Trinity House; and soon after my return from America they requested me, as their official adviser in scientific matters, to superintend an investigation of the entire subject. They had appointed a committee under whose auspices two stations had been established at the South Foreland. I entered upon the inquiry with such ardour as I could derive from a sense of duty, rather than from the pleasure of hope, for I knew it would be long and difficult, and that I was at the mercy of a medium, the earth's atmosphere, which could not be put into the witness-box and cross-examined scientifically. The experimenter can usually impose his own conditions upon Nature, and force her to reply. In the present case we were forced to accept the conditions which Nature imposed.

Nevertheless, if the student only holds on faithfully to any natural problem, intending his mind upon it, and not falling into hasty despair, he is sure to be rewarded in the end; and after a time results, important not only in a practical but in a purely scientific point of view, appeared to grow out of the investigation. I mentioned this to the Deputy Master of the Trinity House, saying that I thought such results might, without impropriety, be communicated to the Royal Society and the Royal Institution. His response was prompt and cordial, and he was seconded by his colleagues in this response. They gave not only the requested permission (which on various pleas they might have withheld), but they have aided me in every way in the preparation

of this discourse.

I would add that the Elder Brethren themselves have had a large share in the executive portion of this investigation, and whatever success has attended the inquiry is in a greit measure due to the cheerful promptness and thoroughness with which my wishes and suggestions were carried out by the gentlemen with whom I had the honour to act. It is not necessary to mention names when all have been so sympathetic and so helpful, but I should like to refer to a few gentlemen on the working staff of the Trinity House, who have aided me with all assiduity and all zeal. They are the able Trinity House engineer, Mr. Doug-i lass, his assistant engineer, Mr. Ayres, and Mr. Price Edwards, the private secretary of the Deputy Master of the Trinity House. On Monday, May 19, the experiments began. The instruments employed had been previously mounted at the top and bottom of the South Foreland Cliff. They were two brass trumpets, or horns, 11 ft. 2 in. long, 2 in. in diameter at the mouth-piece, opening out at the other end to a diameter of 22 in. They were provided with vibrating steel recds, 9 in. long, 2 in wide, and in. thick, and were sounded by air of 18 lbs. pressure. They were mounted vertically on the reservoir of compressed air; but within about 2 ft. of their extremities they were bent at a right angle, so as to present their mouths to the sea. These horns were constructed by Mr. Holmes. There were also two whistles shaped like those in use on locomotives, one 6 in. in diameter, sounded by air of 18 lbs. pressure; the other constructed by Mr. Baily of Manchester, 12 in. in diameter and sounded by steam of 64 lbs. pressure.

We embarked on the steamer Irene, and placed ourselves abreast of the signal-station, halting at a distance of half a mile from it. The wind was strong, the sea rough. The superiority of the trumpets to the whistles was very marked, and I may

Royal Institution, Friday evening Discourse by Prof. Tyndall, D.C.L. LL.D.. F.R.S. Jan. 16.

say continued marked throughout. Their sound was exceedingly fine and powerful. At I mile's distance their sound was clear and strong; at 2 miles they were heard distinctly, though not loudly. The whistles were also heard, but as fog-signals they had become useless. At 3 miles the horns became also useless. It required great attention to hear them distinctly. At a distance of 4 miles, with the paddles stopped, we listened long and attentively, but heard nothing.

On May 20, at 3 miles' distance, the steam whistle was not at all heard, the horns but faintly. At 4 miles' distance, the air being very light, the sea calm, and the circumstances generally to all appearances highly favourable, we halted and listened. The horns were so heard as to render it unmistakeable that a sound was there. At 48 miles the sounds were faintly heard ; at 5 miles an occasional murmur reached us. At 6 miles the faint hum of a horn was wafted to us at intervals. A little farther out, though local noises were absent, and though we listened with stretched attention, we heard nothing.

This position, clearly beyond the range of whistles and trumpets, was chosen with the view of making a decisive comparative experiment between horns and guns as instruments for fog-signalling. Through the courtesy of General Sir A. Horsford we were enabled to carry out this comparison. At 12:30 precisely the puff of an 18-pounder, with a 3-lb charge, was seen at Dover Castle, which was about a mile farther off than the South Foreland. Thirty-six seconds afterwards 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 84 miles, where the report of a second gun was well heard. At 10 miles the report of the gun was heard by some and not by others. At 97 miles a fourth report was heard by all observers.

There was nothing, far as I am aware of, in our knowledge of the transmission of sound through the atmosphere, to invalidate the founding upon these experiments of the general conclusion that, as a fog-signal, the gun possessed a clear mastery over the horns. No observation, to my knowledge, had ever been made to show that a sound once predominant would not always be predominant; or that the atmosphere on different days would show preferences to different sounds. A complete reversal of the foregoing conclusion was therefore not to be anticipated; still, on many subsequent occasions, it was completely reversed.

On June 2 the maximum range, at first only 3 miles, afterwards ran up to about 6 miles.

Optically June 3 was not at all a promising day; the clouds were dark and threatening; and the air filled with a faint haze, nevertheless the horns were fairly audible at 9 miles: An exceedingly heavy rain-shower approached us at a galloping speed.

expressed opinions, should have deadened the sound. It rather aided the sound, and this added to my perplexity.

On June 10 the maximum range was 9 miles. An extraordinary sinking of the sound was, however, noticed on the Dover side of the Foreland. At a mile's distance from the station the sounds rapidly fell. Surprised at the suddenness of the effect, and thinking it might be due to some peculiarity of the horns, at 2 miles' distance I signalled for the guns. With a 3 lb. charge

not one of them was heard.

On June II we steamed towards the South Sound Head lightship. At the distance of 2 miles, and even at 2 miles and less from the station, the sounds were not so strong as at 3 miles. We steamed abreast of the station and on to the line joining the South Foreland to the end of the Admiralty Pier. At threequarters of a mile from the station the sound fell, and a little farther on was scarcely audible. This weakening of the sound between the pier and the Foreland was invariable. This needs a word of explanation. The fall of the sound is not caused directly by an acoustic shadow, for it occurs when the instruments are in view, but the limit of an acoustic shadow is close at hand. A little within the line joining the Foreland and the pier end, the instruments are cut off by a projection of the cliff near the station; all the sea space between this limit and the cliff under Dover Castle is in the shadow. Into this, however, the direct waves diverge, and lose intensity by their divergence, the portion of the wave nearest the shadow suffering most. this must be added the effect of interference.

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On June 25 the range was 5 miles. On June 26 the range was 10 miles. The former day the wind was in the direction of the sound; on the latter the wind was opposed. Plainly there must be something besides the wind which determines the sound-range. This something was now the object of search.

Is it the clearness of the atmosphere? All previous writers have extolled a clear atmosphere as best for sound; but on July 18 we steamed out to a distance of 10 miles and heard sounds, the white cliffs of the Foreland being at the same time entirely hidden in thick haze. Nay, more: we spoke the Triton tender on its way from the Varne lightship, and took the master of the Varne on board. He reported that the sounds had been heard at the lightship, though it is 12 miles from the Foreland. It was, moreover, dead to windward of the Foreland, so that both haze and wind were then in opposition; still the sound ranged at least twice as far as it had done on days when neither haze nor wind was there to interfere with the sound.

On July 2, a sudden acoustic darkness, if I may use the term, settled upon the atmosphere. The range was only 4 miles. The magnitude of the fluctuations, from 34 to 12 miles, observed up to this date, was striking: but I was unable to fix upon any meteorological element that could be held accountable for them. The wind, the clearness of the air, the barometer, the thermometer, the hygrometer, gave me no help. All was perplexity. I longed for light, but saw little prospect of obtaining it.

July 3 was a lovely morning: the sky was of a stainless blue, the air calm, and the sea smooth. I thought we should be able to hear a long way off. We steamed beyond the pier end and listened. The steam clouds were there, showing the whistles to be active; the smoke puffs were there, attesting the activity of the guns. Nothing was heard. We went nearer ; but at two miles horns and whistles and guns were equally inaudible. This however being near the limit of the sound shadow, I thought that might have something to do with the effect, so we steamed right in front of the station, and halted at 33 miles from it. Not a ripple nor a breath of air disturbed the stillness on board, but we heard nothing. There were the steam-puffs from the whistles, and we knew that between every two puffs the horn sounds were embraced, but we heard nothing. We signalled for the guns: there were the smoke puffs apparently close at hand, but not the slightest sound. It was mere dumb show on the Foreland. We steamed in to 3 miles, halted, and listened with all attention. Neither the horns nor the whistles sent us the slightest hint of a sound. The guns were again signalled for; five of them were fired, some elevated, some fired point blank at us. Not one of them was heard. We steamed in to two miles, and had the guns again fired: the howitzer and mortar with 3 lb. charges yielded the faintest thud; and the 18-pounder was quite unheard.

In the presence of these facts I stood amazed and confounded, for it had been assumed and affirmed by distinguished men who had given special attention to this subject, that a clear, calm atmosphere was the best vehicle of sound: optical clearness and

acoustic clearness were supposed to go hand in hand indeed, it had been proposed to make the one a measure of the other. But here was a day perfectly optically clear, proving itself to be a day of acoustic darkness almost impenetrable. I was driven slowly to the conclusion that all I had read upon this subject was wrong, and that for 165 years, namely since 1708, when Dr. Derham published his celebrated paper on this subject, suc. ceeding generations of scientific men had gone on repeating the same errors. This knowledge, however, did not help me much. The problem was still there challenging solution.

I ventured, two or three years ago, to say something regarding the function of the Imagination in Science, and notwithstanding the care that I took to define and illustrate its real province, many persons, amongst whom were one or two able men, deemed me loose and illogical; in fact, merely poetic, when I referred to the imagination. The history of science, however, numbers many men of strong poetic temperament, who, in the presence of a scientific problem, became as cold and clear as the light of stars. Look at these two pieces of polished steel. Have you a sense, or the rudiment of a sense, to distinguish the inner condition of the one from that of the other? And yet they differ materially, for one is a magnet, the other not. What enabled that noble philosopher, and pure and elevated character, Ampére, 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 visualise his lines of force, to follow them through magnets and through space until his mental picture became a guide to discoveries which have rendered this place immortal? What but imagination? I have reason to know but too well the fantastic, and even scandalous use that is made of the faculty when it is divorced from the disciplined understanding and handed over to the undisciplined passions and emotions. But this is not the scientific use of the imagination.

Let

And now to return. Figure yourself on the deck of the Irene, with the invisible air stretching between you and the South Foreland, knowing that it contained something which stifled the sound, but not knowing what that something is. Your senses are not of the least use to you; you are unable to see, or hear, or feel, or taste, or smell the object of your search; nor could all the philosophical instruments in the world, as it now is, render you the least assistance. You cannot take a single step towards the solution without the formation of a mental image, in other words, without the exercise of the imagination. me unfold my own exact course of thought and action. Sulphur in homogeneous crystals is exceedingly transparent to radiant heat, whereas the ordinary brimstone of commerce is highly impervious to it. Why? Because the brimstone of commerce does not possess the molecular continuity of the crystal, but is a mere aggregate of minute grains not in pefect optical contact with each other. When this is the case, a portion of the heat is always reflected on entering and 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 in the substance. A snowball is opaque to light for the same reason. It is not optically continuous ice, but an aggregate of grains of ice, and the light which falls upon the snow being reflected at the limiting surfaces of the snow granules, fails to penetrate the snow to any depth. Thus by the mixture of air and ice, two transparent substances, we produce a substance as impervious to light as a really opaque one. same remark applies to foam, to clouds, to common salt, indeed to all transparent substances in powder. They are all impervious to light, not through the real absorption or extinction of the light, but through internal reflection.

The

Humboldt, in his observations at the Falls of the Orinoco, is known to have applied these principles. He found the noise of the Falls three times louder by night than by day, though in that region the night, through beasts and insects, 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 30° higher than that of the grass. Over every heated rock, he concluded, rose a column of air rarefied by the heat, and 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 expla nation made it generally known that a non-homogeneous atmosphere is unfavourable to the transmission of sound

But what on July 3, over a calm sea, where neither rocks nor grass existed, could so destroy the homogeneity

Carpenter, F.R.S., refuting the old theories of the non-existence of animal life at great depths, and bringing to light (with others) animals such as Pentacrinus wyville-thomsoni and Bathyerinus gracilis, pedunculate star-fishes allied to the Comatula, from 2,435 fathoms, whose very existence was unsuspected, and who were supposed "in the lapse of ages to have been worsted in the struggle for life." And following these investigations came the exploring expedition of H. M.S. Challenger, the most important mission of its kind that Great Britain had ever engaged in, from whence we now and then got stray tidings of yet more remarkable animals,-animals of comparatively high organisation, allied to the lobster, dredged up from 2,000 fathoms in one individual, a gigantic amphipodal crustacean, "the eyes of the creature extended in two great facettic lobes over the whole of the anterior part of the cephalo-thorax, like the eyes Eglina among Trilobites" (NATURE, vol. vii. p. 388). In another, Deidamia leptodactyle, there were no traces whatever of eyes of sight or their pedicels (NATURE, vol. viii. p. 52). Further, from the enormous depth of 3,000 fathoms, or nearly 34 statute miles, "a depth which does not appear to be greatly exceeded in any part of the ocean," have been taken a tube-building annelid, its tube formed of the gritty matter which occurs but sparingly in the clay at the bottom, affording, in fact, as Prof. Wyville Thomson remarks, "conclusive proof that the conditions of the bottom of the sea to all depths are not only such as to admit of the existence of animal life, but are such as to allow of the unlimited extension of the distribution of animals high in the zoological series, and closely in relation with the characteristic fauna of shallower zones (NATURE, vol. viii. p. 53).

Contemporaneously with these expeditions and what would appear to be not an inappropriate sequitur, marine aquaria of extensive size, and constructed on scientific principles, had been established in some of our principal towns, thus affording a new source of enjoyment and an intellectual gratification to the people-that of the examination of living marine animals as they exist “in the depths of the sea"-and combining with this in some instances a source of pecuniary benefit to the promoters. The public interest in these establishments seemed so great that the arrival of the octopus had attracted almost as much attention as the visit of a foreign emperor, and the death of the porpoise was mourned as a national calamity.

In conclusion the president said he hoped he had said something suggested by the recent marine excursion to interest the members in marine zoology. For his own part he could say that the little attention he had been able to devote to it had been a most acceptable relief to official duties (as Treasurer of the Borough), always laborious and responsible, and at the same time it had brought him in contact with fellow-workers in natural history from whose friendship and kindly intercourse he had derived the most lasting pleasure.

Mr. Hughes ventured to express his opinion that a Marine Aquarium, if constructed properly and managed efficiently-for instance like that beautiful one at the Crystal Palace under the direction of his friend, Mr. W. Alford Lloyd, who had done more for the advancement of public aquaria than any man living would be most acceptable to Birmingham, where the great Priestley carried on his scientific researches, and in 1773 obtained the Copley medal of the Royal Society for the discovery of the mutual dependence of plants and animals on each other-the grand principle of all aquaria. It occurred to him after much thought, and as a successful student of marine aquaria for many years, that no greater attraction or means of intellectual recreation for the working classes and their neighbours in the mining districts could be devised, because it would be so utterly different from any other exhibition now existing, and so suitable as a relief and mental refreshment for

It

The

those in crowded courts to whom the sea was but a name. was indisputable that "the sea and its living wonders" had irresistible fascination to us far away from it. peculiar central inland position of Birmingham would be highly advantageous by allowing ready and rapid modes of conveyance of rare animals from almost any part of the coast. He commended the project to the earnest consideration of the many wealthy and intellectual men in the town, and could not help believing and hoping--although his views as a naturalist might be sanguine-that Birmingham would sooner or later possess a marine aquarium worthy of it, and follow the example of London, Brighton, Liverpool, Manchester, Plymouth, and other large towns, which, besides others on the Continent, had already taken the matter up.

We understand that Mr. Hughes' suggestion has been acted u pon, and that an influential committee has since been appointed to make inquiries with the view to the promotion of an efficient marine aquarium for Birmingham.

SCIENTIFIC SERIALS

Astronomische Nachrichten, No. 1,969, Jan. 9.— This number contains a paper by Prof. Spoerer on M. Faye's theory of solar spots. He refers to M. Faye's statement that spots are below the surface of the sun, and to his reliance on his treatment of Carrington's observations; for if a spot be observed for two or more revolutions its distance from the limb can be calcu lated on the assumption that the spot is on the surface of the sun. Should, however, the observed plan of the spot not agree with the calculated position, the assumption will be that the spot is below or above the vurface of the sun. Prof. Spoerer informs us that, on the whole, his observations show that the observed distance from the limb of the sun are too great; this he ascribes to the effect of refraction altering the position of the sun's limb to a greater degree than that of the spot.-On the identity of Coggia's Comet of Nov. 10, with Comet 1818 I. by Prof. Weiss. In this paper the author gives the elements of Coggia's comet as lately determined, together with the recorded observations of Comet 1818 I. from which he considers the two comets to be identical.-In a second paper by Prof. Weiss he gives elements calculated on a parabolic orbit and on two elliptic orbits of 55.82 and 6.9775 years respectively, being on the assumption in the first case thai one revolution has happened since 1818, and in the other that eight have taken place.-Observations on variable stars, by G. T. G. Schmidt. From observations up to the end of 1873, given at length in his paper, we extract the epochs of maxima and minima of the following stars :

Wilhelm Schur gives an opposition ephemeris for Arethusa. The opposition happening Jan. 21, the R. A. being then 8. 2.52.78 and Dec. +4.1.54

The fourth line of the spectrum of the nebula in Orion, by D'Arrest. The author refers to a note on this line by Dr. Vogel in Astron. Nach., No. 1963, mentioning the fact that the fourth line of the nebula coincided with Hy, and goes on to mention that this line was known to Huggins in 1864, and by Capt. Herschell in 1868, and was brought to the notice of the Royal Society in 1872 by Huggins. Its wave-length he gives as 4343. The author also mentions a peculiarity in the spectrum of B. Durchm+ 22° 4203.-Mr. J. Birmingham states in a note that he believes that 252 Schjellerup has disappeared, and he thinks this star a remarkable variable.

Medizinische Jahrbücher: k.k. Gesellsch. d. Aerzte: Vienna, 1873, Heft 3 and 4-The last two issues of this quarterly journal for 1873 contain the following papers of scientific interest :Researches into the minute structure of the Tendon, by Arnold Spina, with an illustration; the Nerves of the Knee-joint in the Rabbit, by Dr. C. Nicoladoni (with a figure); contributions to the Anatomy of the Human Bladder, by Dr. Gustav Jurie; Quarantine in Cholera, a report to the International Medical Congress, by Dr. Oser.

SOCIETIES AND ACADEMIES

LONDON

Zoological Society, Jan. 20.-Prof. Newton, F.R.S., vicepresident, in the chair.-Mr. Sclater exhibited two skulls of Baird's Tapir (Tapirus bairdi) received from Mr. Constantine