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Experiments and observation lead to the conclusion that these anomalies in the penetration and direction of sound from fog-signals are to be attributed mainly to the want of uniformity in the surrounding atmosphere, and that snow, rain, fog, and the force and direction of the wind have much less influence than has generally been supposed." (Lighthouse Report, p. 104.)
Professor Tyndall follows his quotation of these passages with this comment: “ The Report of General Duane is marked throughout by fidelity to facts, rare sagacity, and soberness of speculation. The last three of the paragraphs just quoted exhibit, in my opinion, the only approach to a true explanation of the phenomena which the Washington Report reveals.” (Preface, p. 18.)
Now the rationale offered by Professor Henry, — the flexure of sound-rays by the unequal velocity of their wave-fronts, certainly embraces a larger variety of obscure and puzzling phenomena than that proposed by Professor Tyndall. The observed facts of sound being sometimes heard far beyond an intermediate region of inaudibility, of its being heard through a given region of atmosphere in one direction, while wholly unheard in the reverse direction,- of its being heard at the mast-head of a vessel, after being lost on deck,- and even of its being occasionally heard with increased distinctness against the sensible wind, - are quite inexplicable on the "acoustic-cloud " hypothesis, while they receive a satisfactory explanation on the hypothesis of acoustic refraction.
Professor Henry, in publishing the Report of General Duane, thus comments on the passages above quoted : “In the foregoing, I differ entirely in opinion from General Duane, as to the cause of extinction of powerful sounds being due to the unequal density of the atmosphere. The velocity of sound is not at all affected by barometric pressure; but if the difference in pressure is caused by a difference in heat, or by the expansive power of vapor mingled with the air, a slight degree of obstruction of sounds may be observed. But this effect, we think, is entirely too minute to produce the results noted by General Duane and Dr. Tyndall, while we shall find in the action of the currents of wind above and below a true and sufficient cause." (Lighthouse Report, p. 104.)
Professor Tyndall quotes this paragraph (Preface, p. 18), and espouses very positively the opposite view of General Duane : “The observations at the South Foreland, where the sound has been proved to reach a distance of more than twelve miles against the wind, backed by decisive experiments, reduce to certainty the surmises of General Duane." (Preface, p. 19.) His presentation, however, of the
view of his competitor, hardly exhibits his usual accuracy or courtesy.
“Professor Henry thus accounts for the fact that the northeast snow-wind renders the sound of Cape Elizabeth audible at Portland. In the higher regions of the atmosphere he places an ideal wind, blowing in a direction opposed to the real one, which always accompanies the latter, and which more than neutralizes its action. In speculating thus he bases himself on the reasoning of Professor Stokes, according to which a sound-wave moving against the wind is tilted upwards. The upper and opposing wind is invented for the purpose of tilting again the already lifted sound-wave downwards. Professor Henry does not explain how the sound-wave recrosses the hostile lower current, nor does he give any definite notion of the conditions under which it can be shown that it will reach the observer.” (pp. 19, 20.)
This passage certainly does not indicate a very clear or correct comprehension of the doctrine of refraction as resulting from simple differences in the force or direction of the wind; and the critic has permitted himself to employ the word “is invented," notwithstanding that in the Lighthouse Report reasons are given, both from experiment and observation, fully justifying the assumption of an upper opposing wind.”
Professor Tyndall, with an air of "authority,” proceeds : “This, so far as I know, is the only theoretic gleam cast by the Washington Report on the conflicting results which have hitherto rendered experiments on fog-signals so bewildering. I fear it is an ignis fatuus instead of a safe guiding light. Professor Henry, however, boldly applies the hypothesis in a variety of instances. But he dwells with particular emphasis upon a case of non-reciprocity which he considers absolutely fatal to my views regarding the flocculence of the atmosphere." (p. 20.)
Professor Henry may here be quoted at length: “That a flocculent condition of the atmosphere, due to the varying density produced by the mingling of aqueous vapor, is a true cause of obstruction in the transmission of sound, is a fact borne out by deduction from the principles of wave-motion, as well as by the experiments of the distinguished physicist of the Royal Institution of Great Britain ; but from all the observations we have made on this subject, we are far from thinking that this is the efficient cause of the phenomena under consideration. A fatal objection, we think, to the truth of the hypothesis Professor Tyndall has advanced, is that the obstruction to the sound, whatever may be its nature, is not the same in different directions. We think we are warranted in asserting that in the
cases of acoustic opacity which he has described, if he had simultaneously made observations in an opposite direction, he would have come to a different conclusion. That a flocculent condition of the atmosphere should slightly obstruct the sound, is not difficult to conceive; but that it should obstruct the ray in one direction and not in an opposite, or in a greater degree in one direction than in another, the stratum of air being the same in both cases, is at variance with any fact in nature with which we are acquainted. We would hesitate to speak so decidedly against the conclusions of Professor Tyndall, for whose clearness of conception of physical principles, skill in manipulation, and power of logical deduction, we entertaiu the highest appreciation, were the facts which were obtained in our investigations of a less explicit character.” (Lighthouse Report, pp. 116, 117.)
Among several similar observations, the one mainly referred to is the case of a ten-inch steam-whistle at the Whitehead Light-station (about one mile and a half off the coast of Maine), suddenly ceasing to be heard by an approaching vessel within the distance of about three miles. Professor Henry notes that “the wind during this time was from the south, or approximately in an opposite direction to the sound”; a smaller steam-whistle on the vessel being meanwhile distinctly heard at the station. (Lighthouse Report, pp. 107, 108.)
The champion of "flocculence,” still undaunted, replies : “But the atmosphere was not still, and a sufficient reason for the observed nonreciprocity is to be found in the recorded fact that the wind was blowing against the shore-signal, and in favor of the ship-signál.” (Preface, p. 21.) How this favors the acoustic-cloud hypothesis is not very apparent. Professor Henry had previously remarked : “At first sight, the explanation of this might seem to be very simple, namely, that the sound is borne on in the one direction and retarded in the other by the motion of the wind. But this explanation, satisfactory as it might appear, cannot be true. Sound moves at the rate of about 780 miles an hour, and therefore, on the above supposition, a wind of 7.8 miles per hour could neither retard nor accelerate its velocity more than one per cent,- an amount inappreciable to ordinary observation; whereas we know that a wind of the velocity we have mentioned is frequently accompanied with a reduction of the penetrating power of sound of more than fifty per cent." (Lighthouse Report, p. 106.)
Professor Tyndall continues : “But the argument of Professor Henry, on which he places his main reliance, would be untenable, even had the air been still. By the very aerial reflection which he practically ignores, reciprocity may be destroyed in a calm atmos
phere. In proof of this assertion, I would refer him to a short paper on Acoustic Reversibility' printed at the end of this volume.” (Preface, p. 21.) The condition, “even had the air been still,” is certainly curious, in view of the fact that “the argument of Professor Henry is based directly on the existence of differences in the velocity of the wind !
The chapter on “ Acoustic Reversibility" so confidingly referred to, forming “ Appendix II.," and the conclusion of the work (pp. 403 – 411), is probably the least satisfactory essay in the volume. The card-board screen experiment, permitting sound to be diffracted around the edge of the card to a nearer point in one direction than in the reverse direction, appears quite foreign to the remarkable case of
non-reversibility” observed by Arago in 1822; and on no known physical principle can it be shown that a flocculent veil in the atmosphere will be “diacoustic" on one side, and “catacoustic” on the opposite side. It is the conception, of a transparent mirror, with a valve so cunningly arranged, that all the rays falling on one side will pass through without obstruction, while all the rays falling on the other side will be reflected back.
And yet, with a complacency that almost seems infatuation, Professor Tyndall continues : “ The most remarkable case of non-reciprocity on record, and which, prior to the demonstration of the existence and power of acoustic clouds, remained an insoluble enigma, is there shown to be capable of satisfactory solution. These clouds explain perfectly the abnormal phenomena' of Professor Henry. Aware of their existence, the falling off and subsequent recovery of a signal-sound, as noticed by him and General Duane, is no more a mystery than the interception of the solar light by a common cloud, and its restoration after the cloud has moved or melted away. The clew to all the difficulties and anomalies of this question is to be found in the aerial echoes, the significance of which has been overlooked by General Duane, and misinterpreted by Professor Henry." (Preface, p. 21.)
It is believed that the real cause of the abnormal phenomena noticed lies much deeper than is here supposed ; and that to every unbiassed physicist it can be satisfactorily shown that the true solution of the enigma has been discovered by Professor Henry, and has been as signally missed by Professor Tyndall ;* nay, that it has even failed to be rightly understood by him, after having been propounded.
* This subject has been quite recently discussed, in the American Jour. Science for January and February.
“ If we accept for a moment,” says he, “a statement which Professor Henry seems to indorse, that sound-waves of great intensity, when they impinge upon a solid or liquid surface, do not obey the law of incidence and reflection, but roll along the surface like a cloud of smoke,' it only increases the difficulty.” (Preface, p. 23.) There was no occasion for attributing this inexact comparison to Professor Henry. The phrase quoted is from General Duane, who says in his Report : “Upon a large scale, sound does not appear, ou striking a surface, to be reflected after the manner of light and heat, but to roll along it like a cloud of smoke." The only “seeming indorsement” in Professor Henry's comment is : “This statement is in a measure in accordance with results which I have previously found," that "powerful sounds tend to diverge laterally to such & degree as to render reflectors of comparatively little use.” (Lighthouse Report, p. 100.) The statement of Professor Henry is simply one of fact, ascertained by very decisive experiments, the details of which are given in his Report. Nor is this so surprising as might appear, if we consider that the dimensions of a sound-reflector cannot conveniently exceed an average wave-length of sound. Were it possible to construct such a reflector, comparable in this respect to even the very smallest ever employed optically, we should probably have that distinct relation of incidence and reflection observed in light, and proportionately in ordinary sounds within small distances.
If we may judge from Professor Tyndall's earnest invective (rather oddly interpolated into his “aerial echoes "), he has at last triumphantly burst asunder the shackles of "authority” which had previously bound him, and now stands forth disenthralled and exultant.
“ The clew to all the difficulties and anomalies of this question is to be found in the aerial echoes, the significance of which has been overlooked by General Duane, and misinterpreted by Professor Henry. And here a word might be said with regard to the injurious influence still exercised by authority in science. The affirmations of the highest authorities, that from clear air no sensible echo ever comes, were so distinct that my mind for a time refused to entertain the idea. Authority caused me for weeks to depart from the truth, and to seek counsel among delusions.* On the day our observations
* On page 258 occurs this remarkable statement : “ 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.” This systematic inquiry does not however appear to have been read; for “ the affirmation of this authority” (however “injurious its influence") is certainly “distinct," one hundred and sixty-seven years ago. “I will add also an