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most stringent system which popular opinion can be made to tolerate, not fifty per cent of the personal property subject under law to assessment ever comes within the cognizance of the assessors.

DAVID A. WELLS.

ART. VI.- CRITICAL NOTICES.

1. Sound. By John TYNDALL, D. C. L., LL. D., F. R. S., Profes

sor of Natural Philosophy in the Royal Institution of Great Britain. Third Edition. London : Longmans, Green, & Co. 1875.

The chief differences between this new edition of a popular and excellent text-book and the former issues consist in the insertion, before the last two chapters, of a new chapter (numbered VII.), of sixty-seven pages, entitled "Researches on the Acoustic Transparency of the Atmosphere in Relation to the Question of Fog-Signalling"; and in a Preface of nineteen pages, mainly occupied in combating the views promulgated by Professor Henry in the Appendix to the last U. S. Lighthouse Board Report, for 1874.

It is well known, from various publications, that Professor Tyndall was engaged under the auspices of the Trinity House, during a great portion of the year 1873 (beginning in May), in investigating the subject of fog-signals, - a subject which had been under investigation by the U. S. Lighthouse Board from a much earlier date and for a much longer period; and Major Elliot, in his recent valuable Report of observations on European Lighthouse Systems, was struck with the fact that “our shore fog-signals are vastly superior, both in number and power,” to those of England or of Europe.

Notwithstanding that the results of acoustic observations made from time to time by Professor Henry and his coadjutors during the past ten years, and communicated to the Lighthouse Board, must have been known to its Engineer Secretary, -- Major Elliot, -- he has occupied one eighth of the Report alluded to, with the detail of Professor Tyndall's observations, mainly embracing the same results; being struck probably with the differences of explanation offered on some of the observed phenomena. The natural tendency, however, of such extended references to foreign investigations, in a narrative addressed to the U. S. Lighthouse Board by its own agent, is obviously as its chairman has stated in the official Report of 1874,

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"to convey the idea that the facts which it states were new to the Board, and that the latter had obtained no results of a similar kind; while a reference to the Appendix to this Report will show that the researches of our Lighthouse Board have been much more extensive on this subject than those of the Trinity House, and that the latter has established no facts of practical importance which had not previously been observed and used by the former.”

This passage is quoted by Professor Tyndall, and he remarks upon it (Preface, p. 11, foot-note) : “ It will be borne in mind that the Washington Appendix was published nearly a year after my Report to the Trinity House." While this may be accepted as an evidence of the originality of said Report, it does not establish priority of its subject, since corresponding Reports were made to the U. S. Lighthouse Board in 1855, by Professor J. H. Alexander; in 1865, 1867, and in recent years, by Professer Henry; and in 1871, by General Duane. Professor Tyndall, however, does not appear to be quite satisfied with the broad statement of Professor Henry. He

says : desire is to be strictly just; and this desire compels me to express the opinion that their Report fails to establish the inordinate claim made in its first paragraph. It contains observations, but contradictory observations; while as regards the establishment of any principle which should reconcile the conflicting results, it leaves our condition unimproved." (Preface, p. 15.)

That the observations were “contradictory” is one of their merits, as the essential prerequisite to "the establishinent of any principle” that should have a permanent value; and this merit is shared by the observations of Professor Tyndall, who records that “the most conflicting results were at first obtained ” (p. 321). . The failure to apprehend the complexity of the conditions controlling the propagation of sound, leading to hasty but natural generalizations from a single well-marked occurrence, or from a small number, has tended to impair the confidence of subsequent investigators in the accuracy of previous "contradictory observations."

There is no doubt that were the striking experiment made by Delaroche in 1816, to be repeated under the same essential circumstances, it would be found that a particular sound is carried farther at right angles to the direction of the wind than in its direction. But this is certainly not the general fact, though sometimes so stated.* And the “contradictory observations” as to the effect of

*“The strangeness of Delaroche's results consisted in his establishing by quantitative measurements, not only that sound has a greater range in the direction

wind upon sound generally have led some persons to deny its influence altogether. Thus General Duane in his instructive Report to the U. S. Lighthouse Board, January 12, 1872, detailing the results of experiments made at Portland in 1871, concludes that “the force and direction of the wind have much less influence than has generally been supposed,” since he had previously noticed that while the signal sound is often heard at a much greater distance in one direction than in another, “this is not the effect of wind, as the signal is frequently heard much farther against the wind than with it.”

On this subject there is in the researches of Professor Tyndall a singular lack of any original observation or suggestion; and as to the true character of the perturbations of sound by aerial currents, he leaves our condition unimproved.” In Chapter VII. Section 3, “Contradictory Results," he says: “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 six and a half miles, the wind was favorable ; on the 26th, when the range exceeded nine and one fourth miles, it was opposed to the sound” (p. 268). Not a thought occurs here that aerial currents are seldom uniform for any great extent; not a suspicion that, above the observer's head, the wind might differ either in direction or in force, or, if it did, that either of these circumstances could in any way affect the radiations of sound; no idea of differences of temperature in atmospheric strata exercising any action of acoustic refraction ; but only the most cursory notice is taken that “the wind was favorable," or was opposed to the sound.”

The most significant remark in his work, on the “ Action of Wind" is the following from Chapter VII. Section 7: “On the leeward side of the Foreland, on the 23d of October, the sounds were heard at least four times as far as on the windward side. ... This wellknown effect of the wind is exceedingly difficult to explain. Indeed, the only explanation worthy of the name is one offered by Professor Stokes. . . . . In a short, but exceedingly able communication presented to the British Association in 1857, the eminent physicist above mentioned points out a cause which, if sufficient, would account for the results referred to." (pp. 309, 310, 311.)

of the wind than in the opposite direction, but that the range at right angles to the wind is the maximum.” (Sound, pp. 310, 311.) Professor Tyndall adds in a footnote (p. 311): “Experiments so important as those of Delaroche ought not to be left without verification. I have made arrangements with a view to this object."

VOL. CXXIII. NO. 251. 27

To Professor Henry belongs the credit of having first established the principles reconciling the “contradictory observations" made by himself and by others, and demonstrating by a carefully discriminated series of facts the "sufficiency” of the theory proposed by Professor Stokes.

During Professor Tyndall's visit to this country, he was present December 11, 1872, at the reading of a paper by Professor Henry, “On certain abnormal Phenomena of Sound in Connection with FogSignals," before the Philosophical Society of Washington. (Bulletin, Phil. Soc. W., Vol. I. p. 65.) Almost immediately on his return to England, in 1873, he began a series of investigations on the same subject. “On the 19th of May, 1873, this inquiry began. The South Foreland, near Dover, was chosen as the signal station.” (Sound, p. 260.) Yet throughout the numerous channels of publication in which his results were spread, one looks in vain for an allusion to any similar work having been done in this country.

In the new Preface (dated June, 1875), after noticing somewhat superfluously, though with an air of impartial commendation, the experiments of Professor Henry, published in the Lighthouse Board Report, on the character of the instruments employed as fog-signals (Preface, pp. 12, 13), he adds vaguely: "I was quite aware, in a general way, that labors like those now for the first time made public had been conducted in the United States, and this knowledge was not without influence upon my conduct.” (p. 14.) What acknowledged influence these earlier labors of Professor Henry in the same field had does not appear from anything to be found in Professor Tyndall's writings. That the Preface in its present form is due to the dissent from his favorite theory of invisible acoustie clouds in the atmosphere, expressed by Professor Henry in the Appendix to the Lighthouse Report of 1874, is sufficiently obvious. Indeed, its polemical tenor very clearly indicates that the question involved therein is simply one of explanation, — the acceptance or non-acceptance of certain views, — in no respect one of priority of observation of the fundamental facts. And, in fact, no anomaly of sound has been referred to, not embraced and discussed in the previous investigations of the U. S. Lighthouse Board, and no practical recommendations given for improving the fog-signal service with a view to the protection of life and of property at sea, not already adopted in this country.

After briefly reciting some of the instrumental experiments undertaken by the chairman of the U. S. Lighthouse Board, Professor Tyndall continues : “Further observations were made by Professor Henry and his colleagues in August, 1873, and in August and Sep

tember, 1874. In the brief but interesting account of these experiments, a hypothetical element appears which is absent from the record of the earlier observations." (Preface, p. 13.) This tribute at least illustrates the caution of the true philosopher in framing hypotheses.

The key-note of Professor Tyndall's contribution to the theory of sound is atmospheric “flocculence," or non-homogeneity. On the remarkable variability in the range or penetration of sound observed on different days, he gives the following summary of his conclusions : “ These discrepancies were proved to be due to a state of the air which bears the same relation to sound that cloudiness does to light. By streams of air differently heated, or saturated in different degrees with aqueous vapors, the atmosphere is rendered flocculent to sound. Acoustic clouds in fact are incessantly floating or flying through the air. They have nothing whatever to do with ordinary clouds, fogs, or haze. The most transparent atmosphere may be filled with them ; converting days of extraordinary optical transparency into days of equally extraordinary acoustic opacity. . . . . Aerial echoes of extraordinary intensity and of long duration are thus produced. They occur, contrary to the opinion hitherto entertained, in the clearest air. - . . . The existence of these aerial echoes has been pr red by observation and experiment. They may arise either from air-currents differently heated, or from air-currents differently saturated with vapor.” (pp. 321, 322.)

This is simply an extension of the received doctrine of a non-homogeneous atmosphere being unfavorable to the transmission of sound; familiar for the last three quarters of a century, or since the observations of Humboldt on his tour in South America. Two years before Professor Tyndall's investigations, General Duane, in a Report to the U. S. Lighthouse Board, of observations made in 1871, had arrived at similar conclusions. This, however, was first published in the Report of the Lighthouse Board for 1874. The following are his remarks on the subject : “Now it frequently occurs that a signal, which under ordinary circumstances would be audible at the distance of fifteen miles, cannot be heard from a vessel at the distance of a single mile. This is probably due to the reflection mentioned by Humboldt. The temperature of the air over the land where the fog-signal is located being very different from that over the sea, the sound, in passing from the former to the latter, undergoes reflection at their surface of contact. The correctness of this view is rendered more probable by the fact that when the sound is thus impeded in the direction of the sea, it has been observed to be much stronger inland.

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