the water, and thus producing a current from the latter to the former, while the wind observed in the morning from the west was the landwind due to the cooling of the latter.

In the morning the wind was blowing from the west both in the higher strata and at the surface of the earth, and in this condition the sound was heard farther with the wind than against it.

The wind at the surface about midday gradually ceased, and shortly afterward sprang up from an east direction; in this condition the sound, with the wind at the surface, was heard at a greater distance. This is also in strict conformity with the theory of a change in the form of the sound-wave, as in the latter case the lower portion would be retarded, while the upper portion of the wave would be carried forward with the same velocity, and hence the sound would be thrown down on the ear of the observer. To explain the result of the third trial of the second day, we have only to suppose that the influence of the upper current was less than that of the lower. The conditions for these observations were unusually favorable, the weather continuing the same during the two days, and the change of the wind also taking place at nearly the same hour.

The fact thus established is entirely incompatible with the supposition that the diminution in the sound is principally caused by a want of homogeneity in the constitution of the atmosphere, since this would operate to absorb sound equally in both directions.

In May, 1873, Professor Tyndall commenced a series of investigations on the subject of the transmission of sound, under the auspices of the Trinity House, of England, in which whistles, trumpets, guns, and a siren were used, the last-named instrument having been lent by the Light-House Board of the United States to the Trinity House for the purpose of the experiments in question. The results of these investigations were, in most respects, similar to those which we had previously obtained. In regard to the efficiency of the instruments, the same order was determined which has been given in this report, namely, the siren, the trumpet, and the whistle. Professor Tyndall's opinion as to the efficiency of the siren may be gathered from the following remarks. Speaking of the obstruction of sound in its application as a fog-signal, he says, "There is but one solution of this difficulty, which is to make the source of sound so powerful as to be able to endure loss and still retain sufficient residue for transmission. Of all the instruments hitherto examined by us the siren comes nearest to the fulfillment of this condition, and its establishment upon our coasts will, in my opinion, prove an incalculable boon to the mariner." Professor Tyndall arrived at the conclusions which the information we had collected tended to establish, that the existence of fog, however dense, does not materially interfere with the propagation of sound; and also that sound is generally heard farther with the wind than against it, although the variation of the in

tensity of the sound is not in all cases in proportion to the velocity of the wind. The result of his investigations in regard to the pitch of sound was also similar to those we have given; and, indeed, all the facts which he has stated are, with a single exception as to the direction of the echo, in strict accordance with what we have repeatedly observed. We regret to say, however, that we cannot subscribe to the conclusions which he draws from his experiments as to the cause of the retardation of sound that it is due to a flocculent condition of the atmosphere, caused by the intermingling with it of invisible aqueous vapor.

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 entertain the highest appreciation, were the facts which were obtained in our investigations of a less explicit character.

While the phenomena in question are incompatible with the assumption of a flocculent atmosphere as a cause, they are in strict accordance with the hypothesis of the refraction of the waves of sound due to a difference in velocity in the upper and lower portions of the currents of air. We do not say, however, that the transmission of sound in the atmosphere is fully investigated, or that the abnormal phenomena which are said to have been observed in connection with fog-signal stations have been fully explained. So far from this, we freely admit we are as yet in ignorance as to how the hypothesis we have adopted is applicable to the critical explanation of the obstruction to sound in the abnormal cases mentioned by General Duane. We feel, however, considerable confidence in its power to afford a rational explanation of these phenomena when the conditions under which they exist shall have been accurately determined.

We are farther confirmed in our conclusion by the publication of an interesting paper in the proceedings of the Royal Society by Professor Osborne Reynolds, of Owens College, Manchester, intended to show that sound is not absorbed by the condition of the atmosphere, but refracted in a manner analogous to the hypothesis which has been adopted in the preceding report.

Much further investigation is required to enable us to fully understand the effects of winds on the obstruction of sound, and to determine the measure of the effect of variations of density in the air due to inequality of heat and moisture. But such investigations can only be made under peculiar conditions of weather and favorable localities, with the aid of a number of steamers, and a series of observers, by whom the transmissibility of the air may be simultaneously observed in different directions. The position which we were so fortunate to obtain in our experiments in the lower bay of New York at the season of the prev alence of land and sea breezes was exceptionally favorable for the study of the action of wind upon sound. It is the intention of the LightHouse Board to continue observations in regard to this matter, and to embrace every favorable opportunity for their prosecution under new and varied conditions.

LIGHT-HOUSE BOARD, October, 1874.

PART IV.-INVESTIGATIONS IN 1875.*

PRELIMINARY REMARKS.

In the Appendix to the Light-House Report of 1874 I gave an account of a series of investigations relative to fog-signals, which had been made at different times under the direction of the chairman of the committee on experiments.

These investigations were not confined to the instruments for producing sound, but included a series of observations on sound itself, in its application to the uses of the mariner. In the course of these investigations the following conclusions were early arrived at:

1st. That the rays of a beam of loud sound do not, like those of light, move parallel to each other from the surface of a concave reflector, but constantly diverge laterally on all sides; and, although at first they are more intense in the axis of the reflector, they finally spread out so as to encompass the whole horizon, thus rendering the use of reflectors to enforce sound for fog-signals of little value.

2d. That the effect of wind in increasing or diminishing sound is not confined to currents of air at the surface of the earth, but that those of higher strata are also active in varying its transmission.

3d. That although sound is generally heard farther with the wind than against it, yet in some instances the reverse is remarkably the case, espec

* From the Report of the Light-House Board, for 1875.

ially in one locality, in which the sound is heard against a northeast snow-storm more distinctly than when the wind is in an opposite direction. This anomaly was referred to the action of an upper current in an opposite direction to that at the earth, such a current being known to exist in the case of northeast storms on our coast. But in what manner the action of the wind increased or diminished the audibility of sound was a problem not solved. It could not be due, as might be thought at first sight, to the acceleration of the sonorous impulse by the addition of the velocity of the wind to that of sound, on the one hand, nor to the retardation of the latter by the motion of the wind, on the other. The inadequacy of this explanation must be evident when we reflect that sound moves at the rate of 750 miles an hour, and therefore a wind of 7 miles an hour would only increase its velocity one per cent.; whereas the actual increase in audibility produced by a wind of this intensity is in some instances several hundred per cent.

In this state of our knowledge, a suggestion of Professor Stokes, of Cambridge, England, which offered a plausible explanation of the action of the wind, became known to us, and was immediately adopted as a working hypothesis to direct investigations.

This suggestion, the importance of which appears to have escaped general recognition, is founded on the fact that the several strata into which a current of air may be divided do not move with the same veloc ity. The lower stratum is retarded by friction against the earth and by the various obstacles it meets with, the one immediately above by friction against the lower, and so on; hence the velocity increases from the ground upward-a conclusion established by abundant observation. Now, in perfectly still air, a sounding instrument, such as a bell, produces a series of concentric waves perfectly spherical; but in air in motion the difference of velocity above and below disturbs the spherical form of the sound-wave, giving it somewhat the character of an oblique ellipsoid, by tending to flatten it above-to the windward, and to increase its convexity above-to the leward; and since the direction of the sound is perpendicular to the sound-wave, against the wind it will be thrown upward above the head of the observer, and in the opposite direction downward toward the earth. A similar effect will be produced, but with some variations and perhaps greater intensity, by a wind above, opposite to that at the surface of the earth.

These propositions will be rendered plain by the following illustrations (Figures 1, 2, and 3), for which I am indebted to an article in the American Journal of Science, by William B. Taylor.

W

SA

Fig. 1.

E

In these, Figure 1 represents the effect of a favorable wind in depressing the waves of sound, S being the signal-station and O the point of observation. The wind blowing from W to E, as the spheroidal faces of the sonorous waves become more pressed forward by the greater velocity of the wind above, assuming it to be retarded at the surface by friction, and the direction of the acoustic beam being constantly normal to the wave-surfaces, the lines of direction of the sound will gradually be bent downward and reach the ear of the observer with an accumulated effect at the point O.

W

Fig. 2.

E

Figure 2 represents the ordinary effect of an opposing wind blowing from E to W against the sound; the wave-faces being more resisted above than below, assuming as before a retardation at the surface, the sound-beams are curved upward, and the lowest ray that would reach, in still air, the distant observer at O, is gradually so tilted up that it passes above the ear of the listener, leaving him in an acoustic shadow.

W

Fig. 3.

E

Figure 3 represents the disturbing effect of two winds, the lower in opposition to the sound at the surface, and the upper with it. In this case the principal effect will be a depression of the sound-beam, similar to that shown in Figure 1, but more strongly marked, as the difference of motion will be greater as we ascend. Attending this action, says Mr. Taylor, there will probably be some lagging of the lower stratum by reason of the surface-friction, the tendency of which will be to dis tort the lower part of the sound-waves, giving them a reverse or serpentine curvature. In this case the upper ray of sound would only have a single curvature, similar to that shown in Figure 1, while the lower rays would be represented by the lower line S O, rendering the sound less audible at an intermediate point, t, than at the more distant station O. This hypothetical case of compound refraction offers a plausible explanation of the paradox of a nearer sound being diminished in power by the wind which increases the effect of a more distant one.

S. Mis. 59-33