erable increase of temperature in the lower strata of the air, the lower portion of the sound waves is projected in advance of the upper portion. (p. 71.) Atmospheric vapor also, though exercising but little direct influence on the velocity of sound, "nevertheless plays an important part in the phenomena under consideration; for it gives to the air a much greater power of radiating and absorbing heat, and thus renders it much more susceptible of changes in the action of the sun. . It is a well-known fact that the temperature of the air diminishes as we proceed upward, and that it also contains less vapor. Hence it follows that, as a rule, the waves of sound must travel faster below than they do above, and thus be refracted or turned upward." (p. 72)

The variation of temperature will be greatest in a quiet atmosphere when the sun is shining. The report of Mr. Glaisher "On eight Balloon Ascents in 1862" showed that "The decline of temperature [upward] near the earth with a partially clear sky is nearly double that with a cloudy sky."* "During the night the variations are less than during the day. This reasoning at once suggested an explanation of the well-known fact that sounds are less intense during the day than at night. This is a matter of common observation, and has been the subject of scientific enquiry." (p. 73.) The opinion must here be hazarded that this familiar phenomenon has first received its true and satisfactory explanation from Professor Reynolds.

Assuming that for a few hundred feet upward, the diminution of temperature on a clear summer day is 1° for each hundred feet, a horizontal sound-ray would be bent up in an arc having a radius of about 20 miles. From a cliff 235 feet high, a sound should be audible from 1 to 2 miles on the sea, and the ray should then begin to rise above the observer's head. This is shown to accord very closely with the observation of Tyndall [6]. Professor Reynolds after quoting the observation at length, remarks: "Here we see that the very conditions which actually diminished the range of the sound were precisely those which would cause the greatest lifting of the waves. And it may be noticed that these facts were observed and recorded by Professor Tyndall with his mind altogether unbiased with any thought of establishing this hypothesis. He was looking for an explanation in quite another direction. Had it not been so he would probably have ascended the mast and thus

* Mr. Glaisher remarks: "From these results we may conclude that in a cloudy state of the sky, the decline of temperature is nearly uniform up to the clouds; that with a clear sky the greatest change is near the earth, being a decline of 1° in less than 100 feet, gradually decreasing as in the general law indicated in the preceding section, till it requires 300 feet at the height of 5,000 feet, for a change of 1° of temperature." (Rep. Brit. Assoc, 1862, p. 462.) AM. JOUR. SCI.-THIRD SERIES, VOL. XI, No. 62.-Feb., 1876.

แ

found whether or not the sound was all the time passing over his head. On the worst day an ascent of 30 feet should have extended the range nearly one quarter of a mile." (Phil. Mag., p. 76.)

V.

The instructive result, brought into view by the foregoing summaries, is that the differences noticed are essentially those of interpretation, and not to any important extent, of observation: an illustration if any were needed, of the high and rare order of imaginative insight requisite to the successful investigation of the more recondite operations of natural law. The differing actions of acoustic reflection and acoustic refraction suggested by the ingenious hypotheses of Humboldt and of Stokes, and espoused respectively by Tyndall and Henry, are probably both operative but their relative importance has yet to be established. It is certain, as already indicated, that some of the phenomena observed lie quite beyond the reach of the acoustic cloud hypothesis.

A particularly interesting case which is claimed with equal confidence for either theory, is the remarkable observation of General Duane, that at Portland, Maine, the steam whistle on Cape Elizabeth, nine miles distant, "can always be distinctly heard" with "the wind blowing a gale directly toward the whistle" or against the sound. (L. H. Rep., p. 100.) At Portland Head, about midway between this fog-whistle and the point of observation is another signal, -a Daboll trumpet. While both these signals are better heard with an adverse wind ("a heavy northeast snow storm") than at other times, yet "as the wind increases in force, the sound of the nearer instrument-the trumpet-diminishes, but the whistle becomes more distinct." (Rep., p. 92.) The abnormal influence of the wind in reversing the order of these two signals is not the least surprising feature of the general phenomenon.

Professor Tyndall believes that this curious observation only proves the snow-laden air from the northeast to be a highly homogeneous medium;" (Sound, Preface, p. 19,) the intervening air at other times being acoustically less transparent.

Professor Henry supposes "that during the continuance of the storm, while the wind was blowing from the northeast at the surface, there was a current of equal or greater intensity blowing in an opposite direction above, by which the sound. was carried in direct opposition to the direction of the surface current:" (Rep., p. 92)-somewhat in the nature of a vertical cyclone. He adds: "The existence of such an upper current is in accordance with the hypothesis of the character of a northeast storm, which sometimes rages for several days at a given

point on the coast without being felt more than a few miles in the interior, the air continuously flowing in below and going out above. Indeed in such cases a break in the lower clouds reveals the fact of the existence above of a rapid current in the opposite direction." (p. 92.)

6

Professor Henry's attention had been directed to this point as early as 1865, by discovering that a signal was audible against the wind at the mast-head of a vessel, after ceasing to be audible on deck: Obs. [4]. "This remarkable fact at first suggested the idea that sound was more readily conveyed by the upper current of air than the lower, and this appeared to be in accordance with the following statement of Captain Keeney, who is commander of one of the light-house vessels, and has been for a long time on the banks of Newfoundland in the occupation of fishing: When the fishermen in the morning hear the sound of the surf to leeward, or from a point toward which the wind is blowing, they take this as an infallible indication that in the course of from one to five hours the wind will change to the opposite direction from which it is blowing at the time. The same statement was made to me by the intelligent keeper of the fog-signal at Block Island. In these cases it would appear that the wind had already changed direction above, and was thus transmitting the sound in an opposite direction to that of the wind at the surface of the earth.” ~(Rep., p. 92.) The full significance of this idea however was not apprehended until the hypothesis of Professor Stokes (already alluded to) was taken up and considered. This appeared to furnish a satisfactory explanation of the observed effect of an upper current,-not on the actual range, but on the direction of the sound waves

Professor Tyndall thus comments on the rival hypothesis of Professor Henry: "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 soundwave moving against the wind is tilted upward. The upper and opposing wind is invented for the purpose of tilting again the already lifted sound-wave downward." (Pref. to Sound, pp. 19, 20.)

The word "invented" is scarcely the most appropriate term for an hypothesis derived from such patient research and careful induction. While in the case considered, the reversed upper wind of a local circulation is rendered so probable by the circumstances presented. it is proper to remark that this condition is not at all essential to the refraction doctrine. The hypothesis of Professor Stokes by no means assumes that "a

sound-wave moving against the wind is tilted upward." (Rep. Brit. Assoc., 1857, pp. 22, 23, of Abstracts.) An opposing wind exercises no sensible influence on either the velocity or the range of sound, nor (if uniform) on the direction of sound. Ordinarily indeed, a wind (which may be likened to an aerial river) is retarded at the earth precisely as the current of a stream is, over its bed.* When, however, the mouth of the aerial chimney of ascent is low, it may very well happen that the lower current of air (excepting inmediately at the surface of the earth) is considerably swifter than the successive layers of wind above it; and in such a case the effect of the opposing wind will be not to tilt upward the sound-beam, but to tilt it downward. In like manner a "favoring" wind, if more sluggish above, will tilt the sound-beam upward, and thus prove unfavorable to its audibility. In short, the postulate required for acoustic refraction is simply that there shall be a difference of amount between the upper and the lower currents of wind. And as this condition is certainly not an unusual one, we have here apparently a true and satisfactory account of the seeming anomalies of sound with reference to the influence of the wind.

But if the natural tendency of a mere diminution of velocity in the upper strata of an adverse wind is thus to bend an advancing sound downward, "a precisely similar effect" as Professor Henry has well remarked, "will be the result but perhaps in a considerably greater degree, in case an upper current is moving in an opposite direction to the lower, when the latter is adverse to the sound." (Rep., p. 107.) In September, 1874, when a signal near Sandy Hook, N. J., was observed to be audible at a greater distance against the afternoon sea-breeze than with it, Professor Henry ascertained by the employment of small toy balloons, that the upper current was opposed to the lower one, and in the direction of the maximum sound range: Obs. [11.] He was enabled thus to demonstrate experimentally the reality of the "ideal wind" which had been so confidently accepted before, from other conspiring intimations.

The critical commentary above cited, which postulates for this doctrine of acoustic refraction the super-position of "an ideal wind blowing in a direction opposite to the real one," as a condition "which more than neutralizes its action," quite fails to apprehend its true import. No action analogous to "neutralization" is assumed by the doctrine. There is no solution

*Professor Henry determined by experiment in 1865, when the velocity of the wind was not more than six miles per hour, that the speed of the clouds as indicated by their moving shadows, was several times this rate. (L. H. Rep., p. 93.) And Professor Reynolds in 1874, by observations with the anemometer, ascertained that near the ground the retardation of the wind rapidly increased; so that the lower sound rays move more nearly in the arc of a parabola, than of a circle. (Phil. Mag., pp. 64 and 70.)

of continuity between opposing currents; but every gradation of movement in each successive intermediate stratum. And as it is wholly improbable that the sound-beam which reaches the observer's ear, ever passes high enough to approach the upper "ideal wind," nothing is neutralized. Obedient to the law of instantaneous resultants, the beam of acoustic impulse presses on ever at right angles to the wave-surface which is conditioned by compounded factors.

As wide of the mark is the supposition that the upper and opposing "ideal wind" is "for the purpose of tilting again the already lifted sound-wave, downward." As has been just contended, the one wind is as incapable of depressing the soundwave, as the other is of lifting it.

The misconception culminates in the objection that "Professor Henry does not explain how the sound-wave re-crosses 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." (Loc. cit., p. 20.) There is no "hostile lower current," since as above pointed out, an opposite wind may be just as favorable to the propagation of sound, as a concurrent

one.

To give, however, a more definite notion of the conditions. under which it can be shown that the sound wave will reach the observer without crossing currents, the accompanying diagrams are submitted.

W

1.--Favoring Wind.

Fig. 1 exhibits the more ordinary effect of a favorable wind in depressing the beam of sound: s being the signal-station, and o the point of observation; the wind blowing from W. to E. As the spheroidal wave-faces become more pressed forward above by the freer wind (assuming it to be retarded at the surface by friction), and as the direction of the acoustic beam is constantly normal to the successive aerial surfaces of impact, it follows that very minute differences of concentricity in the successive waves, will by constant accumulation gradually bend the line of dynamic effect downward, as shown in the sketch on a very exaggerated scale. Of the sound rays below the line represented, some will by reflection from the sea, reach the observer's ear and thus increase the sound.

Fig. 2 represents the ordinary effect of an opposing wind here blowing from E. to W. The wave faces being more resisted