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seven and three-quarter miles. Toward the close of the day the signals were heard at twelve and three-quarter miles. (p. 196, 197.)

[7.] On the same day at one o'clock, the echoes from the direction of the open sea were very distinct at the signal station. “ The instruments hidden from view, were on the summit of a cliff 235 feet above us, the sea was smooth and clear of ships, the atmosphere was without a cloud, and there was no object in sight which could possibly produce the observed effect. From the perfectly transparent air, the echoes came, at first with a strength apparently but little less than that of the direct sound, and then dying gradually and continuously away.” (p. 198.) These remarkable echoes are supposed by Professor Tyndall to be returned from the invisible surfaces of the vaporous striæ, which thus render the air opaque to the sonorous waves. Subsequently, on the 8th of October, the American siren being just received and set up, its loud echoes were observed to be “ far more powerful than those of the horn," and to last eleven seconds, while those of the horn had eight seconds duration (p. 199.) On the 15th of October, the direction of the echoes was found to correspond with the principal axis of the direct or primitive sound; the direction of the return sound changing with the rotation of the horn. (p. 200.)

[8.] On October 8th rain and hail were found not to obstruct sound. While in the morning (after a thunder storm) from Dover and the South Foreland across the English channel " for a time the optical clearness of the atmosphere was extraordinary, the coast of France, the Grisnez lighthouse, and the Monument and Cathedral of Boulogne being clearly visible in positions from which they were generally quite hidden; the atmosphere at the same time was acoustically opaque;' and the horn was feebly heard at six miles. (p. 205.) But in the afternoon a storm arose, and although the rain was falling heavily all the way between the signal station at Foreland and the point of observation on the steamer, “the sound instead of being deadened, rose perceptibly in power. Hail was now added to the rain, and the shower reached a tropical violence.” “In the midst of this furious squall both the horns and the siren were distinctly heard,” and as the shower lightened, diminishing the local pattering on the deck, they were heard "at a distance of seven and a half miles distinctly louder than they had been heard through the rainless atmosphere at five miles.” (p. 206.) On the 23d of October, a similar experience was noticed on land, and, contrary to the usual impression, snow was also observed to offer no serious obstacle to sound. (p. 207.)

It must be borne in mind that the investigations by Profes. sor Tyndall were concluded before the publication of the United States Lighthouse Report. And it is noticeable that these two series of original observations thus independently made on the opposite sides of the Atlantic, in the main quite strikingly confirm each other.

Tyndall's notice [1] of the inconstant relative range of different instruments corresponds with Henry (2), though indicating a much more marked variability.

Tyndall's notice [2] of the sound shadow, corresponds generally with Henry [7], and Duane [E], but assigns a sharper definition to iis limit; probably in consequence of the intervention of a larger obstacle (a cliff), and an observation within a shorter distance.

Tyndall [3] confirms Henry [3] and [11].
Tyndall [4] corresponds with Henry [7] and Duane [E].

Tyndall 15) confirms by a series of careful observations, the opinion of Henry [5] and Duane [I].

Tyndall [6] confirms Duane (A and F], and in like manner adopts and extends the suggestion of Humboldt as to the cause of acoustic opacity. Professor Tyndall's admirable skill in experimental physics enabled him to illustrate and fortify his hypothesis by exbibiting in a popular lecture an apparatus for producing in an elongated box or tunnel, aerial laminæ of unequal density, through which the sound from a small alarm box failed to excite a sensitive flame. That this mottled condition of the air is therefore a true cause of acoustic ob. struction is no longer doubtfu). To what extent a similar condition of the atmosphere actually prevails, in view of the law of the diffusion of gases, and how far such usual or unusual inequalities of density in the air are capable of entirely dispersing the powerful sound of a steam trumpet or siren, at the distance of a quarter of a mile, are not so positively determined. With a continuous wind any such condition of aerial “flocculence" might be expected to be very speedily dissipated.

This theory, however, fails entirely to explain the interesting observations of Henry [4, 8, and 9). It is scarcely credible that a local screen of aerial flocculence could obliterate on the deck of a schooner, a fog.signal audible at the mast-head. Atmospheric refraction on the other band, completely satisfies the observed condition ; an opposing wind blowing at the time. Still less successful is the theory, in dealing with the abnormal phenomenon of simultaneous audibility at long range, with the intermediate “belt” of acoustic opacity, first observed by Duane [D]. And lastly, the assumption of simultaneous transmission of sound through a flocculent air-screen in one direction and its absorption or dissipation by the screen in the opposite

direction, (acoustic “non-reversibility,") is obviously inadmissible. Nor is the supposition of acoustic * diffraction” around the defined edge of a vapor cloud, more available.

Professor Tyndall in bis recent Preface to the last edition of “Sound” remarks upon this observation of Henry [9]—"a sufficient reason for the observed non-reciprocity is to be found in the recorded fact that the wind was blowing against the shore-signal, and in favor of the ship-signal.” (Preface, p. xxi.) But he offers no suggestion how this “sufficient reason” is supposed to apply. As it is well-known that an ordinary wind cannot increase the range of sound more than two or three per cent (an amount quite inappreciable), this circumstance alone is wholly inadequate to account for the complete suppression of the shore-signal (a ten-inch steam-whistle) from the distance of three miles to a quarter of a mile, while the feebler sound of the ship-signal (a six-inch steam-whistle) was making itself distinctly heard throughout the three miles. Something more therefore than the direct or convective action of the wind must be invoked to explain the facts.

Tyndall's observation [7] on the aerial or ocean echoes, corresponds with Henry (10) excepting as to the direction of the principal echo. This difference is doubtless due to the special arrangement of the surfaces or points of reflection in the respective cases observed. Professor Tyndall connects this phenomenon with that of acoustic opacity [6]; and here again his fine experimental skill is brought into requisition to demonstrate the reality of artificial "aerial echoes.” By so simple a device as the employment of the flat side of a “bat-wing” gasjet, the sound beam from a reed instrument was shown to be entirely deflected from one sensitive flame, and reflected back toward another.

This view of a relation between the acoustic opacity outward or seaward, and the reinforcement or reflection of sound inward, is in striking accord with Duane [G], who however in referring to the “reflection' of sound, does not specifically allude to the ocean “echo.” On the refraction theory also, a necessary result is that a deflection of the sound-beam upward in one direction, must be attended with a downward deflection and consequent increase of sound in the opposite direction.

Professor Henry bad referred these mystic echoes to the crests and slopes of distant waves; (in conjunction probably with a curvature of the sound-beams, constituting a kind of acoustic “mirage.”) To this suggestion, Professor Tyndall op. poses the observation that “the echoes have often manifested an astonishing strength, when the sea was of glassy smootbness.” (Sound, Pref., p. xxiii.)

That this very interesting subject presents features requiring still further and more refined investigation is sufficiently obvious from the single consideration that aerial opacity and echo have not been shown to bear that direct relation which the vapor theory requires. Professor Tyndall has recorded that, on the 17th of October (1873), “It is worth remarking that this was our day of longest echoes, and it was also our day of greatest acoustic transparency, the association suggesting that the duration of the echo is a measure of the atmospheric depths from which it comes. On no day, it is to be remembered, was the atmosphere free from invisible acoustic clouds; and on this day when their presence did not prevent the direct sound from reaching to a distance of 15 or 16 nautical miles, they were able to send us echoes of 15 seconds duration." (Trans., p. 202.) If these echoes were not "folded,” this would represent an extreme limit of about a mile and a half. Our most powerful sounds cannot afford to waste much of their energy on echoes, if under the inexorable law of increasing attenuation as the square of the distance they are to be audible through a range of 16 miles : less than tbe 400th of the intensity at one nautical mile, that is heard at the distance of 100 yards from the source; and one 256th of this at the distance of 16 nautical miles, or less than the hundred thousandth of the intensity at 100 yards. And the inference is strong that in sucb a case accompanying echoes must be derived from sound beams in a somewhat different direction.

Further observations are needed also to ascertain whether these aerial screens of unequal density and acoustic opacity are capable of returning echoes on opposite sides, as is to be expected if we may accept the analogy of catoptrics: and whether the echoes are as frequently heard from steamers in mid-ocean, or whether they mainly attach themselves to coast lines. As Professor Henry bas well stated: “Much farther 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.” (L. H. Rep., p. 117.)

As the last of the series here selected, Tyndall's observation [8] agrees well with the observation of Duane [1].

[To be concluded.]

ART. IV.- Effect of Temperature on the Power of Solutions of

Quinine to rotate Polarized Light. The corrections to be applied for the same. Suggestions regarding the preparation to be used when Quinine is employed as a Medicine; by JOHN C. DRAPER, Professor of Natural History, College of the City of New York.

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In an admirable article on “The Action of the Solution of certain Substances on Polarized Light," by (). Hesse, in the Annalen der Chemie for 1875, the writer after dealing at length with the varying action of the alkaloids on a beam of polarized light says: “If we now take into consideration the fact that transparent bodies, as water and alcohol, are able, under the influence of electro-magnetism to deflect the plane of polarized light, although this property does not otherwise belong to them ; and that the optical powers of a substance can be influenced by mere mechanical means, as Scheibler has proved in certain kinds of glass; we must admit, that There is no real relation between the rotating power of a substance and its molecules.' " He then adds, “The rotating power of a substance is simply the result of the variable action of its factors, viz: the arrangement of the molecules as regards the volume, the solvent, the temperature, the concentration, the chemical combination, the dissociation and other things.”

The importance of utilizing the rotation power of quinine for the practical purposes of analysis has induced me to endeavor to determine, as far as possible, the corrections to be applied for the variations in question, and especially for those dependent on temperature. Concerning this, A. Bouchardat says, “variation in temperature causes variation in the rotation power of quinine.” In the paper mentioned above, O. Hesse says, “in the case of Thebaine and Quinine the rotation diminishes under an increase of temperature;" but he afterward adds, “I found that the variation between 15° C. and 25° C. was insignificant."

In my experiments the polariscope employed belonged to my friend, Dr. Ř. A. Witthaus. It was made by Laurent, of Paris, and read by verniers to two minutes. The tube was of glass 220 millimeters in length, with a lateral aperture near the center, through which a thermometer was introduced for the determination of temperature. Around this tube I placed a water jacket, the temperature of which was easily raised to and kept at any required degree, by the injection of steam through a pipe which passed to the bottom of the jacket. Having satisfied myself by a series of experiments that extreme variations of temperature in the water of the jacket, or bath, did not produce

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