OBSERVATIONS ON THE ECHO AT LITTLE GULL ISLAND. The first observations to be mentioned are those relating to the echo; the results, however, in regard to this are not very satisfactory. The sirens were of the second order, and therefore the echoes produced were not as distinct as those from the larger instrument at Block Island. The echo from the horizontal trumpet was distinct, and in the prolongation of its axis; the interval however between the blast of the siren trumpet and the commencement of the echo was very brief; so short, indeed, that the ending of the one and the beginning of the other were generally difficult to distinguish. A slight leak in the apparatus of the siren produced a continuous hum, which interfered somewhat with the distinct appreciation of the sound of the echo. The keeper thought the weather was not favorable for the production of echoes. He thinks they are heard most distinctly during a perfect calm, which did not occur during the course of these investigations. The axis of the siren with the movable trumpet being directed to the zenith, strict attention was given by all the observers to any echo which might be produced from it; but in this case, as in that at Block Island, the slight echo which was heard came from all points of the horizon. On one occasion General Woodruff called attention to a small cloud passing directly over the zenith, from which a few drops of rain fell upon the platform on which the light-house is erected. Advantage was taken of this occurrence to direct strong blasts of the siren toward the cloud, but no perceptible echo was returned. We have failed, therefore, in this series of investigations, to obtain any positive facts in addition to those already known as to the character of the echo. In regard to the hypothesis offered for its explanation, if we found little in its support, we have met with nothing to invalidate it. But whatever may be the cause of the phenomenon, we do not consider it an important factor in explanation of the results we have obtained, since it was too feeble to produce any effect in the way of absorbing any notable part of the original sound. Its importance from Dr. Tyndall's point of view is its apparent support of the hypothesis of a flocculent condition of the atmosphere. OBSERVATIONS ON EFFECT OF ELEVATION ON AUDIBILITY. The next class of experiments at Little Gull Island had relation to the effect of elevation on sound. The conditions here, however, for ar riving at definite results on this point were by no means as favorable as those at Block Island. The height which could be commanded was only that of the tower of the light-house, the gallery of which is 74 feet above the platform upon which the buildings are erected, and 92 feet above the level of the sea, much less than that at Block Island. Besides this, the variableness of the wind at the surface of the ocean and at heights above was not favorable for the illustration of the point in question. The theoretical conditions in order that the sound may be heard with greater distinctness at an elevation than below are, as we have said before, that the wind be moving with a greater velocity in a given direction at an elevation than at the surface of the earth, and that the difference in the velocities may be against the sound-wave, so that its upper part may be more retarded than the lower. In this case the direction of a beam of sound will be curved upward, leaving as it were a vacuum of sound beneath. The distance of the origin of sound, however, must not be too great relatively to the elevation of the observer; otherwise it will pass over his head, as well as over that of the observer at the surface of the earth. In most instances the sound was not continuous, but was interrupted-heard for a time, then lost; again becoming audible, it was heard until it finally became imperceptible. Besides this, it was difficult to determine when the sound ceased to be heard, since this depended on the sensibility of the ear and the greater or less attention of the observer at the time of the observation. To obviate these difficulties as well as the unfavorable condition of too great a distance of the origin of sound from the observer, it was concluded to adopt as the duration of the sound the elapsed time between its beginning and the period when it was first lost. The observer on the tower was Mr. P. Barnard, while the one below was General Woodruff. From the records of the observations of these gentlemen the following tables are compiled, the first of which indicates the relative duration of sound on the top of the tower and at the bottom, the sound moving against the wind; the second, the same duration, the sound moving with the wind; and the third, the same with the sound at right angles to it. It appears from Table 1 that without a single exception the duration of the sound was greater at the top of the tower than at the bottom, although the difference in favor of the top of the tower in the several experiments is very variable. These results are in accordance with what was anticipated. In these observations the duration of the sound at the bottom and top are nearly the same, from which we might infer that the elevation of the observer has little effect on the hearing of sound moving with the wind. Were it not for the result of the first experiment of this class at Block Island, we should not hesitate to adopt this as a general conclusion. TABLE 3-Sound heard nearly at right angles to the wind. From the result of this table it would appear that the sound can be heard moving at right angles to the wind better at an elevation that at the surface-a result not anticipated. OBSERVATIONS ON THE EFFECT OF WIND ON SOUND. This series was commenced on the 2d of September. Barometer, 30.3 inches; thermometer, dry-bulb, 700.5 F. wet-bulb, 670.5. Wind at the surface of sea was six miles per hour, and variable; at 3 p. m. the velocity was eight miles at the surface. (See Fig. 7.) Fig. 7. 6Th 4 miles 4th 信 S.S.W. The experiments were made by means of the steamer Mistletoe, which proceeded from the light-house, as a center, in different directions, blowing the whistle every half-minute, and returning when, from a sig nal, the sound was lost; the time being noted by different observers, and the distance estimated by the position of the steamer in reference to known objects on the Coast-Survey chart, as well as by angles of azimuth and time of sailing. The steamer was directed to proceed, as indicated in Fig. 7, 1st, against the wind, so that the sound would come to the observers with the wind; 2d, at right angles to the wind; 3d, in an intermediate direction between the last course and the direction of the wind; 4th, approximately with the wind, so that the sound would come to the ears of the observers against the wind; 5th, in an intermediate direction; and, 6th, again at right angles to the wind. It was |