fluid has no perceptible influence on the time of vibration.Lubeck's Inaugural Dissertation, Berlin, 1873.

THE CAUSE OF WOLF IN THE VIOLINCELLO.

Mr. Kingsley, in a communication to the Cambridge Philosophical Society, states that the wolf, a name given to a well-known defect in the violin, occurs somewhere about low E or E flat, and has been attributed to the finger-board having the same pitch, so that it becomes, as it were, a portion of the string stopped down on it, and vibrates with it. Another explanation is given by Savart, viz., that the violincello is constructed of such dimensions that the mass of air included within the instrument resonates to a note making 85.33 vibrations in a second, a number which formerly represented the lowest F on the C string; but which now, owing to the rise of pitch since the beginning of the eighteenth century, nearly represents the note E immediately below it.-Nature, XII., 40.

THE PYROPHONE.

In 1873 Mr. Kastner brought forward his new invention, the pyrophone, which consists essentially of a flame of hydrogen gas, burning within a tube in such a way as to produce the well-known singing sounds on a large scale. If in the tube of glass or any other material, we introduce two or more isolated flames of proper size, and if we place them at a distance from each other one third of the length of the tube, these flames will vibrate in unison. This phenomenon is produced as long as the flames remain separated, but ceases as soon as the flames are brought into contact. It is upon this principle that his pyrophone is based; and the principal objection to the original instrument, which consisted in the necessity of employing hydrogen gas, he has recently overcome, and states that he is now able to employ ordinary illuminating gas; but to do this he is obliged to eliminate the carbon, whereas at first it was impossible to make the tube vibrate with illuminating gas, although the flames were placed in the proper position. According to him, sonorous flames of illuminating gas are in fact enveloped by a photosphere which does not exist when the flame is simply luminous. This photosphere contains a detonating mixture of

hydrogen and oxygen, which determines the vibration of the air in the tube. In order that the sound be produced in all its intensity, it is necessary and sufficient that the number of detonations produced by the molecules of oxygen and hydrogen in a given time shall be in accord with the number of vibrations corresponding to the sound produced by the tube. He finds it sufficient then to increase the number of his flames, substituting four, five, six, or more jets of illuminating gas for his two jets of hydrogen, and diminishing the height of these flames correspondingly, until the sum total of the surfaces of the photospheres suffices to produce the vibrations of the air in the tube.-Bull. Hebd., 1875, 266.

RELATIVE EFFICIENCY OF VARIOUS FOG-SIGNALS.

The principal instruments employed on the American coast as fog-signals are the Daboll reed trumpet, the locomotive whistle, and the siren. In a report on the relative efficiency of these instruments, General Duane states in reference to all of them that, while they are frequently heard at distances of twenty miles, yet as frequently they can not be heard a distance of two miles, and this with no perceptible difference in the state of the atmosphere. It is therefore very difficult to determine the relative powers of fog-signals, unless they are placed side by side, under exceptionally favorable atmospheric circumstances. The sound from the whistle is equally distributed in all horizontal directions, and is most powerful in a horizontal plane passing through the whistle. The sound from the siren is most distinct in the axis of the trumpet with which it is provided. The sound given by the Daboll reed trumpet is usually strongest in a plane perpendicular to its axis. In the average of a great number of experiments, General Duane concludes that the powers of the first-class siren, the 12-inch whistle, and the first-class Daboll trumpet may be expressed by the numbers 9, 7, and 4. The extreme limit of the audibility of the sound of the trumpet is twelve miles; that of the 12-inch whistle about twenty miles. That of the siren has not been ascertained. The relative expenditure of fuel by the steam-engines working these instruments at their full capacity is, for the siren, 9; the whistle, 3; and the trumpet, 1. As regards the skill and attention required in the management of these signals, the

siren seems to require the most, while the steam-whistle gives the least trouble. As to the anomalies observed in relation to the penetration and direction of sound from fogsignals, General Duane holds that they are to be attributed mainly to the want of uniformity in the surrounding atmosphere, and that snow, rain, fog, and wind have much less influence than has generally been supposed.-Rep. Light-house Board, 1874, Appendix.

FOG-SIGNALS.

In the appendix to the recent report of the Light-house Board, Professor Henry gives the first account that has, as yet, appeared of the experiments and observations made by him in reference to fog-signals, and especially in reference to the acoustic phenomena exhibited on a large scale in the atmosphere. Among other matters, he states that Professor Bache adopted a very ingenious plan for an automatic fog-signal, which consisted in taking advantage of a conical opening in the rocky coast, generally designated as a blow-hole. On the apex of this hole he erected a chimney, which was terminated by a tube surmounted by a whistle. By this arrangement a loud sound was produced as often as a wave entered the mouth of the indentation. The penetrating power of the sound was, under favorable circumstances, due to the pressure of a column of water twenty feet high, giving a pressure of about ten pounds to the square inch. The effect of the percussion, however, sometimes added considerably to this. In practice it was found that this arrangement, which continued in operation for several years, did not entirely supersede the necessity of occasionally producing sounds of greater power. It is stated that Professor J. H. Alexander, of Baltimore, in his investigations on the use of the locomotive steam-whistles, experimentally demonstrated that the power of the sound depends upon the pressure of the steam in the boiler, and the pitch of the sound depends upon the distance between the edge of the whistle and the circular orifice through which the steam issues. Among the various steam fog-signals, one consisting of a double whistle, improperly called a steam gong, seems of interest. This consists of two bells of the ordinary steam-whistle upon the same hollow axes, mouth to mouth; the upper bell has a

length of axis of twenty inches; the lower whistle is of the same diameter, but of a length of axis of fourteen inches. The note of the shorter bell is a fifth of that of the longer. This arrangement gives a melodious sound, unlike that of ordinary locomotive whistles, and on that account has extraordi nary merit; its character being strongly distinct from that of steamboat whistles. In reference to the audibility of signals in different kinds of weather, it was found that a sound moving against the wind, and inaudible to the ear on the deck of a schooner, was heard by ascending to the masthead. In general, it was stated that when the fishermen in the morning, on the Banks of Newfoundland, hear the sound of the surf to the leeward, or from a point toward which the wind is blowing, they take this as an inevitable indication that in the course of from one to five hours the wind will change to the opposite direction from that in which it is blowing at the time. General Duane states that the fogsignals at Cape Elizabeth, and at Portland Head, which are respectively nine and four miles southeast of Portland, can be heard in the latter city much better during a heavy northeast snow-storm than at any other time, although the sound comes to the city in nearly direct opposition to the course of the wind. The most perplexing difficulty, however, arises from the fact that the signal often seems to be surrounded by a belt in which the sound is entirely inaudible. Thus, in moving directly from the station, the sound is audible for the distance of a mile, is then lost for about the same distance, after which it is again distinctly heard for a long time. This action is common to all sound signals, and has been at times observed at all the stations; even at one where the signal is situated on the bare rock, twenty miles from the mainland, with no surrounding objects to affect the sound.-Rep. Lighthouse Board, 1874, Appendix.

ON CELESTIAL PHOTOMETRY.

Professor Thury communicates to the Physical Society of Geneva a very full description of a new photometer adapted to astronomical purposes, and also some general considerations upon photometry. It is not at present necessary, as it was fifty years ago, to insist upon the importance of photometric observations in astronomy. We know that the problems

of the distribution of the stars in space, and the gradual modifications that celestial bodies undergo in their own nature, are intimately connected with the intensity of the light we receive from them or that they send out; but it would be impossible to find a collection of photometric observations sufficient to serve as a basis for safe deductions. Either the accuracy of the observations is too small, or there are not enough of them. The observations that we have are due, for the most part, to experienced observers, and the differences between their methods of research fully explains the diversity of their results. In order to make their observations comparable with each other, and to eliminate causes of error peculiar to each method, it is necessary to institute comparative observations by making use of each of the methods employed hitherto in photometry. The results of such an investigation, which has already been commenced in Germany, will probably be, first, a general accordance of the figures obtained by different methods, sufficient to give confidence in their exactness. Second, the knowledge of the means proper to bring about such an accordance; that is to say, a knowledge of the universal corrections and of the improvements necessary to be introduced into the apparatus, and the methods of employing it. Finally, we shall know which of the photometric methods permits the greatest degree of exactness, and which offer special advantages. In general the photometers hitherto employed may be divided. into two categories: First, those where the object affected by the light is the eye itself. Second, the physical and chemical photometers, where some inert body is modified by the light. Of these latter, that of Leslie and the photographic photometers are instruments especially adapted to measure separately the intensity of different kinds of radiations. Visual photometers are divided into two classes. In one we diminish the brightness of the light until it disappears from the sight, or, rather, until it becomes too feeble to enable us to distinguish certain definite details of objects, and we then calculate the quantity of the diminution by knowing the methods employed to produce it. These are the photometers of extinction; such are those of Arago, Xavier, and Maistre. The second class of visual photometers is that of comparison, where the two lights present themselves at