Abstract
WHEN lighthouse lights and all other seamarks are obscured by fog, sound is the only medium by which warning signals can be conveyed to mariners. It has been thought that it might be possible to transmit such signals by means of etheric vibrations; but assuming such intercommunication were established, it would fail in two most essential requirements for assisting the mariner in foggy weather, as it would not give him any information as to the direction from which the warning message came, nor would it tell him how far distant the signalling station was. Further developments may in the course of time remedy these defects, but from present-day knowledge and experience it cannot be said that etheric vibrations are available for fog-signal purposes at sea. In a paper recently read before the Society of Arts, Mr. E. Price Edwards discusses the present position of this question of sound signals and gives some interesting particulars of the trials carried out at St. Catherine's Point, in the Isle of Wight, last summer. From this it appears that for many years past sound-producing instruments of various kinds have been employed for uttering warning sounds at points of danger on our coasts, and that constant efforts have been made to develop instruments yielding sounds of great loudness and penetrating power, so as to overcome the numerous obstructive influences affecting the propagation of sound through the atmosphere. The instrument which has proved most effectual for this purpose is the sireji, sounded by means of air forced through it at a pressure of about 40 lb. on the square inch. It is used in the form of a double cylinder, one cylinder fixed, the other (inside it) rotating, each cylinder having longi. tudinal slits corresponding in number and area, through which, as often as they coincide, the air passes. In the trials at St. Catherine's, two flat circular discs with radial slits were tried, with very satisfactory results; but this arrangement involves a separate motor to rotate the movable disc, whereas the rotation of the cylinder siren is effected by the air pressure which produces the sound. It is considered that some loss of power [and a more or less defective blast result from the self-driving arrangement, and ithat the use of a separate motor will remedy these defects. The trials referred to were made with various forms and sizes of siren and several instruments sounded on the reed principle, the result being that the reed instruments proved greatly inferior to the siren instruments in loudness and penetrating power. It is contended by some that the reed principle as applied for the production of loud sounds has never yet been done justice to, and that with proper development a reed instrument could be made to yield sounds as powerful and penetrating as those of the siren; but, as Mr. Price Edwards points out, the reed instruments tried, and which were supposed to be the most effective types of that form of sound producer in existence, were not able to approach the sirens as regards efficiency for coast fog-signal purposes. If a reed instrument could be brought up to an equality with a siren in respect of sound power, it would probably be more economical than a siren in working. The question of trumpets received some special consideration at St. Catherine's, a new form of trumpet designed by Lord Rayleigh having been experimentally tried there. Lord Rayleigh had observed that with the conical trumpets of circular section usually employed there was a liability to some interference of the sound waves issuing from the mouth, caused by the difference in distance of the nearest and furthest parts of the mouth, whereby the waves were likely to get out of step and thus cause interference. He also pointed out that a good deal of sound| was sent to the zenith from the mouth of circular section, which| sound was certainly wasted. To remedy these defects, Lord Rayleigh's idea is to make the horizontal diameter at the mouth only half the length of the sound wave generated by the sounding j instrument, and that the vertical diameter should be elongated j to two wave-lengths or more, thus producing a mouth of ellip- i tical section. The tendency for the waves issuing from the mouth to get out of step would thus be reduced to a minimum, and the narrowness of the mouth at top and bottom would offer but little scope for the sound to be projected upward or immediately downward. So far as the trials went, Mr. Price Edwards tells us that the effects produced were most encouraging, and it is now intended to set up this elliptical trumpet for practical trial at a fog-signal station. The mushroom form of trumpet for an all-round signal has been largely used for lightships. Instead of a long horizontal trumpet, or a vertical one with the head bent over (capable of being turned in any direction), the trumpet is fixed vertically with its mouth directed upward. Just above and in the centre of this open mouth is fixed an inverted cone, and the sound issuing from the trumpet strikes the curved sides of the cone and is reflected out with equal force all round the horizon. The trials made with this form of trumpet showed that it was well adapted for the purpose for which it had been designed.
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Coast Fog Signals . Nature 66, 115–116 (1902). https://doi.org/10.1038/066115b0
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DOI: https://doi.org/10.1038/066115b0