Lightships and Lighthouses. Frederick Arthur Ambrose Talbot
one end, the whole being enclosed in an outer casing. Air under pressure is admitted into the outer casing, and drives the piston backwards and forwards with great rapidity. The result is that the air effects its escape through the orifices, when they come into line, in intermittent puffs.
While the broad principle is not unlike that of the conventional siren, the main difference is that in the latter there is a rotary motion, whereas in the diaphone the action is reciprocating. The great advantage of the latter is that all the vibrations are synchronous, owing to the symmetrical disposition of the slits, and consequently the note produced is very pure. The mechanism is so devised that the piston’s motion is controlled to a nicety, and the sound is constant. Experience has proved that the best results are obtained by using air at a pressure of 30 pounds per square inch. The sound thus produced is intensified to a markedly greater degree by means of a resonator properly attuned.
This instrument has displaced the siren among the stations upon the St. Lawrence River. The general type of apparatus has a piston 4½ inches in diameter, and uses 11 pounds of air per second during the sounding of the blast. But at more important stations a far larger and more powerful class of apparatus is used, the diaphone at Cape Race having a piston 8½ inches in diameter and using 27 feet of air per second while sounding. This does not indicate the limit of size, however, since the builders of this terrible noise-producer are experimenting with an apparatus having a piston 14 inches in diameter. The sound issuing from such a huge apparatus would be almost as deafening as the report of a big gun and should succeed in warning a mariner several miles away.
The atmosphere, however, plays many strange pranks with the most powerful sound-producing instruments. To-day, for instance, a fog-signal may be heard at a distance of ten miles; to-morrow it will fail to be audible more than a mile away. This aberration of sound is extraordinary and constitutes one of the unsolved problems of science. Innumerable investigations have been made with the object of finding the cause of this erratic action, but no conclusive explanation has been forthcoming. Another strange trick is that, while a sound may be audible at distances of two and four miles during a fog, it fails to strike the ear at three miles. It is as if the sound struck the water at a range of two miles, bounded high into the air, and again fell upon the water at four miles, giving a second leap to hit the water again farther on, in much the same way as a thin flat stone, when thrown horizontally into the water, will hop, skip, and jump over the surface. This trick renders the task of the lighthouse engineer additionally exasperating and taxes his ingenuity to the utmost, as it appears to baffle completely any attempt towards its elimination.
Recently another ingenious and novel system has been perfected by Messrs. D. and C. Stevenson. This is an acetylene gun which acts automatically. Hitherto an unattended fog-signal—except the bell-buoy tolled by the movement of the waves, which is far from satisfactory, or the whistling buoy, which is operated upon the same lines and is equally ineffective except at very short range—has found little favour. The objections to the bell and whistle buoys are the faintness of the sounds, which may be drowned by the noises produced on the ship herself; while, if the wind is blowing away from the vessel, she may pass within a few feet of the signal, yet outside its range. Thus it will be recognized that the fog-gun serves to fill a very important gap in connection with the warning of seafarers during thick weather.
As is well known, even a small charge of acetylene, when fired, will produce a loud report, and this characteristic of the gas induced Messrs. Stevenson to apply it to a fog-signal. They have developed the automatic acetylene system of lighting to a very high degree around the coasts of Scotland, and there are now more than twenty lights of this class, mostly unattended, in operation, some of which have been established for many years. These lights have proved highly satisfactory. There has never been an accident, a freedom which is due to the fact that Moye’s system is used, wherein the possibilities of mishap are surmounted very effectively. Accordingly, the engineers saw no reason why a similar system should not be adapted to the emission of sound instead of light signals, or, if desired, of both simultaneously. Their experiments have been crowned with complete success, and, as the gun uses no more gas than would be consumed if a flashing light system were used, the cost of operation is very low.
The general features of the acetylene fog-gun may be observed from the illustration (facing p. 64). The acetylene, dissolved in acetone, is contained under pressure in a cylinder, and thence passes through a reducing valve to an annular space, where it is ignited by an electric spark. A trumpet is attached to the firing chamber, so that the sound becomes intensified. If desired, the explosion can be effected at the burner, so that, in addition to a sound-signal, a flashing light is given.
The applications vary according to the circumstances. Suppose there is an unlighted bell-buoy at the bar of a port. Here the procedure is to install a gun and light combined, so that the flash of the explosion may give visual and the report audible warning. Or, should there be a lighted buoy already in position, its effectiveness may be enhanced by adding the gun, the detonation alone being employed for warning purposes. The size of the cylinder containing the dissolved acetylene may be varied, so that renewal need only be carried out once in one, two, or more months, according to conditions. If the increasing traffic around a certain rock demand that the latter should be marked, a combined sound and light apparatus can be installed. It may be that the head of a pier which is accessible only at certain times, or a beacon which can be reached only at rare intervals, may require improved facilities. In this case the gun can be set up and a cable laid to a convenient spot which may be approached at all times by an attendant. Then the latter, by the movement of a switch, can bring the gun instantly into action upon the alarm of fog, and it will keep firing at the set intervals until, the fog lifting, the gun is switched off.
In some cases, where the apparatus is set upon a lonely rock, a submarine cable may be laid between the marked point and the control-station. The cable is not a very costly addition. There are many lights where wages have to be paid merely for a man to bring the fog-signalling bell machinery into action. In such cases a fog-gun can be installed and the annual cost of maintenance decreased enormously, thereby enabling the outlay on the gun to be recouped within a very short time; while the light may be improved by using the flashes, so that the warning can be rendered more distinctive.
The invention is also applicable to lightships, many of which are manned by four men or more at a large cost per annum. In the majority of cases an unattended Stevenson lightship—such as described in another chapter, six of which are in use around the coasts of Scotland, and which give, not only a first-class light, but, by the aid of the fog-signal gun, can be made to give an excellent fog-signal as well—offers a means of reducing the heavy maintenance charges arising in connection with a manned light-vessel. In many instances existing lightships can be converted to the automatic system and completed by the gun. Each case must, of course, be decided upon its merits as regards the time the gun and light are required to work upon a single charge of acetylene, but there are no insuperable obstacles to its utilization.
Of course, in an isolated station lying perhaps some miles off the mainland, it may be necessary to keep the gun going night and day in fog and in clear weather alike. In this case, naturally, the great number of explosions involves considerable expense; but the inventors are carrying out experiments with a view to switching the gun on and off, as required, from a distant point by means of wireless telegraphy, so as to effect a saving in the expenditure of acetylene when there is no need on account of fine weather to keep the gun going. Still, it must not be supposed that the detonations even during clear weather are altogether abortive, inasmuch as a sound-signal at sea, where the atmosphere has a long-distance-carrying capacity as a rule, in conjunction with a light, draws double attention to a danger spot. Under such circumstances the waste of acetylene gas during periods of clear weather is more apparent than real.
The contest against the elements is still being waged, and slowly but surely engineering science is improving its position, and is hopeful of rendering audible signals as completely effective as those of a visual character.
CHAPTER V
THE EDDYSTONE LIGHTHOUSE