Lightships and Lighthouses

by Frederick Arthur Ambrose Talbot

THE SIRENS OF THE LIZARD.

Owing to the importance of the Lizard Station and the fact that the coast often is obscured by fog, a powerful fog-signalling station is imperative.

In the early seventies an American investigator, Mr. C. L. Daboll, contrived an entirely new system, which developed into the foundation of one of the most successful fog-signalling devices for lighthouses which has been discovered—the siren. The Daboll invention was a huge trumpet, recalling a mammoth phonograph horn. It was 17 feet in length, and its mouth was 38 inches in diameter. In the lower end of this trumpet—the throat—was placed a tongue of steel measuring 10 inches in length and secured at one end to form a reed. It was blown by air compressed in a reservoir to the desired degree, and then permitted to escape through the trumpet. The mad rush of the expanding air through the constricted passage set the reed vibrating violently, causing the emission of a penetrating, discordant bellow. When Daboll commenced his experiments, he suffered from the lack of a suitable mechanical means for compressing the air, and made shift with a donkey for this purpose until the hot-air engine was improved, when the latter was substituted.

Trinity House adopted the idea and found it serviceable; but the Canadian authorities, after four years’ experiment, dissented from this view, remarking that the trumpet was expensive to maintain, unreliable in working, and liable to break down when most urgently needed. In fact, they characterized the Daboll trumpets which they had installed as “sources of danger instead of aids to navigation.”

From the trumpet to the siren was not a very big step. The history of the latter’s invention is somewhat obscure, but it was brought before the United States Government in a primitive form. The American engineers, recognizing its latent possibilities, took it up, and endeavoured to improve it to such a degree as to render it suitable for lighthouse work. Their efforts were only partially successful. The solution of the many difficulties attending its perfection was effected in Great Britain by Professor Frederick Hale Holmes, whose magneto-electric machine brought electricity within reach of the lighthouse as an illuminant, and it was due to the efforts of this scientist that the siren became one of the most efficient sound-producing instruments which have been discovered for this class of work.

The reason that made Professor Holmes bring his energies and knowledge to bear upon this subject was somewhat curious. The siren in its first form made its way from the United States to Great Britain. The British Admiralty realized the power and penetration of its sound, and forthwith adopted it in the navy, operating it by steam instead of by air. At this there arose a great outcry from the mercantile marine. Captains argued that the similarity of the signals confused and often misled them, as they could not tell in the fog whether the sound proceeded from a warship or a lighthouse. The Board of Trade was forced to intervene, but, as it had no jurisdiction over the Admiralty, it sought to extricate itself from an awkward situation by inviting Professor Holmes to perfect a siren which would emit a distinctive sound. His efforts were crowned with complete success.

Fig. 15.—The Fixed (A) and Revolving (B) Parts of the Siren.

Professor Holmes exhibited his wonderful device at the Paris Exhibition of 1867. He installed it in working order, and the visitors displayed an anxiety to hear it. It was brought into action, and those around never forgot the experience. It was the most diabolical ear-splitting noise which had been heard, and, apprehensive that serious results might arise from its demonstration when the buildings were thronged with sight-seers, the authorities refused to permit it to be sounded again. The humorous illustrated papers did not suffer such a golden opportunity to escape. Grotesque and laughable cartoons appeared depicting the curious effects produced by the blast of the instrument, one showing the various statues being frightened off their pedestals proving exceptionally popular.

The siren in its simplest form is an enlarged edition of the “Deviline” toy whistle. There is a Daboll trumpet with a small throat, in which is placed horizontally, not a reed, but a metal disc, so as to fill the whole circular space of the throat. The sheet of metal is pierced with a number of radial slits. Behind this disc is a second plate of a similar character, and likewise pierced with radial slits of the same size, shape and number; but whereas the first disc is fixed, the second is mounted on a spindle. The free disc rotates at high speed, so that the twelve jets of air which are driven through the throat are interrupted intermittently by the blanks of the revolving disc coming over the openings in the fixed disc, while when the two slits are in line the air has a free passage. If the revolving disc completes 3,000 revolutions per minute, and there are twelve slits in the discs, then a total of 36,000 vibrations per minute is produced while the instrument is in operation. The speed of the revolving disc, as well as the number and size of the openings, varies according to the size and class of the siren; but in any case an intensely powerful, dense and penetrating musical tone is emitted, which can be heard a considerable distance away. The blast of a high-powered large siren has been heard at a distance of twenty to thirty miles in clear weather, though of course in thick weather its range is reduced.