Lightships and Lighthouses. Frederick Arthur Ambrose Talbot
prism, and the dioptric mirror. Another noteworthy fact is that, by an exceedingly ingenious arrangement, the absorption of the rays by the glass used in producing the red flashes is neutralized to such a vast degree that the white and red flashes are of equal intensity.
The subsidiary light is another striking feature which the lighthouse engineer has introduced. For instance, a light may be shown from a dangerous reef, and give the mariner all the warning desired. But some distance away may lurk another isolated rock, which it is just as imperative to indicate, and yet on which another tower cannot be erected. This necessity is met by the subsidiary light. A portion of the light from the main apparatus is deflected and thrown to the desired spot by an ingenious arrangement of the prisms. On the west coast of Scotland, at Stornoway, a stream of light used to be deflected from the lantern in a vertical direction down the tower, and there bent at right angles, to be thrown through a lower window and fall upon a prism placed on the crest of a rock several hundred feet distant. From the deck of a vessel, the effect of the light striking the prism was akin to that produced by a beacon. Similarly in the case of St. Catherine’s light in the Isle of Wight: a portion of the light, which would otherwise be wasted over the area on the landward side, is carried vertically down the tower by a disposal of lenses and prisms, and is projected horizontally through a small window, after being coloured into a red ray by passing through some glass of the desired tint, to mark a danger spot some distance away. This method, however, is not favoured now, as the peril can be more efficiently marked by means of an independent beacon, a system which has become feasible owing to the vast improvements that have been made in automatic lights requiring no attention for several weeks or months at a time.
But in those instances where the latter expedient is not adopted, the practice is to cover the danger with a ray thrown from an entirely different light. When the present Eddystone tower was completed, a “low-light room,” as it is called, was incorporated, and a low-powered light was thrown from two Argand burners and reflectors through a window to mark a dangerous reef some three miles distant. But perhaps the best example of a subsidiary light is that which was carried out by Messrs. Chance in connection with the Cap de Couedie lighthouse. In this instance two dangers had to be indicated in a subsidiary manner, one being covered with a red, the other with a green, ray. The red sector marks a danger spot known as Lipson’s Reef, lying 8¾ miles distant, while the green light indicates Casuarina Island, 1¾ miles away. This installation, it may be pointed out, has proved highly successful, and certainly is very economical.
Fig. 14.—The Means whereby the Rays are deflected from the Main Light to form a Subsidiary Light.
(By permission of Messrs. Chance Bros. and Co., Ltd.)
There is another point which deserves mention—the duration of the flash in a revolving light. There was considerable discussion and difference of opinion upon this question some years ago. It was maintained that the shorter the duration of the flash, and the more rapidly it were thrown, the better it would be for the mariner. The Scottish engineers realized the significance of this problem, and, despite the hostile criticism of contemporary engineers, adopted a specific principle which was to give a flash of two and three-quarter seconds’ duration. Subsequently it was reduced to one second. The introduction of the mercury float enabled the optical apparatus to be revolved faster, and also facilitated the reduction in the number of panels or faces, so that ultimately the Scottish engineers reduced the flash to one of four-tenths of a second.
When Mr. Bourdelles devised the mercury float which enabled rotation to be accelerated, the French authorities rushed to the opposite extreme. They reduced the faces to four, and arranged for the apparatus to be revolved at a high speed, so that the duration of the flash was only one-tenth of a second at rapidly-recurring intervals. This type of light was called the feu-éclair, and was adopted as a result of prolonged laboratory investigation. But this was an instance where laboratory experiments and scientific reasoning failed to go hand in glove with practical experience and navigation, where the mariner has to contend with all sorts and conditions of weather. The seafarer expressed his opinion of the one-tenth of a second flash in uncomplimentary terms, displaying an indifferent appreciation of artificially-produced sheet-lightning.
Eventually there was a general agreement, among all those countries which had investigated the problem closely, that a flash of about three-tenths of a second was the most satisfactory, and this has since become tacitly standardized. The French authorities recognized the fallacy of their idea, and soon came into line with the other countries.
CHAPTER IV
FOG-SIGNALS
Notwithstanding the wonderful ingenuity that is displayed in the concentration of light into powerful beams, these all count for nothing when fog settles upon the sea. The ray of 1,000,000 candle-power is almost as futile then as the glimmer from a tallow dip.
Fog is the peril of the sea which the mariner dreads more than any other. The blanket of mist, descending upon the water, not only shuts everything from sight, but deadens every sound as well. The sea is absolutely calm, so that no intimation of danger ahead is conveyed by the breaking of the waves upon rock, shoal, sandbank, or iron-bound coast.
It is in times of fog that the navigator must be given the greatest protection. As this is impossible to accomplish visually, appeal must be made to his ear. In the early days of lighthouse engineering the methods of conveying audible warning were very crude. The discharge of a gun was the most popular, but it was neither serviceable nor reliable, and was made upon somewhat haphazard lines. Thus, in the case of a dangerous headland on the North American coast, which the Boston steamer had to round on its journey, the keepers mounted guard at the probable time of the vessel’s arrival off this point. They listened eagerly for the steamer’s whistle, and when it came screaming over the water they began hurriedly firing a carronade, keeping up the blank-cartridge bombardment until another shriek told them that those on the vessel had heard their signals. Sometimes the whistle was heard from a distance of six miles; at others from not more than two miles away. It depended upon circumstances. Obviously, such a primitive system was attended with considerable danger, as an accident was liable to happen to the men in their feverish haste to load and discharge the gun, while the plight of the boat was far from being enviable at times.
By permission of Messrs. Chance Bros. & Co., Ltd.
A MODERN LIGHTHOUSE SIREN PLANT.
Showing gas engines and air-compressors in duplicate, with siren at side.
In the early days every lighthouse tower was provided with a heavy bell. Indeed, the ponderous dome of metal projecting from the lantern gallery was considered indispensable. The bell varied in weight from 1,200 to 2,240 pounds, was fitted with a massive clapper, and when struck emitted a deep musical note. In order to enable the seafarer to gain some idea of his whereabouts, the fog-signals were given a sound-characteristic somewhat upon the lines of those in connection with the light. Thus, one lighthouse would give one stroke every ten seconds; another would give two strokes in quick succession, followed by a long silence, and so on. This system suffers from the severe handicap that the sound does not travel very far during foggy weather.
Another ingenious engineer recommended the utilization of the locomotive whistle, giving a high-toned, ear-piercing shriek, but the same objection as attended the use of the bell prevailed: the sound could not be heard more than a short distance away. The British lighthouse authorities submitted the idea to a series of searching investigations to ascertain its possibilities, but eventually were compelled to conclude that it was not superior to, if as good as, the other systems then in vogue. The United States authorities, as a result of their independent experiments, expressed a similar opinion; but in Canada practical application gave this whistle a favourable verdict.
Rockets