Читать книгу Lightships and Lighthouses - Frederick Arthur Ambrose Talbot - Страница 6

By permission of the “Syren and Shipping.”

LOOKING UP THE LANTERN OF THE NEEDLES LIGHTHOUSE.

So far as the candle-power of any light is concerned, the method of determining this factor, varying according to the calculating methods adopted, is somewhat misleading. So far as Great Britain is concerned, the practice of setting out the candle-power of any light in the official list has been abandoned, the authorities merely stating that such and such a light is of great power. The United States and Canada, on the other hand, indicate the approximate candle-power.

By courtesy of Messrs. Chance Bros. & Co., Ltd.

FIXED APPARATUS OF THE FOURTH ORDER FOR SARAWAK.

The focal distance is 250 millimetres, and the diameter of lantern inside glazing 6 feet 7¾ inches.

By combining and arranging the integral parts of the optical apparatus, the lighthouse engineer is able to accomplish many astonishing results. Thus, while the various types generally follow accepted broad lines, coinciding with the order which they represent, here and there some very striking divergences are made. The Bell Rock light is perhaps the most interesting example in this direction. It was designed by Messrs. D. and T. Stevenson, and built by Messrs. Chance Brothers and Co. The light is alternating, the colours being white and red. Externally the optical apparatus appears to be bizarre, yet it is one of the most perfect which has ever been installed. In its design and construction almost all the known lighthouse optical elements are incorporated, including the equiangular refractor, the reflecting prism, the double-reflecting 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.