Lightships and Lighthouses. Frederick Arthur Ambrose Talbot
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Fig. 12.—Apparatus showing a Double Flash, followed by a Single Flash.
(By permission of Messrs. Chance Bros. and Co., Ltd.)
Obviously, owing to the great weight of the glass, the support must be heavy and substantial. A massive cast-iron pedestal is employed for this purpose. When the light is of the revolving character, means have to be incorporated to secure the requisite rotation. In the early days the turntable upon which the lens is mounted ran upon rollers, but now a very much better system is universally employed. This has been brought to a high standard of perfection by Messrs. Chance of Birmingham, who have carried out unceasing experiments in this field. The objection to rollers was the enormous friction that was set up, and the great effort that was required, not only to set the lenses revolving, but to keep them rotating at a steady pace. In the modern apparatus the rollers are superseded by an iron trough filled with mercury, upon which floats the turntable carrying the lenses. When the apparatus is properly built and balanced, the friction is so slight that the turntable can be set in motion by the little finger, notwithstanding that several tons have to be moved. Although the optical part of the apparatus floats upon the bed of quicksilver in the same way as a cork lifebelt floats upon water, it is provided with rollers which serve to hold the whole apparatus steady and to overcome any oscillation.
In the case of an immense apparatus such as a hyperradiant lens, which, together with the turntable, may have a total weight of 17,000 pounds, an enormous quantity of mercury is required. The trough of the Cape Race hyperradiant light carries 950 pounds of quicksilver, upon which the lantern is floated. In such an instance, also, the pedestal is a weighty part of the apparatus, representing in this case about 26,800 pounds, so that the complete apparatus utilized to throw the 1,100,000 candle-power beam from the guardian of the Newfoundland coast aggregates, when in working order, some 44,000 pounds, or approximately 20 tons.
Within the base of the pedestal is mounted the mechanism for rotating the optical apparatus. This is of the clockwork type driven by a weight. The latter moves up and down a tube which extends vertically to a certain depth through the centre of the tower. The weight of the driving force and the depth of its fall naturally vary according to the character of the light. In the Cape Race light the weight is of 900 pounds, and it falls 14½ feet per hour. Similarly, the length of time which the clock will run on one winding fluctuates. As a rule it requires to be rewound once every sixty or ninety minutes. A longer run is not recommended, as it would demand a longer weight-tube, while many authorities prefer the frequent winding, as the man on duty is kept on the alert thereby. As the weight approaches the bottom of its tube it sets an electric bell or gong in action, which serves to warn the light-keeper that the mechanism demands rewinding.
Fig. 13.—The Classification of Lights, showing the Respective Radius or Focal Distance of Lens from 150 to 1,330 Millimetres.
(By permission of Messrs. Chance Bros. and Co., Ltd.)
The weight and clockwork mechanism perfected by Messrs. Chance is regarded as one of the best in service. The rotation is perfect and even, owing to the governing system incorporated, while the steel wire carrying the weight is preferable to the chain, which is subject to wear and is noisy in action. In the Chance clockwork gear the weight is just sufficient to start the apparatus from a state of rest, the advantage of such a method being that, should the apparatus be stopped in its revolution from any untoward incident, it is able to restart itself.
Of course, the clockwork mechanism is required only in those cases where the lenticular apparatus has to be revolved. This introduces the question of avoiding confusion between lights. When beacons were first brought into service, the lights were of the fixed type, and the navigator, although warned by the glare to keep away from the spot so marked, was given no information as to his position. Accordingly, lighthouse engineers sought to assist him in this direction during the blackness of the night by providing a ready visual means of identification. Owing to the ingenuity which has been displayed, it has been rendered possible to ring the changes upon a light very extensively.
These may be subdivided broadly as follows:
| Type of Light. | Symbol. | Characteristics. |
|---|---|---|
| Fixed | F. | A steady continuous light. |
| Flashing | Fl. | A revolving light showing a single flash at regular intervals, or a fixed light with total eclipses. |
| Fixed and flashing | F.Fl. | A fixed light varied at regular intervals by a single flash of greater brilliancy. |
| Group flashing | Gp.Fl. | Various combinations of flashes shown at regular intervals. |
| Occulting | Occ. | A steady light suddenly and totally eclipsed at regular intervals. |
In the foregoing classifications only a white light is used. But it may so happen that the lighthouse, owing to its position and the dangerous character of the spot which it marks, carries a light which changes colour from white to red or green, which are shown alternately in various combinations. These characteristics are indicated as follows:
| Type of Light. | Symbol. | Characteristics. |
|---|---|---|
| Alternating | Alt. | White and colour alternating. |
| Alternating flashing | Alt.Fl. | Flashing alternations by revolving mechanism. |
| Alternating fixed and flashing | Alt.F.Fl. | Fixed and flashing alternating. |
| Alternating group flashing | Alt.Gp.Fl. | Group flashing alternating. |
In timing a revolving or flashing light, the cycle is taken from the beginning of one flash to the beginning of the next. In these readings the flash is always shorter than the duration of the eclipse, while an occultation is shorter than, or equal to, the length of the light interval. Since flashing and occulting may be carried out with a fixed light suddenly extinguished or eclipsed, the characterization is determined solely according to the relative duration of light and darkness, irrespective of the type of apparatus employed or the relative brilliancy. There is one peculiarity of the flashing light which may be remarked. At short distances and in clear weather a faint continuous light may be shown.
Hand in hand with the development of the optical apparatus has been the wonderful improvement in regard to the illuminants and the methods of producing a brilliant clear flame. The fuel first used upon the introduction of the oil lamp was sperm or colza oil, the former being obtained from the whale, and the latter from seeds and a wild-cabbage. Both were very expensive, so that the maintenance of a light was costly—so much so that the United States authorities devoted their efforts to the perfection of a high-class lard-oil. This proved highly satisfactory, possessing only one drawback. In winter it congealed so much under the low temperature that it had to be heated before it could be placed in the lamp; but once the light was set going, the heat radiated from the burner served to keep the oil sufficiently fluid to enable it to mount the wick to the point of combustion under capillary action.
So far as the American authorities were concerned, the advantages of lard-oil sufficed to bring a cheaper medium than colza-oil into vogue. A company, which had been induced by the Government to install an elaborate and expensive plant for the production of colza-oil, after prolonged experiment and efforts to reduce the cost