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: