[Illustration: 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. | | | | | | Group occulting | Gp.Occ. | A steady light suddenly and totally |