(_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 In the modern apparatus the rollers are superseded by an iron trough filled with mercury, upon which floats the turntable carrying the lenses.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.