But if friction can be almost eliminated the apparatus can revolve at a high speed without throwing undue burden upon the men. But how can friction thus be got rid of? Messrs Chance Bros., the great lighthouse constructors, of Birmingham, have done it, almost entirely, by floating the apparatus on mercury. The turntable has on its under side a large ring which nearly fits a cast-iron trough on the top of the pedestal. In this trough there is mercury, so that upon the liquid metal the apparatus floats as if upon a circular raft. The table with its lenses, prisms and other fittings may weigh six or seven tons, yet it can be pushed round by one finger. The various sizes of optical apparatus are known as "orders." One of the "first order" has a focal distance of 920 millimetres. This means that there is that distance between the centre of the lamp and the bull's-eye. They descend by successive stages down to the sixth order, with a focal distance of 150 millimetres, while the most important lights are of an order superior even to the so-called "first," termed the "hyper-radial," the focal distance of which is 1330 millimetres. A recent example of a hyper-radial light is at the well-known Cape Race in Newfoundland. It revolves once every 30 seconds, giving a flash of 3 seconds every 7-1/2 seconds. The optical apparatus weighs seven tons. [Illustration: _By permission of Messrs. Chance Bros. and Co., Ltd., Birmingham_ DASSEN ISLAND LIGHTHOUSE, CAPE OF GOOD HOPE This lighthouse, 80 feet high, is built of cast-iron plates, bolted together]