This plant possesses many ingenious features. As the light is derived from gas distilled from crude oil, a small gas-making plant is installed on the island, and this is used also for driving a battery of five eight-horse-power gas-engines--four are used at a time, the fifth being in reserve--to supply the thirty-horse-power demanded to operate the fog-signal. The energy thus developed drives two sets of powerful air-compressors, the four cylinders of which have a bore of 10 inches by a stroke of 20 inches, the air being compressed to 80 pounds per square inch and stored in two large air-receivers which hold 194 cubic feet. From this reservoir pipes buried in a trench excavated from the solid rock extend to the two trumpets, placed on the north and south sides of the island respectively. The length of piping on the north side is 3,400 feet, and on the south side 2,500 feet. At places where the pipe makes a dip, owing to the configuration of the rock, facilities are provided to draw off any water which may collect. Extreme care had to be displayed in connecting the lengths of piping, so that there might be no leakage, in which event, of course, the pressure of the air would drop and thereby incapacitate the signal. [Illustration: THE RATTRAY HEAD LIGHTHOUSE. A very exposed Scottish rock tower. It is unique because a full-powered siren fog-signal is installed therein.] Each signal is mounted in a domed house built of concrete, the mouth of the trumpet extending from the crown of the roof. Within the house is an air-receiver 9 feet in height by 4½ feet in diameter, of about 140 cubic feet capacity, which receives the compressed air transmitted through the piping from the compressing-station. It also contains the automatic apparatus whereby the signal is brought into action at the stipulated intervals, so as to produce the requisite sound characteristic. This is a self-winding clockwork mechanism which admits and cuts off the supply of air to the trumpets, its chief feature being that the clock is wound up by the compressed air itself, so that it is entirely free from human control. However, as a breakdown even with the best-designed and most-carefully-tended machinery cannot be circumvented entirely, there is a duplicate electrical mechanism, also automatically controlled from the power-generating station, the electric cables for which are laid in the pipe trenches. This acts as an emergency control. [Illustration: _By courtesy of Messrs. D. and C. Stevenson._ SULE SKERRY LIGHT. A lonely light of Scotland. The nearest land is the Butt of Lewis, 30 miles distant.]