Rockets also have been adopted, and are highly successful. Indeed, this method of conveying audible warning prevails still in many countries. The practicability of such a means of throwing sound over a wide area was advanced by Sir Richard Collinson, when Deputy-Master of Trinity House, and his idea comprised the insertion of a gun-cotton charge, timed to explode at a given height, in the head of the rocket. The height could be varied up to about 1,000 feet, and the weight of the charge fluctuated according to requirements. The rocket system was tested very severely, and in some instances the report was heard as many as twenty-five miles away. It received the approbation of Professor Tyndall, and, although superior methods of signalling have been devised since, there remain one or two lighthouse stations where it is considered to be the most satisfactory fog-signalling device, notably the station on the island of Heligoland, where the rocket is hurled into the air to explode at a height of nearly 700 feet. In many lighthouses the detonation of gun-cotton constitutes the means of conveying warning to passing vessels, but is accomplished in a different manner. The charge, instead of being sent into the air to be exploded, is attached to a special device which is supported upon a simple frame at a point above the lantern, so that no damage may be inflicted upon the glass of the latter from the concussion. The apparatus is fitted with a safety device which prevents premature explosion, so that the keeper is preserved from personal injury, and, unless culpable negligence is manifested, the charge cannot be ignited until it has been raised to its designed position. The report is of great volume, and as a rule can be heard a considerable distance; but in this, as in all other cases, the atmosphere plays many strange tricks. Still, it has not been superseded yet for isolated sea-rock lighthouses, such as the Eddystone, Skerryvore, and Bell Rock, where there is lack of adequate space for the installation of any other equally efficient fog-signalling facilities. [Illustration: _Photo, Paul, Penzance._ THE SIRENS OF THE LIZARD. Owing to the importance of the Lizard Station and the fact that the coast often is obscured by fog, a powerful fog-signalling station is imperative.]