A Treatise On The Principles And Practice Of Harbour Engineering

54 HARBOUR ENGINEERING. For purposes to which no great acouracy is essential, any convenient buoyant object may serve, such as an empty keg or barrel, and in positions where a small object can be easily seen, an angler’s float will do admirably. Small pieces of cork or wood suggest themselves as equally utilisable. An orange makes a good float under convenient circumstances of visibility. Its spécifie gravity is very little less than that of water ; hence it floats nearly wholly immersed, exposing little or no surface to atmospheric action. Diving".—Perhaps the most interesting, not to say romantic, feature of harbour engineering work is the use of diving apparatus in connection with the preparation of submarine foundations. A very large proportion of the operations necessary to the satisfactory stability of breakwaters and quays has to be conducted under water, and rnuch of it would be almost inipracticable without the aid of diving-bells and diving-dresses. Natives of the East Indian Seas engaged in pearl fisheries do, it is true, remain under water for appreciable periods without any special apparatus for the supply of air, but the strain is very great, causing bleeding at the nose, mouth, and ears, and if unduly prolonged, leading to fatal results. Apart from these physical effects upon the agents engaged, there is rauch interruption to the operations, which, in the case of structural work, would be inimical to its satisfactory accomplish- ment, and the haste with which the operations have to be performed would be incompatible with the exercise of care and accuracy. A regular and con- stant supply of atmospheric air to workers below the surface enables them to remain on duty for some time without any serions discomfort, and for this reason alone the use of air chambers and diving suits has become an intégral accompaniment of all maritime operations. The Diving’-Bell (fig. 54) is a metallic chamber of sufficient capacity to accommodate any number of workers, frora one man up to a dozen or more. The chamber, which is formed of mild steel plates, carefully rivetted together and caulked so as to be absolutely water-tight, is suspended from the jib of a crane or overhead traveller, or from any lifting appliance afloat or ashore, and so raised and lowered as the case may be. In the interior are seats and footrests for the occupants during ascent and descent, and shelves for tools. Signalling gear is provided, and many bells are now fitted with electric light and with telephonic apparatus. In addition to this equipment, there are the necessary air-valves and pipes for maintaining a supply of fresh air at the required pressure. At the top and sides of the bell are observation lenses, affording a view of the environment. The bell is ballasted with cast- iron kentledge, placed in a special chamber, to enable the structure to sink without endangering its equilibrium. The size of bells varies very consider- ably. Two instances may be quoted as examples. The diving-bells used on the Dover Harbour Works were 17 feet long by lOj feet wide by 6| feet high, inside measurement. At Marseiiles the dimensions were 66 feet in length, 22 feet in width, and 6^ feet in height. Diving-bells may be kept in comparatively free communication with the upper air by means of a cylindrical tube, carried up above the surface of the