A Treatise On The Principles And Practice Of Dock Engineering

GRANITE. 153 to its extensive use is its expense. A zinc covering has been tried, but it is soon corroded by sea-water. Muntz metal is another substitute. Pipe Casings.—P^ encased in earthenware pipes, such as drain pipes, with the intervening space filled in with sand or cement grout, make a durable combination in situations free from shocks and erosion. A coating of Portland cement will often answer the same purpose, but it is more likely to crack. Tubes of steel wire netting, embedded in conerete on the Monier principle, have been found very effective. Compound Coverings. —The following method, used on the Pacific coast, has attracted attention :—“After removing the bark, the surface of the pile is covered with a prepared compound, some of the ingrédients of which are paraffin, powdered limestone, and kaolin. The pile is then wrapped in jute burlap, and another application of the compound is made. Wooden battens are then nailed along the surface, which receives a final coat of the paint. Piles thus protected have been in use for ten years. The coating protects the piles from the teredo, limnoria, and similar animals, but its duration is not known.”* Close Nailing.—The driving in, very closely together, of broad-headed scupper-nails is an expedient of some antiquity. The heads are apt to rust, and though this is sometimes held to be a further protection from worms, the statement lacks confirmation. The method has been applied to dock gates, but it is troublesome and expensive. A natural protection is very often provided by the accumulation of barnacles, mussels, and other shellfish upon the surface of the wood. Sea thorns act in the same way when the surface has been covered with theii discs. STONE. There are many varieties of stone suitable for constructive woik, but the dock engineer confines his attention to a comparative few, which, by long experience, have gained a reputation for durability and strength. The principal of these is granite. Granite is a very hard and extremely durable rock, of igneous origin, crystalline in structure, and of great value in dock work on account of its heavy and massive proportions. In its true form it is composed of crystals of quartz, felspar, and mica; but there are other—so-called—granites con- taining hornblende (syenitic granite), quartz diorite, &c. The quartz is a very hard substance, witli a vitreous lustre, and prac- tically indestructible. It renders the granite very difficult to work. The felspar is lustrous and granular, and, being present in greatest volume, gives the granite its distinctive colour, which inay be white, grey, pink, red, or brown. It is less hard and less durable than quartz. Mica is a thin, flaky substance, with a bright, metallic lustre. It is easily decomposed. * Snow on “ Marine Woodborers,” Engineering, Oct. 7, 1898.