PILES are squared logs of wood used in engineering operations, such as dams, bridges, roads, etc. They arc sharpened at the point, and, if necessary, protected with iron points, to enable than to cut through the strata they encounter as they are driven into the ground. When used for coffer-damns, or such temporary purposes, they are placed close togcther, and driven firmly into the earth; time water is then pumped out, amid the piles form a dam, to enable workmen to lay foundations of piers. etc. When the force of the water rotund the dam is great, two rows of piles are driven in all round, and the space between the rows filled with clay, and paddled. Piles are also used for permanent works, when they are driven through loose soil till they reach a firm bottom, and thus form a foundation on which buildings, roads, etc., may be placed.
Cast-iron is frequently used for piles, which are cast hollow. Wharf-walls are some times built of piles; they are then cast with grooves on the sides, into which cast-iron plates (forming the walls) are fitted.
A kind of pile has been invented by Mr. Mitchell, which is of great use in very loose and shifting substances. It is called the screw-pile, and consists of a long shaft (of wrought iron), with a broad cast-iron disk, of a screw form at the lower cud. These
piles are especially useful for light-houses, beacons, etc., which have to be placed on sands. They are fixed by means of capstans, which give them a rotatory motion. Com mon piles are driven in by machines called pile-drivers. In these a heavy weight (or monkey) is raised to a considerable height between two guides, and then let fall on the head of the pile. The application of steam to these drivers has made them very power ful engines—Nasmyth's steam-hammer being a well-known instance.
In 1S43 Dr. L. H. Potts obtained a patent for a new kind of pile, which consists of hollow tubes of iron, from which the sand, etc., within them is removed by means of an air-pump, and the pipes are then sunk.
In recent railway bridges, cylinders have been much used to form both piles and piers. They are of cast-iron, and made in pieces (of about 6 ft. in height), which are applied one on the top of another. The sand or -gravel is removed from the inside of the first laid, which thus sinks down; another cylinder is placed above it, and the same process continued till it also has sunk sufficiently; and so on, cylinder over cylinder, till a solid foundation is reached. The requisite number of cylinders is then piled up to form the pier above ground.