Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/493

Rh HARBOURS 465 There is also good reason to believe that slopes of large pierres perdues are preferable to slopes of dressed masonry. The great object to be attained is the maximum amount and uniform distribution of friction. on Piers of cast and of malleable iron are now frequently ers. employed. Examples of these may be seen at Scarborough, Southport, Portobello, and at many other parts of the English coast, and have been found to answer even where there is a considerable sea. The pier at Southport, described 1 by Mr II. Hooper, is of cast iron, and the mode of sinking the piles was peculiar. The piles proper or lowest lengths of the columns are cast in lengths of 8 feet and 10 feet, and are sunk into the sand to the depth of 7 feet and 9 feet respectively. They were provided with circular disks 1 foot 6 inches in diameter, to form a bearing surface ; and, a small hole being left in the centre, a wrought-iron tube 2 inches in diameter was passed down the inside of the pile, and forced about four inches into the sand, a connexion being made by means of a flexible hose between the top of the tube and a temporary pipe con nected with the Water Company s mains, and extended as the sink ing of the piles proceeded. A pressure of water of about 50 Ib per inch was thus obtained, and this was found to be sufficient to force the sand from under the disk. Each disk was provided on the lower side with cutters, which, on an alternating motion being given to the pile, loosened the sand. The piles were gradually lowered, and guided by a small ordinary piling engine. When the pressure of water had been removed about 5 minutes, the piles settled down to so firm a bearing that when tested with a load of 12 tons each no signs of settlement could be perceived. The cast-iron columns are 7 inches in external and 5| inches in internal diameter. All the piles, to the number of 237, were sunk in six weeks, being at the rate of between 6 and 7 in twenty-four hours. The Clevedon pier is of malleable iron. Mr J. W. Grover 2 states that each upright consists of two Barlow rails weighing 80 It) to the yard, riveted back to back, and having a total section for each 100 feet span of 64 inches. They are braced together by diagonal tie-rods from If to 2 inches in diameter. The lower portions of the piles below low water are of solid wrought iron 5 inches in diameter, shod with cast-iron screws 2 feet in diameter, and were screwed down till a 4^-inch rope passed round 6-feet capstan bars parted with the strain. They penetrated the ground to depths varying from 7 to 17 feet, and though made with a thread of 5 inches in pitch seldom descended more than 2| inches or 3 inches in a turn. The solid pile-stems are connected with the Barlow rail piles by cast-iron shoes. Where rock occurred holes were jumped, and a 4-inch wrought-iron bar was inserted and secured by a jagged key. A shoe to receive the Barlow rail was fitted and keyed on this, and the remaining space was caulked with iron cement. The length of the longest pile is 76 feet. The level of roadway is 16 feet above extreme high water, and the height above the ground at the pier-head is about 68 feet. Deposits and Scouring. Deposits arise from two essentially opposite causes, viz., commotion and stagnation of the water. They are also of different kinds of material : (1) those lighter matters, such as mud, silt, and fine sand, which are held for a time in mechanical suspension, and which, as the water approaches stagnation, are gradually dropped in the order of their .specific gravity and mass; and (2) those grosser matters, such as different kinds of sand and gravel, which are rolled along the bottom or driven forward with the water by the action of waves and currents. The first-named are deposited on the lee side of piers or groins which obstruct the progress of the waves or currents, while the second accumulate on their weather sides. But the amount and the positions assumed by the deposits of the second kind depend on the direction^and force of the waves or currents in relation to the foreshore and to obstructions. The amount of deposits of the first kind are little dependent on the direction of the stroke of the waves outside of the basin or sheltered place in which they are deposited. flcial The preservation of the depth of harbours at a level lower than that of the original bottom involves both un certainty and expense. Where the deposit is confined to the space between high and low water-marks, the scouring 1 Min. Inst. Civ. Eny., vol. xx. - Jfin. Inst. Civ. ny.,vo. xxxii. by means of salt or fresh water is comparatively easy ; but where it forms a bar outside of the entrance, the possibility of maintaining permanently a greater depth becomes very doubtful. The efficacy of the scour, so long as it is not impeded by enlargements of the channel, may be kept up for great distances, but it soon comes to an end after it meets the sea. When the volume of water liberated is great compared with the alveus or channel through which it has to pass, the stagnant w r ater which originally occupied the channel does not to the same extent destroy the momentum as where the scouring has to be produced by a sudden finite impulse. In the one case the scouring power depends, cceteris paribus, simply on the relation subsisting between the quantity liberated in a given space of time and the sectional area of the channel through which it has to pass ; while in the other it depends on the propelling head, and the direction in which the water leaves the sluice. Mr Rondel s scheme for Birkenhead was on the former principle, which it must be recollected is only applicable where the soil is easily stirred up. The first example of artificial scouring in England seems ! to be due to Smeaton, who used it effectively at Ilamsgate in 1779. At Bute Docks, Cardiff, designed by the late Sir W. Cubitt, the access to the outer basin is kept open most successfully by means of artificial scouring on a large scale. Minard holds that when a channel has to be maintained I by regular and habitual scouring, the whole effect is gene- ! rally produced in the course of the first quarter of an hour. j This was made the subject of particular investigation at Dunkirk, where sections of the channel were made before and during the scour ; and it was found that there was no alteration in the sectional area after the first quarter of an hour. Causes of Increase of Height of Waves. The following causes increase the height of waves in a harbour : (1) the removal of a spending beach either at the entrance channel or within the harbour by building a quay or talus wall across it ; (2) deepening the entrance channel or the basin of the harbour itself, thus the diffi culty of keeping vosseLs to their moorings at the quays of a harbour increases during flood tide, just as the tide rises and the depth is thereby increased ; (3) removal of an artificial breakwater or a protecting point of land or spit of gravel outside of the harbour ; (4) filling up side creeks or lateral basins in a harbour or its entrance channel ; (5) shortening the distance between the sea and the entrance nearer the sea entrance, or projecting the mouth of a harbour farther seawards ; (6) removal of a jetty or filling up an artificial side chamber or converting talus into plumb walls ; (7) straightening an entrance channel or an outer side breakwater or walls within a harbour ; (8) widening the entrance or the entrance channel ; (9) reduction of the area of the basin of the harbour ; (10) making smoother the surface of a natural sloping beach either within or outside of a harbour by pitching its surface; (11) lower- ! the sea breaks; (12) converting an open timber quay into a close quay; (13) lengthening a breakwater so as to intercept a greater portion of the waves which come in certain directions. II. DOCKS. Docks are of two kinds, viz., dry or graving docks, and wet or floating docks. The dry or graving dock is a small water-tight chamber, fitted with gates of timber or iron, which are shut against the rising tide after a vessel has XI. - 59
 * to an inner basin or dock by projecting the basin or dock
 * farther out from the shore-line so as to bring the gates
 * ing the top of a protecting outside breakwater over which