There is, however, no fixed rule as to the profile of any sea-work, which must neces sarily depend upon a variety of local peculiarities, such as the nature of the bottom, and the size and quality of the materials. While a long, sloping breakwater does not offer the same amount of resistance to the waves, neither is it in itself so strong, for the weight resting on the face-stones is decreased in proportion to the sine of the angle of the slope. On the other hand, the tendency of the waves to produce horizontal displace ment, supposing the direction of the impinging particles to be horizontal, is proportional to the cube of the sine of the an,* of elevation of the wall.
In tidal harbors, or those in shoal-water, it is admitted by all that the waves break, and therefore exert an impulsive force. Such works have to withstand (1.) The direct horizontal force which tends to remove the masonry; (2.) The vertical force acting upwards on projecting stones or protuberances, and against the lying beds of the stones; (3.) The vertical force acting downwards upon the talus wall, or passing over the para pet, and falling upon the roadway; and (4.) The back-draught, which is apt to remove the soft bottom in front of the work.
In designing the ground-plan of harbors, some rules should be kept in view: (1.) The entrance should be always kept seawards of the works of masonry; (2.) Long straight piers arc not so safe as those of horizontal curvature; (3.) There should be a good "loose," or point of departure free of rocks or a lee shore; (4.) The relation of the width of entrance to the area of a harbor should be a matter of careful study, as upon this depends the tranquillity of the interior, or what has been called the reductive power of the harbor. Mr. Stevenson's formula for the reductive power is as under: H = height of wave at entrance; b = breadth of entrance; B = breadth of harbor at place of obser vation; D = distance from mouth of harbor to place of observation; x = reduced height of wave at place of observation.
(H H — b VD) H 44.1 .
VB The late Mr. J. M. Renders plan of depositing rubble from open stages of pile-work is now universally used in the construction of deep-water piers. Sir J. Hawkshaw's method, adopted at Holyhead, consists of huge, irregular, undressed masses, set in hydraulic mortar, and resting upon pierces perdues.
The commercial value of a harbor increases, according to Mr. Stevenson, not simply as the depth of the water is increased, but as the cube of the depth. Hence the great
expense which is willingly incurred for securing even a foot or two of additional depth. The greatest feat • in deepening is at the Tyne, where Mr. Ure, c.E., dredged out the channel to 20 ft. at low water all the way up to Newcastle, Scouring is also employed for increasing the depth, as by sir W. Cubitt at Cardiff, where 2,500 tons of water a minute are let off. The late Mr. Renders scheme for Birkenhead was based simply on the quantity liberated and the sectional area of the channel, and was therefore operative for any distance, and did not depend on the propelling head, or on the direction in which the water left the sluices, which conditions regulate ordinary scouring on the small scale, and which is efficacious for only short distances from the outlet.—Docks (q.v.) of various kinds are connected with harbors.
Pine timber is admirably adapted for soft soils, when the exposure is not great, but owing to the ravages of the tondo navalis and limnoria terebro ns in localitieS where there is no admixture of fresh water, it is,soon destroyed. Greenheart, African oak. and bullet-tree are little affected by the wotm, its shown by experiments made in 1814 at the Bell Rock by Mr. Robert Stevenson. Even limestone and sandstone are perfo rated by the pholades and saxicavre. Metals also suffer from chemical action when immersed in salt-water. Mr. George Rennie's experiments showed that wrought-iron resists this action better than cast in the ratio of 8 to 1; while Mr. Mallet's experiments show that from to of an inch in depth of castings 1 inch thick, and about Ysof f wrought-iron, will be destroyed in a century in clean salt-water. A cannon-ball 4i- in. in diameter became oxidized to the extent of of an inch in the century. The use of Portland ceinent may be regarded as the most of recent improvements in harbor construction. Blocks of any size can be formed of sand, gravel, or stones, mixed with cement in the proportion of 1 of cement of the other materials. This will set readily in still water, and in a week will be nearly as hard as any ordinarysandstone. Sea walls of cement work can•also be built continuously, so as to constitute a single monolithic mass.
Reference may be made to sir J. Rennie's book on harbors, to that of Mr. Thomas Stevenson on the same subject, and to the minutes of institution of civil engineers, passim.