A Treatise On The Principles And Practice Of Dock Engineering

DEFINITIONS. 269 acquired, and hence its application to maritime structures, the connection of which with the mainland is of a slight and restricted nature. This feature, however, is equally characteristic of jetties and moles. The word mole is evidently derived from the Latin moles, a mass, and is indicative of a large mound, or long ridge of material, heaped more or less regularly, in such a way as to constitute some protection from rough external seas. In this respect it fulfils the functions of a breakwater, with which it is closely allied, though, in later times, it has acquired the special significance of a breakwater provided with a broad superstructure capable of being used as an ordinary quay. Perhaps the position may be best summarised thus:— Outlying works in exposed situations, used for protective purposes alone, are breakwaters. When joined to the shore, and equipped for commercial operations, they become, almost indifferently, piers, jetties, and moles. Accordingly, the latter terms will be employed, in the present chapter, as practically synonymous. A wharf may be defined as a continuons structure, occasionally acting as a retaining wall, along the open margin of a sea, or along the banks of a river, canal, or other waterway. The application of the word is some- what loose, and it is sometimes taken as identical with quay, though its use in connection with dock and basin walls is rare. Wharfs have obviously provided the most natural sites for the berthing of vessels from the earliest times, being employed for this purpose long before the ideas of outlying jetties and enclosed basins were conceived. In this connection, they are subdivisible into two classes—legal wharfs and suffer- ance wharfs. The former are certain wharfs, in all seaports, at which goods were required to be landed and shipped by Act 1 Eliz., cap. 11 (now repealed), and subséquent acts. Some wharfs, as at Chepstow, Gloucester,. &c., are deemed legal from immemorial usage; others have been made legal by spécial Acts of Parliament. Sufferance wharfs are places where certain goods may be landed and shipped—as hemp, flax, coal, and other goods—by spécial sufferance, granted by the Crown for that purpose.* These legal distinctions, however, have no bearing on the engineering aspect of the question. From their close relationship to ordinary quays, much that has been said in Ohapter v. is equally applicable to wharfs, but need not be repeated here. As part of a dock system, external jetties and piers serve a twofold purpose. In the first place, they act as protective works, by means of which vessels are guided and sheltered during their entry. Secondly, they serve as directive agencies for the deflection or régulation of currents. Whether intentionally on the part of the designer or not, this second function is one which must inevitably be performed by any artificial projection beyond the normal contour of a littoral. Hence it behoves the engineer to exercise great care in determining the location and disposition of a proposed jetty or * Dr. Ogilvie.