Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/435

F R F B 421 upwards of 200 feet ; and there are extensive alluvial for mations along the lower part of its course, and in the upper reaches of its estuary. The tides are felt 4i miles above Stirling and at Stirling harbour the spring tides rise 7 feet 9 inches. At Leith and Kinghorn their average height is 17i feet. Steamers go as far up the river as Stirling, the" only important obstructions to the navigation between that town and Alloa, the " Town Ford " and the " Abbey Ford," having been removed since 1843. The shallowness of the channel seems to have been occasioned by the prac tice in vogue from 1732 among the Stirlingshire pro prietors of using the river to carry off the peat from their lands; Mr Drummond of Blair-Drummond, for example, between 1783 and 1839 floated away upwards of 1000 acres of this substance. Further down in the firth the principal obstructions are the Drumsands near Cramond and the Sand-end to the east of Burntisland harbour. The anchorages in the firth are excellent ; and it thus forms the most important harbour of refuge to the north of the Humber. Several of its ports carry on a large foreign trade, more especially Leith, Granton, Bo uess, and Grangemouth. The fisheries of herring, white fish, and salmon are all of great economical importance, not only giving employment to a considerable local population, but attracting English and even foreign fishermen. The traffic between the two sides of the firth has been an object of legislation since 14G7. Both the principal ferries that between North and South Queensferry, and that between Granton and Burnt- island are now in the hands of the North British Railway Company. In connexion with their system they have undertaken to construct a magnificent viaduct across the firth, the foundation stone of which was laid on the 30th of September 1878. Thomas Bouch, the engineer entrusted with the enterprise, has practically revived a plan proposed as early as 1818 by James Anderson. The bridge is to cross a little to the east of the present line of traffic between North and South Queensferry, advantage being taken of the small island of Inchgarvie, which lies about mid channel. Its most striking features will be two spans of 1GOO feet, constructed on the suspension principle, at a height of 150 feet above high water of spring tides. In order to secure sufficient stability against the lateral pres sure of the winds, which sometimes blow with great violence in the firth, these central portions will be split into two branches, each about 14 feet wide, which will lie 100 feet apart, but be bound together by horizontal braces. Both the great spans will be laid on a dead level, but the shore ward sections will have gradients of about one in 77 and one in 80. The total cost is estimated at 1,250,000, and it is expected that the works will be completed in 1882. FORTIFICATION T710RTIFICATION is the art of strengthening, by works J_ of defence, positions in which it is proposed to place troops so as to render them secure from the attack of an enemy. Such positions may contain within them towns, dockyards, arsenals, and ports, or may serve merely for shelter for an army in the field; but the object to be attained is always the same, and the works of defence are so disposed round the position that, while they offer the greatest obstacles to the advance of their assailants, they afford the utmost shelter to their defenders. Fortifica tion is, in short, the art of enabling the weak to resist the strong. Various terms have been adopted in connexion with the general term of Fortification, but none of them are of much use in considering the subject, and some may lead to error, by inducing the engineer to restrict himself under some cir cumstances to a very limited view of his subject. Thus, Fortification Natural and Fortification Artificial imply a use less distinction, as every engineer must avail himself of the natural advantages or obstacles of ground, as well as of the obstacles his science and genius enable him to add to them ; and thus in every Fortification nature and art must act together. Fortification Regular and Fortification Irregular are defective terms, as no Fortification can possibly be regular unless it should so happen that the ground it occupies, as well as the ground surrounding it on all sides, is uniform in level and general character. Again Fortification Permanent and Fortification Field or Temporary have reference only to the immediate object of the works, or to the application of the science, and in no way affect its principles, which remain the same whether the work is a simple earthen intrenchment, or a great for tress surrounded by masonry walls. Fortification Offensive and Fortification Defensive are, however, of all terms the most objectionable, since they imply a contradiction to fact, for the perfection of defence depends as much on its active offensive operations as on the protection of its covering works, while the perfection of attack depends as much on the skill with which its passive protective works are pushed forward as on the fire of its batteries. The principles of Fortification then should be studied un shackled by any of these distinctions, and the engineer should apply his means to his end, using without restriction the works best suited to his purpose ; and it is in this way that the study of the subject will be here treated. ELEMENTARY FORTIFICATION. It is desirable to examine the exact meaning of technical words, in order to acquire a distinct notion of the ideas they were intended to convey, and to obtain a glimpse of the historical progress of the science in which they are used, Fortify, Fortifications, Fortress, Fort, are all derived from fortis, strong; and the idea the first two convey is that, by artificial arrangement, additional strength is bestowed upon one combatant over another, or upon one party of combatants over another party. A rock, the trunk of a tree, a bank of earth, or any natural object, which shelters the body of a man from the missiles of his opponent whilst it leaves him free to discharge his own, may be considered the simplest form of fortification. A bank of earth, when reduced to the requisite thickness, and moulded into proper form, with such slopes as the particular tenacity of the earth may require to insure stability, or which the intended direction of the fire over its summit may render necessary, becomes a Parapet, so called from the Italian words para, a defence or guard, and petto, the breast, or, in English, a breastwork. If the Breast work or Parapet be made only sufficiently high to permit the soldier to fire over it, he will be exposed after firing, and will be forced to crouch in order to obtain cover. The parapet is therefore made high enough to cover the soldier when standing up so that he can load with ease and security. and can move with safety from place to place behind it. This increased height renders it necessary to introduce a Banquette or step (accessible by an easy slope), standing upon which the soldier can fire over the parapet and from which he can retire by the interior slope to the lower ground behind it; the name Banquette is derived from banchetta, a little bench or step. As Parapets are usually formed arti ficially, the earth for their construction is derived from a ditch, which being dug immediately in front of and parallel