ahead, or afforded every means of suspending temporary staging from it. The greater portion of the work as erected could be securely fixed at once and rivetted up, and this close up to places where new parts were in course of erection. Great rigidity was thereby insured, and less temporary work required than in any other mode of construction, while it gave confidence to the workmen engaged, and offered every facility in providing for their safety, and for that of the structure itself.
Great stability is obtained by straddling the sides of the structure, as viewed in cross-section that is, making it considerably wider at the base than at the top. In the central towers, the width at the base is somewhat more than one-third of the height, and a uniform batter is maintained throughout the structure. This feature conveys a sense of great security against the action of violent gales tending to overturn the bridge. Finally, the arrangement of cantilevers and central girders admits of the simplest and most effective form of expansion-joint, and this problem is solved here in the happiest manner, as will appear from the detailed description given later on.
On December 21, 1882, the contract for the construction of the Forth Bridge was let to the firm of Tancred, Arrol and Co. Both the contract sum and the time specified, have been exceeded, for reasons which will be fully apparent to the reader who follows attentively the development of this work, and who gives intelligent consideration to the conditions under which it had to be carried out.
Site of Bridge and Profile on Centre Line.
Surrounding Country.
The site which was finally fixed upon as the most suitable in all respects for carrying the bridge across, is in its natural features singularly well adapted for that purpose. The general level of the country on both sides of the Firth lies at about the height at which it was required to carry the rails in order to afford sufficient headroom for the largest vessels in the Navy or merchant service. Should the project of making a ship canal between the Forth and Clyde lately brought to the notice of the public ever be carried into execution, and should the Forth thus become an international highway, as well as one of the finest natural harbours of refuge in the world, the largest vessel yet built would have to do no more than strike its topmasts should it happen to pass at the hour of high water of an extra high spring tide. The river bottom, in all places where the foundations of piers had to be laid, consists of either the hard whinstone rock or of hard boulder clay, both of great soundness and solidity. At no point where foundations had to be placed is there a greater depth of water than that well within the capabilities of sinking caissons by pneumatic process. The only contraction in nearly 50 miles of river is to be found here, and it reduces the width, elsewhere never less than fully two miles to one mile and about 150 yards. (See Fig. 1C. ) On the North or Fife shore, a rocky promontory, somewhat triangular in shape, projects southwards for fully a mile and a quarter into the river, and affords not only sound building material in its bedrock, but also sheltered corners for discharging vessels and anchorages for a small fleet of barges and launches connected with the works. From the apex of this promontory, nearly due south, the small island of Inchgarvie lies distant exactly one-third of a mile, and between the two runs the Main or North Channel with a depth of over 200 ft. This is the channel almost exclusively used by shipping, because it is safer and easier to navigate than the South Channel, and also because it is the shorter road of the two. Inchgarvie is a peak of basaltic trap-rock or whinstone, and lies to the east side of the centre line of the bridge. It rises abruptly from the bed of the Firth except at its western extremity, where it widens into a broad toe with a tolerably regular slope to S.W. of about 1 in 7. On the south side of Inchgarvie lies the South Channel, which is of about the same depth and width as the North Channel. Its southern edge, however, lies still about 30 ft. under water, and from it to the South or Queensferry shore there is a distance of 2000 ft., one-fourth of which becomes uncovered at low water.
In the South Channel the whinstone rock disappears about the centre, being from that point forward overlaid by a bed of very hard boulder clay of great thickness, this being in turn overlaid by about 40 ft. of a softer clay, gravel, silt, and soft mud. The ground rises from the edge of the deep water channel towards the shore with a gentle slope, the clay disappearing about half-way up, and giving place to ledges of freestone rock. It will thus be seen that the natural configuration of the ground on the centre line of the bridge, offers points or three main supports, of which the two outer ones are about equidistant from the central one, thus indicating two large spans of equal length, while the remaining spaces to be traversed on both shores, offer every facility for the construction of satisfactory foundations. As a matter of fact there has been no single instance of the ground on which the foundations were placed being uncertain or in any way doubtful, and no anxiety need be felt in regard to this part of the Forth Bridge.
Indications of the nature of the ground on the centre line of bridge are given in Fig. 2 on Plate III. A general view across the river is shown in Plate V., from a photograph taken on September 11, 1883. The latter view shows the Hawes Inn and garden in immediate foreground, also the Hawes Pier, the Queensferry jetty, with the half tide cofferdam of No. 6 pier, and the commencement of the large cofferdam for the South cantilever end pier. It also shows the commencement of staging on Inchgarvie, and the same on Fife, with the new coastguard houses on the first elevation to the right.
The country immediately surrounding the site upon which the bridge now stands is strikingly beautiful. Whatever opinion may be held in regard to the lines of the bridge itself, it must be conceded that this bridge or any other bridge must be a discordant feature in a pastoral landscape. Standing on Mons Hill in Dalmeny Park, and looking down over its thickly wooded slopes into the broad expanse of the Forth, with the island of Inchgarvie and its old castle breasting the swift current and cutting it into two arms, which below it, unite again in a whirlpool glittering in all the colours of the rainbow, the whole backed by the Fifeshire Hills, the Ochills, and the great peaks in Dumbarton, Stirling, and Perthshire, is a view hard to be excelled in any part of the world. Hardly less fine and perhaps more grand still is the view down the estuary into the limitless ocean, from the grounds round Hopetoun House.
In the last case the horizon falls in with the line of the rails of the internal viaduct, and thus shuts out all view most completely, while the lines of the bridge itself in geometrical repetition with severe regularity of triangles and squares, cannot be made to harmonise in the least degree with the soft and undulating lines of the adjoining landscape. Thus the best view of the landscape is from the bridge, because the disturbing element is left out, while by far the best view of the bridge is obtained from the river, whether above or below, at a distance of a mile or so, the structure rearing itself to a great height, and being backed only by the sky. Thus viewed, its simple lines, its well-proportioned parts, its impressive air of strength and solidity and yet of lightness and grace, never fail to strike the mind of the beholder. Four-square to the wind and immovable it stands!
The view from the summit of the central tower on a clear day is magnificent. The broad river itself, with craft of all sorts and sizes, in steam or under sail, running before the wind, cutting across the current on the tack, or lazily drifting with the tide, is always a most impressive spectacle upon which one can gaze for hours with an admiring and untiring eye. And such it is, whether viewed in the glory of sunrise or sunset, in broad daylight with the cloud shadows flying over the surface, and a thousand ripples reflecting the sun's rays in every conceivable shade of colour, or in the soft haze of a moonlight night. The sunsets in summer are always magnificent, whether due to Krakatoan volcanic dust or to the vapours of the distant Atlantic, but there have also been many sunrises in early autumn when a hungry man could forget the hour of breakfast, and one could not find the heart to chide the worker who would lay down his tools to gaze into the bewildering masses of colour surrounding the rising light of day. An unbounded view more than 50 miles up and down river! Far away to east the May Island, often so clearly defined, though 35 miles distant, that the sun-lit cliffs are clearly visible, the Bass Rock and North Berwick Law, and the coast line of Haddingtonshire with the Lammermuir range fading into the sky, nearer Inchkeith with the white walls of the coastguard station and the lighthouse, Inchmickry and Cramond Island, the long jetties of Leith Harbour and the shorter of Granton and Newhaven, the roads full of shipping, the masses of houses in the marine suburbs of Edinburgh, Arthur's Seat and Corstorphine Tower just peeping over Mons Hill and the woods of Dalmeny Park. To the south, the fertile districts of the Lothians gradually rising to the imposing range of the Pentland Hills, and to south-west Dundas Hill and Castle, Hopetoun House, and the old palace and church of Linlithgow, the harbours of Bo'ness and Grangemouth and the Campsie Hills closing in the upper Firth, still many miles wide with beautifully wooded shores, and many towns and villages upon its banks. Nearly 60 miles to the west, as the crow flies, stands the massive cone of Ben Lomond, and behind it a formidable array of hills and mountains, clothed in the summer-time in the tenderest shades of purple and blue, in the winter showing forth boldly in a coat of purest snow. In the north-west appear the Ochills, in the north and north-east the Fife Lomonds and the beautiful coast of Fife running down into the horizon, where, glancing over the old priory on Inchcolm, the eye catches the May Island again.
At night too a sight is presented not easily forgotten; the flashing lights of the May and of Inchkeith, and many others stationary, such as the harbour lights of Granton, Leith, Newhaven, and Burntisland combine to form a beautiful picture. At times of continued east wind, when large and small craft run for shelter in the Firth, it is not unusual to see from 150 to 200 vessels anchored in the roads, and the long straggling lines of their masthead lights give the appearance of a busy town of many streets having suddenly risen from the waters. On Jubilee night (21st June, 1887), although the atmosphere was somewhat thick, 68 bonfires could be counted at one time on the surrounding hills and isolated points, while the great masses of the central towers of the bridge lighted up by hundreds of electric arc lights Lucigen and other lamps at various heights where work was carried on, formed, with their long-drawn reflections in the waters of the Firth, three pillars of fire, and afforded a truly wonderful and unique spectacle.
Tides, Wind, Wind Pressures, and Gauges.
Climate Generally.
The tidal rise at Queensferry, that is, the difference between high water and low water during ordinary spring tides, is 18ft., rising occasionally to 21 ft. and even 22 ft. Owing to the contraction in the river, already spoken of, the velocity of the tide flow is considerable, more especially so in the North Channel. The strong currents running to each side of Inchgarvie have given a good deal of trouble, both during the erection of the extensive iron girder staging between the four main supports, and between them and the rock, and during the founding of the piers. Still more was this difficulty felt during the erection of the Inchgarvie north cantilever, when it was necessary to lift all material out of steam barges up to the structure direct, and when the combined influences of tide flow, set of current and wind, made it next to impossible to keep the barges in place for a sufficient time to allow the lifting tackle to be attached even with a most skilful and experienced skipper at the helm.
The only other drawback due to tidal action, was due also to the want of proper pier accommodation upon all three points. Until the timber stages and jetties were built, none of the landing places could be approached at low water except by small boats, and there was consequently a grievous waste of time from that cause in the early days.
The prevailing winds are from the S.W. and tho highest pressures recorded upon the wind gauges have invariably proceeded from that quarter; next in point of frequency occurs an E.N.E. wind, which brings up heavy seas from the German Ocean, and which is as unpleasant to the senses and as trying to the temper as the proverbial east wind in London. From the N.W. come occasional blasts which have the effect of completely clearing the atmosphere, so that the most distant mountains show with considerable distinctness their every form and detail. S.E. winds bring rain and dirty weather invariably, and are fortunately not of frequent occurrence. It is a curious fact that while in spring and summer the east wind brings with it an icy chill, while the west winds are warm and genial, the latter in the winter time bring whatever frosty weather comes to pass, which is immediately broken up into thaw by a change in the wind to east.
