The Forth Bridge/Tides, Wind, Wind Pressures, and Gauges. Climate Generally

by Wilhelm Westhofen

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.

East winds are prevalent generally in April, May, and June, but sometimes continue right through the summer, but for the remainder of the year, often for many weeks without change, south-west wind keeps in possession.

Fig 5a. Living model illustrating principle of the forth bridge.

On three or four days during the year gales blow with such violence as to stop even large paddle boats from attempting the passage. On many other days the smaller barges and launches have to keep within shelter. During such times all outside work was necessarily stopped owing to the impossibility of handling material by the derrick cranes, or of getting about on the exposed stagings. From twenty-two to twenty-three full working days in the month must be considered very satisfactory in this climate; on many days only an hour or two need have been lost but for heavy rains in the early hours, which drenched the men and sent them to their homes. When such happened no power of persuasion was great enough to bring them back to work again, even if the weather turned fine and continued so for the rest of the day—a curious fact not easy of explanation.

Fig 10. Map of the firth of forth.

Of snow there was but little during the seven winters, and but few days were lost through its covering the ground, but the frost caused much stoppage in a work where hydraulic appliances were so largely used, and where, owing to the enormous extent to which pipe leads had to be carried, it was impossible to effectively protect them all. It was thus the practice to break a number of joints and allow all pipes to drain dry after work stopped at night. Some of those joints were on deck, others on the very top of the structure, and a fire which occurred one night, February 13, 1889, on Inchgarvie spread to an alarming extent, and might have had most serious consequences to the lower portions of the steelwork as well as to the granite piers before the joints could be closed and the pumps made effective. The danger was all the greater that a furious gale was blowing from the south-west, which made it a matter of some danger to get to the island at all, or when there to ascend to the top of the superstructure to find the broken joints.

The variations in temperature are not excessive, and may be said to range between 20 deg. Fahr. and 85 deg. Fahr. minimum and maximum in the shade respectively.

Far greater than storms to the sailing craft passing the bridge is the danger from sudden calms which frequently occur during spring tides when a strong ebb is running, and which cause them to drift about helplessly in the powerful currents. At such times the barges and launches belonging to the works were on the look-out to run to the assistance of any shipping becalmed and tow them into mid-channel. To ward off all craft from the iron staging on Inchgarvie, where they would certainly have come to grief, but where they also might have done considerable injury to the structure in the early days of erection, three timber booms, each boom consisting of three heavy Oregon pine logs, were moored to the west of Inchgarvie. The logs were octagon section, from 2 ft. to 2 ft. 6 in. across, and about 100 ft. long, and were strongly bound together by three heavy iron belts to each boom. Mooring blocks weighing nearly 40 tons each were laid down in the bed of the Firth, some 120 yards west to the booms. On the side of the island heavy iron stakes were fixed in the rock, and to these or else to some of the iron columns, 1+¹⁄₄ in. cable chains were attached. The other ends of these chains were shackled to the pointed ends of large floating buoys, and the same shackles received similar chains coming from the large mooring blocks. On the tops of the buoys were large rings, and to these were attached the chain bridles from the ends of the floating booms, or rather from the iron belts near the ends. Sufficient slack was allowed in all chains for rise and fall of tide. These timber booms have been the means of saving many small boats and sailing schooners from certain shipwreck.

With the calm weather in winter and early spring the sea fogs or eastern haars occur with tolerable frequency, and have caused much anxiety on account of the necessity of having to carry large numbers of workmen—many hundreds every morning and night—from either shore to their destinations. These fogs come up the Firth like a solid wall of dazzlingly white cloud, sometimes leaving the tops of the towers standing out clearly in the sunshine, at other times hanging some 50 ft. to 100 ft. up in the air, and leaving the lower portions quite clear. The effect is well shown in the illustration from a photograph given on Plate XIII.