descent lamps for offices, stores, fitting-shops, and such like. The latter were also used in the caissons and by divers under water, and proved of immense value in such work. The incandescent lamps were generally of 15 candle-power, but within the last few years, lamps with power up to 500 candles were used for particular purposes.
The arc lights were of 1500 to 2000 candle-power, and though every care was used to obtain the best carbons in the market and to keep the machinery going with the utmost regularity, these lights were always unsatisfactory owing to the changes in the colour of the arc and the illuminating power, the glare and flicker and the black and impenetrable shadows thrown by their own framework or by any intervening object. Generally they were arranged in circuit of six or more, and sometimes as many as twenty-one, and the sudden extinction of so many lights caused by the comparatively trivial fact of a belt slipping or a journal heating, was a source of inconvenience and much danger to the men working out on the erection, for while standing one moment in the dazzling glare of these lights they were sometimes suddenly called upon to use their eyes in absolute darkness or sit still.
Where the electric light came in as a great and lasting boon was in the lighting of the interior of the air chambers and air shafts of the large pneumatic caissons while in the process of sinking. The absence of all smoke or filth due to oil lamps of whatever description, the facility with which they could be moved, changed, put up or taken down, was of incalculable advantage in places where breathing even the purest air was a task of some discomfort and difficulty, and here, therefore, the electric light reigned supreme.
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Bearing of Caissons on Rock Foundations
From the drawbacks mentioned above which attached to the electric arc lights, the Lucigen lamps, which were also introduced at an early period, were entirely free, though their shortcomings cannot be overlooked. There can be no doubt that for general work of erection they are the best for light, the simplest to keep in order, and the easiest to attend to. The disadvantages are that they are difficult to keep clean, as a good deal of the oil escapes unconsumed, especially in high winds, and covers girders and staging with a thick coat of slimy oil, making them slippery and unsafe. It is also difficult to get the creosote oil pure and free from sand, grit, and water, which impurities cause the small passages for the oil to choke, while the last frequently extinguishes the light. On the other hand, it is as easily shifted as an arc lamp, and is not nearly so fragile or apt to be broken.
Large incandescent lamps give a steady and most excellent light to work by, but if covered with a fine wire guard or a second globe the power of the light is seriously impaired, and if left unprotected, the smallest chip of metal, or even of wood, thrown against it, suffices to cause fracture to the globe and extinction of the light.
Except in special cases, where the work is confined to a comparatively small area, the writer has no hesitation in saying that, in the work of outside erection, under the best conditions of lighting, and with every care taken to give the men confidence in getting about their job, the amount of work done at night will not exceed 50 per cent. of that done in daylight. As a consequence of the lighting of the piers at various points and heights, and changing these almost nightly, according to circumstances there arose a great deal of confusion to the shipping going down or coming up the Firth, and it must be conceded that on a dark night, when, for some reason or other, work was not carried on upon one of the main piers, it was difficult at the first glance to know the exact position of all the piers. On such a night, with a slight mist on, the captain of a tug-boat, coming down river with a barque in tow, mistook the lights on the Fife erection for those of Inchgarvie, and steered his ship straight for the hamlet of North Queensferry, which was hidden in the mist. He perceived his error in time to back his boat out and slip the tow-rope, but the barque continued its course and did considerable damage to itself and to the landing jetty. It is true that a glance at his compass would have shown him that a northerly course instead of an easterly could not be right; but it was in consequence of this mishap that it was decided to erect a lighthouse at the north-west corner of Inchgarvie with a revolving light giving five-second flashes. This light is at an elevation of about 30 ft. above high water, and can easily be seen for twelve miles up and down river.
As regards the cost of lighting, it was not easy under the peculiar circumstances of carrying on the work at the bridge to make comparative trials of the lights. At first the balance was much in favour of the Lucigens—the oil costing only 1⁄2d. to 5⁄8d. per gallon, but as the demand rose so did the cost of the oil, and ultimately it was difficult to obtain it at four times the original price.
Electric light installations, consisting of steam-engines and dynamos, as well as air compressors for the Lucigen lights, were put up on the Fife shore, at Inchgarvie, on the Queensferry Jetty, and at the workshops above, the latter being, of course, by far the most important.
During the busiest years the lighting arrangements required a separate department, and a large number of hands were kept going to attend to the lamps, the dynamo machines, the cables, and other connections. The conductors varied in size from 7 strands of No. 18 gauge wire to 19 strands of No. 16 gauge; they were supplied by Messrs. W. T. Glover and Co., of Salford.
Materials of Construction for the Masonry Piers.
It was laid down in the specification to the contract, that all piers, abutments, and arches were to be constructed of masonry consisting of a granite facing, the hearting being of concrete or of rubble masonry. The stipulations as to the quality of the stone, the amount of facing and dressing, and the quality of cement and sand, were of the usual description in contracts of this kind.
Granite.—All the granite for facing—whether rock-faced or dressed—was obtained from Aberdeen, with the exception of the large coping-stones, which weighed between 6 tons and 8½ tons each, and the necking courses immediately below the coping. These were of Cornish granite and were dressed.
The granite is grey in colour generally, and of very handsome appearance, with some slight veins of red granite running through, which, however, rather add than detract from its appearance.
The granite was brought, roughly dressed and squared, in specified courses ranging from 21 in. in thickness down to 16 in., and also specified as headers and stretchers, so as to form proper bond with the hearting of rubble masonry or concrete. Coming by water, the stone could, of course, be delivered at the respective centres at once. The granite coping-stones were 4 ft. 6 in. in depth, not less than 3 ft. in width, and set out alternately as headers and stretchers. The capping—that is, the slightly-curved crown of the piers—was set out after the coping had been built, and the correct measure of every stone was sent to the quarry near Aberdeen, each stone being numbered and marked.
Rubble Work.—For this a very hard flat-bedded and easily-split freestone—in colour from reddish ochre to purple grey—was brought from Arbroath. It was brought in large blocks up to 4 tons each, and in thickness from 3ft. downwards, but it could be split with ease into slabs not more than an inch thick.
Whinstone blocks, roughly levelled on two sides, were also used in rubble work. These were obtained either by quarrying in the open or by the excavations for the piers.
For bond courses in all the viaduct and the cantilever end piers, a hard freestone from quarries in the neighbourhood or extra large whinstone blocks were used.
All the rubble could be obtained at a cheap rate, owing to the inexpensive mode of transport in ships and the facilities of unloading close to the piers.
Concrete.—For making concrete the whinstone found both on Inchgarvie and at North Queensferry was exclusively used. A number of stone-breakers were set up in the last-named locality, where a large quantity of quarry chips were available and close at hand, and the crushers were placed in convenient positions for charging the broken stone into barges or iron skips for transport to the other centres.
The broken metal was passed over screens to obtain the required size for different purposes.
The concrete was mixed dry first, and again after the addition of water, either by hand, or, if required in very large quantities, by a very effective mixer of simple construction.
The concrete used in open foundations, in the caissons and as hearting behind the granite facing, differed somewhat in its composition according to position; but generally speaking the proportion of cement to broken stone was not less than 41⁄2 cubic feet to the yard of stone, nor more than 61⁄2 cubic feet to the yard, an equal amount of sand, or a slight excess over and above, being added. These quantities were considered as making up one yard of concrete. Such concrete had a resistance to crushing of 50 tons per square foot, and it weighed about 37 cwt. to the cubic yard.
Sand.—Some difficulty was experienced in obtaining good sharp sand. It was found in large quantities on both shores of the Firth within half a mile, but objections to its removal being raised by the proprietors of the foreshores, it became necessary to send steam barges down the Firth to Kinghorn and Pettycur, a distance of ten to eleven miles, where the tide lay some banks dry at low water. Owing to the barge having to be grounded
