The Forth Bridge/Lighting

by Wilhelm Westhofen

Lighting.

The efficient lighting of so large a working area, changing every day almost, having not only to be carried forward with the work, but also upward and in all directions, was a task of considerable magnitude, and of the greatest influence upon the rate of progress. It is not so much the lighting of the actual spot where work is proceeding, but that of the approaches to these points, and of the accessory places, such as stores, engine-houses, workshops, jetties, and landing-places, which runs away with so large a proportion of the total illuminating power.

As regards the nature of the lights it was to be expected that a keen rivalry would spring up between the various systems of lighting. South Queensferry has only a small supply of gas, and that at the ruinous rate of 8s. 4d. per 1000 cubic feet; this mode of lighting was therefore out of the race. Electricity was at once resolved upon—arc lamps for the shops, and outdoor work, and incandescent 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.

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.