Page:Forth Bridge (1890).djvu/61

Rh although the bottom members might at the end of the first junctions be the proper distance apart from each other, yet the centre of an imaginary line between them might not coincide with the centre line of the bridge, but fall to one side or the other. Precisely the same thing might happen in the case of the inclined struts, and the top members thus lie to one side or the other of the true centre line.

Nor was the wind the only factor that could produce such a result, for the fact whether the sun was shining on the east side or the west side of the bridge made a material difference. On the sun appearing the plates on that side of the tubular members on which its rays fell would expand, while on the other sides the plates remained as they were. This produced, therefore, a bend away from the sun, to an extent not inconsiderable, but, of course, only temporary in its action; for, with the sun turning to south and passing to west, the members not only became straightened again, but became curved in the opposite direction.

Errors in measurements and faults in construction might, of course, also produce similar and permanent results, and it was therefore necessary to take steps to fix the position so far as it could be done.

The diagonal wind-bracings between bottom members, of which the first pair starts from the skewbacks, now required to be dealt with.

As neither bottom members nor inclined struts had ever been laid together in the relative position, the wind-bracings could not be built completely, but the last or closing lengths outwards were always left to be measured and templeted in situ. The gussets by which they were attached to the bottom members were already fixed and rivetted to the latter, and it remained, therefore, only to erect the girders in their places and take the templets for the closing ends. While they were being erected they were hung by wire ropes to the internal viaduct. Previous to this, however, the exact position of the bottom members had to be ascertained, and, if not correct, wire rope ties attached between bottom members and girders by means of union screws or timber struts, worked with wedges or small hydraulic rams, had to be employed to draw the bottom members in or push them out, according to requirements. This done, the templets of the booms were taken and the girders completed. The same process had to be repeated at every point of attachment, that is, every half-bay out. The position of the inclined struts with regard to the centre line of the bridge was corrected in a precisely similar way, and by this means was achieved the successful building out of these arms, nearly 700 ft. in length and weighing some 5400 tons, with an error of only 2 in. For an error there is, and, curiously, it exists in nearly all the six cantilevers, and in the same direction, namely, to east. Whether this is due to the prevailing westerly winds, and the fact that the total pressure from the west upon the structure during a twelvemonth must be probably fifty times as great as the total from the east, or whether the fact that the lateral deflections due to the sun's rays are always so much greater from the west than from the east has something to do with it, the writer will not take upon himself to say, but appearances decidedly point in that direction.

With the corrections in the positions of the struts and bottom members, the first bays of the cantilevers were now completed, and, except in so far as the elasticity of the steel came into play, all the points were as fixed as if each was resting on a solid masonry pier.

The duties of the survey department in connection with the erection of the cantilevers were of the heaviest kind. The work had to be carried on in the most exposed positions and in all weathers. To Mr. W. N. Bakewell belongs the credit of a great achievement, and it is not too much to say that to his courage and decision and promptitude in fixing points, is due the saving of much time and much expenditure.

From the first bottom junctions struts 2 were started upwards, and from the first top junctions ties 2 were started downwards, in repetition of the operations already gone through in the first bay. The bottom members were built out and the internal viaduct brought forward. Upon the latter were now erected—upon a staging sliding on the rail troughs—two steam cranes with movable derricks standing side by side, each being worked by a steam winch placed some distance behind. Wire ropes were exclusively used by these cranes, which, owing to their position, were called the twins. As the Jubilee crane, resting on the top members, built all the work from near the viaduct level