girders they were left until the time when the permanent way was required to be laid down. One of the views on Plate VIII. shows the girders at full height.
The Steel.
With the exception of a few hundred tons of cast-iron washers and anchor-plates in the piers, and about 2000 tons of deadweight consisting of cast-iron bricks laid in asphalt which are placed in the ends of the two fixed cantilevers terminating in the cantilever end piers the whole superstructure, from holding-down bolts to the ventilators on the extreme top of the vertical columns, and from the granite arches at one end to those at the other end, is built of steel.
_Fig._67,_Page_35.png)
Fig. 67. Plan of Drill Roads.
_Figs._68_to_71,_Page_35.png)
BOTTOM MEMBER ON DRILL ROAD.
The choice of a material for constructing a bridge of novel design, of extraordinary magnitude, and exposed during erection to the effects of powerful atmospheric disturbances, must have been the subject of much anxious thought and reflection to the engineers. But, in whatever way the decision was arrived at, there can be no two opinions that the choice was a happy one. From beginning—and probably a long time before the beginning of this work—to the end, this steel was subjected to every conceivable test, both in a properly scientific manner for purposes of research or investigation, and in an entirely unscientific manner by workmen, whose only excuse can be that they did not know better. But in all cases the steel stood the test, and a more uniform, a more homogeneous, and more satisfactory material could not be wished for. To only quote one instance, the writer has in his possession several pieces of scrap from the shearing-machine, picked up promiscuously and placed under an ordinary diamond-headed drill about 1 in. in diameter. A hole was drilled about 3⁄4 in deep, and the machine stopped while yet the feed was on, the result being one single corkscrew shaving, about one yard in length, started from the very moment the drill touched the steel and attached yet by the end to the piece of scrap out of which it was bored. It would not, probably, be straining a fact to assume that this behaviour of the steel under severe tests had a great deal to do with the confidence with which the workmen regarded every portion of the structure, and with their belief that no possible load they could pile on the temporary platforms could by any chance bring about a collapse. It is true that, in the early days of plate-bending, some thick plates broke near the edges in a seemingly mysterious manner, but the investigations made and the reasons adduced in connection with these fractures were sufficiently convincing to allay any feeling of distrust.
The Board of Trade stipulations in regard to steel for structures, do not go further than to lay down the rule that the maximum working stress should not exceed one-fourth of the ultimate breaking strain of the steel. No difference is made between the tensile and compressive stresses, nor is any regard paid to the differences between stresses due to dead load or live load alone or in combination nor to the circumstances arising from changes, occurring frequently or rarely, in the nature of the stresses.
The engineers therefore laid down, after consultation with the Board of Trade and with their approval, certain rules in regard to the stresses admissible under varying circumstances.
For tension members the steel was to have an ultimate resistance of not less than 30 nor more than 33 tons per square inch, with an elongation in 8 in. of at least 20 per cent. For compression members a resistance not less than 34 tons nor more than 37 tons per square inch, with 17 per cent. elongation.
With regard to varying stresses for a load varying between nil and a maximum, 20 tons per square inch of section to be assumed as the ultimate
