Sept. 3, 1885] 
NATURE 
431 
At Jnchgarvie similar pneumatic caissons are used for two 
out of the four cylindrical piers, and the work on both is in full 
progress. Owing to the slope of the rock bottom, it is necessary 
to cut away as much as 18 feet in thickness of whinstone rock 
to form a level bench for the pier at 70 feet below high water, 
and the most convenient way of doing this was to convert the 
base of the pier practically into a great diving-bell 70 feet in 
diameter. In this case, there being no silt over the rock, the 
pressure of air necessarily is that due to the depth of water out- 
side, and somewhat sensational ‘‘blows” occur with a falling 
tile. Rock drills are provided, and blasting goes on in the 
compressed-air chamber without necessitating the withdrawal of 
the men. 
At North Queensferry, the four main piers were built either 
on dry land or within timber and clay cofferdams. Above low 
water the whole of the main piers are built of Arbroath masonry 
in cement faced with Aberdeen granite, and hooped occasionally 
with 18 inches wrought-iron bands. The cantilever end piers, 
and the viaduct piers, are built of rubble, concrete, and granite 
in cement. 
Superstructure.—Although the piers of the Forth Bridge 
present many points of interest, it is the enormous span and 
novel design of the superstructure that has attracted the attention 
of the engineers of the world to the work now in progress at 
Queensferry. The chief desiderata in the biggest railway bridge 
ever proposed to be constructed are durability, strength, and 
rigidity under express trains and hurricane pressures ; facility 
and security of erection, high quality of material and workman- 
ship, and economy in first cost and maintenance. These, we 
considered, would be best met by a steel ‘‘cantilever’’ or 
‘continuous girder” bridge. Since the commencement of the 
Forth Bridge, American engineers, ever bold and ready, have 
built three cantilever bridges of considerable spans, and practical 
experience has confirmed our anticipations as to the advantages 
of the system; the Niagara Bridge, over 900 feet in length, 
which was manufactured and erected across the rapids in the 
short time of ten months, having stood all the tests of actual 
working in the most satisfactory manner. 
In the Forth Bridge, each span of 1710 feet is made up of 
two cantilevers, projecting 680 feet, and a central girder con- 
necting the same, 350 feet in length. The cantilevers are 343 
feet deep over the piers, and 4o feet at the ends. The bottom 
members consist of a pair of tubes tapering in diameter from 
12 feet {o 5 feet, and spaced 120 feet apart, centre to centre, at 
the piers, and 31 feet 6 inches apart at the ends. 
The top members consist of a pair of box lattice girders, taper- 
ing in depth from 12 feet to 5 feet, and spaced 33 feet apart at 
the piers, and 22 feet 3 inches at the ends. Each tube has a 
maximum gross sectional area of 830 square inches, and each 
girder a maximum net sectional area of 506 square inches. 
Upon each cylindrical masonry pier is bolted a bed-plate carry- 
ing a ‘‘skewback,” from which spring vertical and diagonal 
columns and struts. The former are 12 feet in diameter, and 
from 368 to 468 square inches sectional area; the latter are 
flattened tubes. Horizontal wind-bracing of lattice girders con- 
nect the tubes forming the bottom member of the cantilevers, 
and similar vertical wind-bracing connects the vertical and 
diagonal tubes, so that the whole structure is a network of 
bracing capable of resisting stresses in any direction and of any 
attainable severity. 
The rolling load provided for is (t) trains of unlimited length 
on each line of rails weighing 1 ton per foot run ; (2) trains on 
each line made up of two engines and tenders, weighing in all 
142 tons, at the head of a train of 60 short coal-trucks of 15 tons 
each. The wind provided for is a pressure of 56 lbs. per square 
foot, striking the whole, or any part of the bridge, at any angle 
with the horizon, the total amount on the main spans being 
estimated at no less than 7900 tons. In practice only two trains, 
weighing 800 tons in all, would be on this length of bridge at the 
same time, so the wind pressure (if such a hurricane as 56 lbs. 
per square foot could ever occur) would be ten times as great as 
the train load. Under the combined stresses resulting from the 
test load in the worst position, and the heaviest hurricane, the 
maximum stress on the steel will not exceed 74 tons per square 
inch on any portion of the structure, and on members subject 
to great variation in the intensity and character of stress, the 
maximum will not exceed 4 tons per square inch. For tubular 
columns and struts 34 to 37 ton steel, with an elongation of 17 
per cent. in 8 inches, is specified, and for tension members 30 to 
33 ton steel, with 20 per cent. of elongation. We have now 
about 15,000 tons of steel delivered and worked up, and are 
satisfied that the quality as supplied to us by the Steel Company 
of Scotland and the Landore Company is admirably adapted for 
bridge construction. In making the tubes the plates are heated 
in a gas furnace and bent hot between dies in a powerful 
hydraulic press. A slight distortion takes place in cooling, 
which is corrected by pressing the plates again when cold. After 
bending, all four edges are planed and the plates built up intoatube. 
Travelling annular drill frames surrounding the tube, fitted each 
with ten traversing drills, bore the holes at once through plates, 
covers, and stiffeners, so that when again fitted in place for erec- 
tion every piece comes into exact juxtaposition. Similar 
travelling drill frames deal with the lattice box-girders, every 
hole being drilled as the machine advances. Generally the 
plant designed by Mr. Arrol for drilling the innumerable holes 
in the 42,000 tons of steel-work for the main spans is of signal 
merit and efficiency, and well worthy the attention of practical 
engineers. 
At the present time, although, as already stated, about 15,000 
tons of steel-work is on the ground, only the approach viaduct 
girders and some of the bed-plates of the main spans are erected 
and rivetted up. Ina few weeks, however, the erection of the 
portion of the main spans over the North Queensferry piers will 
be proceeded with. The ‘‘skewbacks” and connecting tube 
will first be rivetted up, and then a platform of temporary 
girders and planking will be constructed, and raised gradually 
by hydraulic rams in the four vertical 12-foot diameter columns 
as the work of erection and rivetting-up progresses. This plat- 
form will carry cranes and other appliances, and the men will be 
thoroughly protected, so that work may be carried on with as 
much confidence at a height of 350 feet as at sea-level. When 
the portion of steel-work over the piers is erected, the first bay 
of cantilever on each side of the same will be added, the work 
forming its own staging. This will be followed by succeeding 
bays until the cantilevers are complete, and the central girders 
will then be erected, probably on the same plan. 
It will be observed that for certain parts of the Forth Bridge 
we use steel of a higher tensile strength than is at present con- 
sidered admissible either for ships or boilers. This has not been 
done without full and mature consideration of the whole ques- 
tion. Our experiments showed that steel, having a tensile 
strength of from 34 to 37 tons per square inch, ‘offered a decided 
advantage over very mild steel, when compressive stresses and 
the flexure of long columns were concerned. Indeed, an inferior 
quality of steel, such as would be used for rails, will stand com- 
pression far better than the best boiler steel or Lowmoor iron. 
Thus, I found a column twenty diameters in length of common 
Bessemer steel carry 27 tons per square inch, where one of mild 
boiler steel has stood but 17 tons. It would be inexpedient, 
however, to use inferior steel, even for the compressive members 
of a bridge, and therefore a high quality and high tensile resist- 
ance were indicated. Although this steel takes a temper and 
becomes brittle if cooled in certain ways, it will stand the ordin- 
ary Admiralty temper tests, bending to a radius of double the 
thickness, after being made red-hot and cooled in the usual way. 
In a boiler the steel plates are subject to great changes of 
temperature and consequent stresses from expansion and con- 
traction. Ina ship almost every plate in the hull is subject to 
alternate tensile and compressive stresses when amongst waves ; 
and, further, a vessel is liable to severe alternating stresses and 
shocks on taking ground, dry docking, and under other cireum- 
stances. In the compression members of the Forth Bridge 
the steel is subject only to a steady pressure of varying 
intensity, and a quality of steel was adopted which combined 
perfect facility in working with a high resistance to compression. 
Although an increased tensile strength is accompanied by a 
decidedly increased resistance to flexure in columns and struts, 
the latter is not proportional to the former. If the thing were 
practicable, what I should choose as the material for the com- 
pression members of a bridge would be 34- to 37-ton steel, 
which had been previously squeezed endwise in the direction 
of the stress to a pressure of about 45 tons per square inch 
—the steel plates being held in suitable frames to prevent 
distortion. 
My experiments have proved that 37-ton steel so treated will 
carry as a column as much load as 70-ton steel in the state in 
which it leaves the rolls, that is to say, not previously pressed 
endwise. It would be a matter of much practical moment to 
ascertain if some conyenient treatment could be devised which 
would endow steel with this greatly increased power of resistance 
