ON THE FORTH BRIDGE WORKS. 875 
The first part of the superstructure is that termed the lower bedplate. 
Several of these are now completed and in position. They are made up 
of a series of longitudinal and transverse plates, securely riveted together, 
and run about 37 feet long by 17’-8’’ wide, with a thickness of from 3 to 
4 inches. The whole plate is bolted on a number of short iron columns 
in situ, and is riveted up by a special hydraulic machine. Two girders 
are employed, one above and the other below the bedplate; and extend- 
ing beyond it are there joined together. On each of these girders slides 
a hydraulic cylinder, one having a little more effective area than the 
other, while both are regulated by the same cock. The result is that 
when water is admitted the total pressure on one cylinder is greater than 
that on the other, thereby holding the rivet-head firmly in place while 
the point is being pressed up. The work thus produced is of the very 
highest quality; since the whole machine moves lengthwise, and the 
cylinders slide crosswise, the full surface of the plate is commanded by 
it. The riveting is also done expeditiously, the machine being capable, 
in ordinary work, of closing during a single shift 600 11” countersunk 
rivets. When finished, the bedplate is finally lowered into position. 
The upper bedplate or base, on which the various connections at the 
foot of the rising column rest (and which collectively constitute what is 
termed the skewback), is preposed to be rivéted in a like manner to the 
lower bedplate. While being riveted it will be secured to heavy steel 
girders instead of columns, as in the case of the lower bedplate, to keep 
it in true form. After lowering the upper bedplate into position, the 
diaphragms and various other parts will then-‘be built on it, and riveted 
up by common hydraulic machines as well’ as by the special hydraulic 
machines designed by Mr. Arrol for the purpose. As many of the spaces 
in which riveting has to be done are very confined and difficult of access, 
high pressures will be used with machines correspondingly small. Thus 
while the ordinary pressure will still be 1,000 lbs. per square inch, it 
will be increased in some cases to as high as 3 tons per square inch, by a 
simple pressure multiplier, wrought by the ordinary 1,000 lbs. pressure. 
This low pressure is admitied to the large end of the compressing ram, 
the smaller end of which produces the increased pressure (proportional to 
the difference in areas) required to close up the rivet properly. 
The riveting machine is very small, each cylinder weighing about halfa 
hundredweight. The smallest proposed cylinder is only 4 inches diameter, 
is of the simplest form, and contains a hollow plunger provided with a single 
cup leather at the inner end. A spring is secured to the plunger and 
back end of the cylinder, for the purpose of drawing back the plunger, 
when the exhaust water is allowed to escape. Whenin place and at work 
the machine will be hung to the one end of a small wire, passing over a 
pulley, while at the other will be fixed a balance weight to relieve the 
operator of the weight of the machine. Two cylinders, one outside and 
one inside, will be required at the closing up of the rivets. Both willbe 
connected to the compressor and wrought by it. 
The horizontal tubes, skewbacks, and lower parts of all the columns 
will be built by ordinary cranes till they attain a height of about 30 feet 
above the bedplates. At this point of the 12-foot rising columns will then 
be commenced the longitudinal channels (through which are drilled the 
holes for the steel pins to pass through them and the cross girders) ; to 
these channels the cross girders will now be attached within the column, 
on the higher of which will be laid the two main lifting girders of the 
