420 
POPULAR SCIENCE REVIEW. 
balanced. Tliese experiments finally determined the propor- 
tions of the bridges. The progress achieved by the experi- 
ments is shown by the following figures. 
Comparative Strength of Tubes of the same weight, depth, and span. 
Cylindrical 130 
Elliptical ... ... ... 15’3 
Rectangular ... ... ... ... 21‘5 
Model tube uith cells ... ... ... ... ... 26'7 
That is, the strength of the cylindrical tubes was more than 
doubled, with a better distribution of material in a rectangular 
form and with a cellular top. 
III. The General Form of the Bridges . — The bridges ulti- 
mately erected consist of huge tubes or tunnels of wrought iron, 
spanning the water horizontally from pier to pier, and carrying 
a single line of rails within each, as shown in Plate XXIII., fig. 
4, which is an elevation of the Britannia Bridge, and in fig. 5, 
which is a section across one tube at the centre. They are 
composed of rolled plates with longitudinal L iron bars, in 
each of the corners, and vertical T iron pillars or struts* to 
stiffen the sides, the whole being united by riveting. This 
construction will be explained by fig. 6, which is a section at 
one corner of the tube (fig. 5) on an enlarged scale. No 
auxiliary chains were used in the erection of the bridges, part 
of the tubes being built on scaffolding and the remainder on 
stages on the shore, whence, when completed, they were floated 
into their proper position and raised by Bramah presses. 
The Conway Bridge has a single span of 400 feet, crossing 
the river close under the walls of Conway Castle, at a height 
of only seventeen feet above high water. Each of the tubes 
weighs 1,200 tons. 
The Britannia Bridge is a much larger structure, having 
four spans, an elevation of which is shown in Plate XXIII., fig. 
4. The entire length of each tube is 1,511 feet, and the length 
of the bridge, inclusive of the abutments, 1,841 feet, or 
more than a third of a mile. At the point selected for the 
bridge, the Britannia rock, uncovered at low water, divides the 
straits into two approximately equal portions (see fig. 4), on 
this the central, or Britannia Tower, was erected. On either 
side, at a distance of 460 feet clear span, two smaller towers 
were constructed on the strand ; and again, at 230 feet distance 
from these, the Anglesea and Carnarvon abutments, each 176 
feet in length. The towers are of very simple but massive 
form, suiting the simple outline of the stupendous structure 
they sustain. The Britannia Tower is 221 feet in height and 
50 feet by 60 feet at the base. It contains 25,000 tons of 
# The parts of a framed structure under compression, 
