THE BRITANNIA AND CONWAY TUBULAR BRIDGES. 421 
masonry and brickwork, and 479 tons of iron built in. The 
side towers are almost as large, rising 1774 feet from tbe 
plinth. Tbe abutments each 176 feet in length are 874 feet in 
height on the Carnarvon side, and 143 on the Anglesea side. 
They are terminated by projecting pedestals, on which are 
placed colossal couchant lions, weighing 30 tons. The entire 
amount of masonry in the bridge is one million and a half of 
cubic feet or 100,000 tons, which it is calculated was built up 
continuously for twelve hours a day, at the rate of three cubic 
feet per minute. 
The two tubes are now continuous over all the spans, the 
greatest depth being thirty feet in the Britannia tower, tapering 
down in a parabolic curve to twenty-three feet at the ends. 
The large sections for the spans of 460 feet have eight cells 
at the top and six at the bottom, as shown in section, Plate 
XXIII, fig. 5. The general dimensions are as follows : — 
Top 
Sides 
Bottom 
Weight. 
Tons. 
1481 
1727 
1472 
Sectional area in centre. 
Square inches. 
648-25 
302-00 
585-43 
The two tubes in their present state, 1511 feet in length, 
contain, 9360 tons of wrought iron, 1015 of cast iron, and 165 
tons of permanent way. They are estimated to have been con- 
structed of 186,000 separate plates and bars united by upwards 
of two millions of rivets. 
In so great a length of iron the effects of change of tempera- 
ture on the length of the tube is very considerable. In fact, 
taking the extreme variation of temperature in our climate to 
be — 5° and + 115° Fahr., or to range over 120°, the variation 
of the length of the tube would amount to fifteen inches. 
To provide for this the tubes are firmly fixed on immoveable 
bed-plates within the Britannia tower, whilst above they are 
also attached to suspension-bars firmly connected with the 
masonry. But in the side towers and abutments they are free 
to move longitudinally on an arrangement of cast-iron rollers. 
Above there are suspension rods, which are attached to girders, 
also rolling freely on gun-metal balls. With these arrange- 
ments the tube is in unceasing motion, the diurnal variation 
of length amounting to from half an inch to three inches. 
Another curious effect is that resulting from the unequal 
heating of the sides of the tubes. They become convex towards 
the sun from the elongation of the warmer side. Hence, as 
the sun moves across the zenith they revolve in a plane at right 
angles to their length in an irregular curve which varies with 
the weather, and shows the effect of the slightest irregularities 
of temperature — from passing clouds, for example. 
no. iv. 2 G 
