THE BRITANNIA AND CONWAY TUBULAR BRIDGES. 425 
water was pumped in, the ram rose through its stroke of six 
feet ; lifting the tube to that extent. The chain was then gripped 
by a second series of clips on the massive beams beneath the 
press ; the ram was finally allowed to fall; and having grasped 
the chain six feet lower, began a new stroke. The tower was 
built up beneath the tube as the raising was effected. Thus, by 
slow stages, between the middle of August and October, the 
first tube was raised to its place. Delay having been caused by 
the failure of one of the presses, an accident which threatened 
the destruction of the tube. The other tubes followed. The 
last was floated July 25, 1850; — the first stone of the bridge 
having been laid September 21, 1846. Thus ends our brief 
account of the design, construction, and erection of these 
bridges, and although we can hardly hope that all the technical 
details of the subject have been clearly explained to our read- 
ers, yet we trust that we have imparted to even the most 
popular inquirer some idea of the great difficulties which have 
been overcome, and of the wonderful work which has been 
achieved in the completion of a magnificent work of engineer- 
ing skill, which is the pride of every Englishman, and elicits the 
admiration of every foreigner who visits our shores. 
tons per square inch. The area of the ram in one of the Britannia presses 
was 270 square inches, and the lifting power about 270 X 3 = 810 tons. 
In order to pump in the water two forty-horse power steam-engines were 
employed, working pumps A inch diameter. The area of each pump piston 
was, therefore, 0886 square inch, and the pressure against this at three tons 
per square inch 2'6 tons ; it was against this resistance only that the power 
of the engine was exerted in producing a lifting pressure in the press of 
800 tons. 
Fig. 3 represents an hydraulic press, partly in section, partly in elevation. 
EXPLANATION OF PLATE XXIII. 
Fig. 1. The elliptical tube with a fin, or first cellular experimental tube. 
2. Rectangular experimental tube, with two cells at the top, familiarly 
known as the “ spectacle tube.” 
3. Side view of fractured part of one of the experimental tubes, to show 
the nature of “ buckling,” or yielding to compression. 
4. Elevation of Britannia Bridge. Scale about l-3210th. 
5. Cross section through the middle of one of the Britannia tubes, 
showing celLs, &c. Scale about 1-1 24th. 
C. Upper comer of fig. 5, on a larger scale, showing arrangement of 
plates, angle iron, and covering strips, to form the cells, &c. Scale 
about l-15th. 
7. End view of one of the tubes resting on the pontoons. 
