THE BRITANNIA AND CONWAY TUBULAR BRIDGES. 423 
The annexed figures illustrate the nature of the riveted 
joints of the bridges. Fig. 1 is a rivet with one head formed 
ready for use. This is put through two corresponding holes 
in the plates to be united, and a second head formed by 
hammering. A section of such a joint is shown in fig. 2. 
Fig. 2. 
Fig. 1. 
In the Britannia Bridge the four land-tubes were erected in 
their permanent position on scaffolding of great strength. The 
four larger tubes, each 472 feet long, were built on the Carnar- 
von shore of the straits, on similar stages to those employed at 
Conway; workshops adjoined to them on the land, and behind 
these were cottages for 500 workmen. 
V. Floating and Elevation of the Tubes . — We have seen that 
the original intention of throwing suspension chains across 
the straits to form a platform on which to erect the girders 
was abandoned. A simpler, less expensive, and more efficient 
plan was devised, that, namely, of building them on the 
shore, where pontoons could be brought beneath them at low 
water, so as to float them with the rising of the tide ; thence 
to guide them safely about 500 yards as they were impelled 
by the current, and to drop them, as the tide again fell, on 
masonry shelves at the bottom of the towers. A description 
of the manner in which this was effected for one of the main 
Britannia girders will suffice for all. The preliminary arrange- 
ments comprised the attachment of two huge twelve-inch 
hempen cables near the tube, and extending them across the 
straits to capstans on the Anglesea shore. These guide-cables 
passed through hawse-holes in the outermost pontoons, and 
guided the ends of the tube. Immensely powerful friction- 
breaks were placed in the pontoons on each of these guide- 
cables, by which the motion of the tube could be retarded in 
any required degree, as it was feared that, left to the current, 
it would become unmanageable. To haul the tube out of its 
position in-shore two eight-inch cables were extended across 
the straits from moorings on the Anglesea shore to capstans 
on the pontoons. Lastly, four other capstans were placed 
2 g 2 
