of the wall were then replaced by an open viaduct, with the piers
placed edgeways to the sea, the openings between them being spanned by
ten cast-iron girders each 42 feet long. This accident induced the
engineer to alter the contour of the sea wall, so that it should present
a diminished resistance to the force of the waves. But the sea repeated
its assaults, and made further havoc with the work; entailing heavy
expenses and a complete reorganisation of the contract. Increased
solidity was then given to the masonry, and the face of the wall
underwent further change. At some points outworks were constructed, and
piles were driven into the beach about 15 feet from the base of the wall,
for the purpose of protecting its foundations and breaking the force of
the waves. The work was at length finished after about three years'
anxious labour; but Mr. Stephenson confessed that if a long tunnel had
been made in the first instance through the solid rock of Penmaen Mawr, a
saving of from 25,000 to 30,000 pounds would have been effected. He also
said he had arrived at the conclusion that in railway works engineers
should endeavour as far as possible to avoid the necessity of contending
with the sea; {324} but if he were ever again compelled to go within its
reach, he would adopt, instead of retaining walls, an open viaduct,
placing all the piers edgeways to the force of the sea, and allowing the
waves to break upon a natural slope of beach. He was ready enough to
admit the errors he had committed in the original design of this work;
but he said he had always gained more information from studying the
causes of failures and endeavouring to surmount them than he had done
from easily-won successes. Whilst many of the latter had been forgotten,
the former were indelibly fixed in his memory.

But by far the greatest difficulty which Robert Stephenson had to
encounter in executing this railway, was in carrying it across the
Straits of Menai and the estuary of the Conway, where, like his
predecessor Telford when forming his high road through North Wales, he
was under the necessity of resorting to new and altogether untried
methods of bridge construction. At Menai the waters of the Irish Sea are
perpetually vibrating along the precipitous shores of the strait; rising
and falling from 20 to 25 feet at each successive tide; the width and
depth of the channel being such as to render it available for navigation
by the largest ships.