416 
THE BRITANNIA AND CONWAY TUBULAR 
BRIDGES. 
BY WILLIAM CAWTHORNE UNWIN, B. SC. 
HPHE Cliestei* ancl Holyhead Railway, planned with a view 
JL to facilitate the journey from Dublin to London by short- 
ening the sea voyage, is one of the most remarkable engineering- 
achievements of this eminently mechanical age, whether we 
regard the importance of a safe and rapid communication be- 
tween the capitals of this country and the sister isle, or the 
magnitude of the difficulties overcome in accomplishing it. 
And the bridges over the Conway River and the Menai Straits, 
the boldest of the works on the line, derive a special interest 
from the novelty of the scientific principles discovered and 
apphed in their construction. 
Three great names cluster about their history, names of men 
who had already achieved a foremost rank, each in his own 
department, and who, as they shared in the labour, must divide 
the honour of success. Robert Stephenson, the engineer of 
the line, had first to cope with the problem of constructing 
bridges of spans never before attempted in railway structures, 
and under embarrassing conditions imposed by the Lords of 
the Admiralty, to preserve unimpaired the navigation of the 
Straits. To him belongs the merit of the first conception and 
courageous carrying out of plans which were elaborated chiefly 
by Mr. William Fairbairn, to whom was intrusted an experi- 
mental investigation of the laws of resistance of wrought-iron 
structures. Harassed by parliamentary business in the exciting- 
period of the railway mania of 1845-6, Mr. Stephenson had not 
the leisure to mature his own project, and Mr. Fairbairn him- 
self, distrusting- his power of dealing with the more refined 
mathematical questions involved in designing such unprece- 
dented structures, called to his aid Professor Eaton Hodgkin- 
son, who had long been associated with him in similar researches, 
but who, though able and enterprising, was more cautious and 
less confident of success than were his coadjutors. 
I. History of the Original Conception. — Speaking- roughly, 
we may divide bridges into three classes : — 1st. Arched 
