THE BRITANNIA AND CONWAY TUBULAR BRIDGES. 417 
Bridges, in which the vertically acting strain of the load is 
transmitted as a force of compression cansing a divergent 
thrust on the abutments. 2nd. Suspension Bridges, in 
which the pressure of the load is transmitted as a tensile 
strain along two or more chains which exert a convergent pull 
on the abutments. 3rd. Beam or Girder Bridges, in which 
the pressure of the load is resolved within the structure itself 
into two horizontal forces, — one of compression acting along 
the upper portion, the other of tension along the lower portion 
of the beam. In this case there is no strain upon the abut- 
ments, except a simple vertical pressure equivalent to the load 
and weight of the structure. 
Mr. Stephenson’s first idea was to employ magnificent cast- 
iron arches of 350 feet span, which, in the case of the Menai 
Straits, he proposed to construct, by a most ingenious method, 
without the aid of centering. The boldness of the project may 
be understood when we recollect that the largest cast-iron 
bridges then erected were those at Southwark and Wearmouth, 
of 240 feet span. To this proposal, however, the Admiralty 
objected that the navigation of the Straits would be seriously 
obstructed. They demanded that any bridge there should have 
spans of 450 feet, with a clear opening of 103 feet above high 
water for the passage of vessels. Requisitions such as these, 
against which there was no appeal, would have vetoed the 
undertaking to any one less conversant with railway difficulties 
than Mr. Stephenson. He, however, at once reverted to the 
idea of adopting the suspension principle. He had, indeed, 
first of all contemplated the strengthening of Telford’s fine 
suspension-bridge which crosses the Straits with one large span 
of 580 feet, to adapt it to railway purposes, a project very soon 
abandoned. But now in his perplexity he returned to his first 
plans. Such a bridge has a perfectly level soffit,* can be erected 
without centering, f and of all descriptionsof bridges, had alone, 
at that time, been employed in great spans. It has, in addition, 
the advantages of lightness and consequent cheapness ; but, 
with the counterbalancing defect of extreme instability, arising* 
from the unequal expansion of the chain and roadway, and even 
more seriously, from a tendency to vertical wave-lilce oscilla- 
tions, with high winds or suddenly applied loads. Of these 
defects Mr. Stephenson was fully aware, having had not long- 
before to replace a suspension-bridge on the Stockton and Dar- 
lington Railway, which had utterly failed from these causes, the 
rails having risen up three feet in front of the engine when the 
* The underside of an overhanging structure. 
t The wooden scaffolding or framing employed to support arches, &c., 
during their erection. 
