AND LONG-CONTINUED CHANGES OE LOAD ON WEOUGHT-IKON GIEDEES. 315 
From the experiments made by the Commissioners it may be inferred — 
1st. That cast-iron bars or girders are not safe when subjected to a series of deflec- 
tions due to one-half the load that would break them. 
2nd. That they are perfectly secure in sustaining a dead weight not exceeding one- 
third of the weight that would break them ; and 
3rd. That these reiterated deflections appear to have no injurious effect upon the 
metal from which the bars were cast. 
As respects wrought iron, it appeared from the experiments that a progressive increase 
in the deflections and permanent set was observable during every depression produced 
by the same cam as that employed on the cast-iron bars, exhibiting great deficiency in 
its elastic powers. Where the bar retained its power of restoration up to 30,000 deflec- 
tions, with 10,000 more changes it took a set of '06 inch, and from that number, with 
810 additional depressions, the set increased to T84 inch, evidently showing that it 
would have continued still further to increase until the bar was rendered useless. 
Comparing these experiments with those obtained from the riveted wrought-iron 
beam in the following experiments, it will be found that a load equivalent to one-fourth 
the breaking-weight produced no visible change nor any permanent set after being sub- 
jected to 1,000,000 depressions of T7 and *22 inch. By increasing the load from one- 
fourth to two-fifths, it sustained 5175 additional deflections of ‘22 inch, when it broke. 
The difference between the experiments on the wrought-iron bar and the wrought-iron 
manufactured girder consists in the greater rigidity of the latter, and in its increased 
power of resistance to vibration and the force of impact, the weight on the girder- 
descending upon it by the force of gravity. 
The institution of experiments for the purpose of ascertaining the value of wrought- 
iron riveted plates, in the form of tubes, through which a railway train should pass, was 
a conception which led to a new era in the history of bridges, and ultimately effected the 
passage of the estuary of the Conway and the Menai Straits. These experiments not 
only gave the form and strengths required for the construction of these colossal structures, 
but they developed an entirely novel system of constructive art, and established the 
principle on which wrought-iron bridges should in future be made. Since then some 
thousands of bridges, many of them of great span, have been constructed, composed 
entirely of wrought iron, and are now in existence supporting railways and common 
roads to an extent hitherto unknown in the history of bridge-building, and such as 
could not have been accomplished by any other description of material than malleable 
iron or steel. 
The construction of the Britannia and Conway bridges in the tubular form led to other 
constructions, such as the tubular girder, the plate and lattice girder, and other forms, all 
founded on the principle developed in the construction of the large tubes as they now 
span the Conway and the Menai Straits. In the tubular bridges, it was first designed 
that their ultimate strength should be six times the heaviest load that could ever be 
laid upon them, after deducting half the weight of the tube. This was considered a 
