AND LONG-CONTINUED CHANGES OF LOAD ON WROUGIIT-IRON GIRDERS. 317 
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equalize the two forces of tension and compression ; and unless these proportions are 
strictly adhered to in the construction, the 5-ton strain per square inch is an error 
which may lead to dangerous results. Again, it was ascertained from direct experiment 
that double the quantity of material in the top of a wrought-iron girder was not the 
most effective form for resisting compression. On the contrary, it 
was found that little more than half the sectional area was required, 
and, when converted into rectangular cells similar to «, a, a, was in 
its powers of resistance equivalent to double the area when formed 
of a solid plate. This discovery was of great value in the con- 
struction of tubes and girders of wide span, as the weight of the 
structure itself (which increases as the cubes, and the strength 
only as the squares) forms an important part of the load to which 
it is subjected. On this question it is evident that the require- 
ment of the Board of Trade cannot be applied in both cases, and 
is therefore ambiguous as regards its application to different forms 
of structure. In the 5-ton per square inch strain there is not a 
word said about the dead weight of the bridge ; and we are not 
informed whether the breaking-weight was to be so many times the 
applied weight plus the multiple of the load, or, in other words, whether it included 
or deducted the weight of the bridge itself. 
These data are wanting in the railway instructions ; and until some fixed principle of 
construction is determined upon, accompanied by a standard measure of strength, it is 
in vain to look for any satisfactory result in the erection of road and railway bridges 
composed entirely of wrought iron. 
I have been led to inquire into this subject with more than ordinary care, not only 
on account of the imperfect state of our knowledge, but from the want of definite 
instructions from the authorities whose duty it is to secure the safety of bridges and 
to protect the public from malconstructions. To accomplish this, I have in the fol- 
lowing experimental researches endeavoured to arrive at the extent to which a bridge 
or girder of wrought iron may be strained without injury to its ultimate powers of 
resistance. I have endeavoured to ascertain the exact amount of load to which a bridge 
may be subjected without endangering its safety, or, in other words, to determine the 
fractional strain of its estimated powers of resistance. 
To arrive at correct results and to imitate as nearly as possible the strain to which 
bridges are subjected by the passage of heavy railway trains, the apparatus specially 
adapted for that purpose was designed to lower the load quickly upon the beam in the 
first instance, and subsequently to produce a considerable amount of vibration, as the 
large lever with its load and shackle was left suspended upon it in the second. The 
apparatus was sufficiently elastic for that purpose, as may be seen on reference to the 
drawings. 
