AND LONG-CONTINUED CHANGES OF LOAD ON WROUGHT-IBON GIRDERS. 325 
From the above it is evident that wrought-iron girders, when loaded to the extent of a 
tensile strain of seven tons per square inch, are not safe, if that strain is subjected to 
alternate changes of taking off the load and laying it on again, provided a certain 
amount of vibration is produced by that process ; and what is important to notice is, 
that from 300,000 to 400,000 changes of this description are sufficient to ensure fracture. 
It must, however, be borne in mind that the beam from which these conclusions are 
derived had sustained upwards of 3,000,000 changes with nearly five tons tensile strain 
on the square inch, and it must be admitted from the experiments thus recorded that 
five tons per square inch of tensile strain on the bottom of girders, as fixed by the Board 
of Trade, appears to be an ample standard of strength. 
As regards compression, we have only to compare for practical purposes the difference 
between the resisting-powers of the material to tension and compression, and we shall 
require in a girder without a cellular top from one-third to three-fourths more material 
to resist compression than to resist tension ; and as the strength of wrought iron in a state 
of compression is to its strength in a state of tension as about 3 to 4^, the area of the 
top and bottom will be nearly in that proportion, or, in other words, it will require that 
much more material in the top than the bottom to equalize the two forces. 
In the experimental beam the area of the top was considerably in excess of that of 
the bottom, it having been constructed on data deduced from the experiments on tubes 
without cells, which required nearly double the area on the top to resist crushing. In 
the construction of large girders, where thicker plates are used, this proportion no longer 
exists, as much greater rigidity is obtained in the thicker plates, which causes a closer 
approximation to equal areas in the top and bottom of the girder ; and from this we 
deduce that from one-third to three-fourths, and in some cases one-third additional 
area in the top has been found, according to the size of the girder, sufficient to balance 
the two forces under strain. 
The foregoing experiments, however, were instituted to determine the safe measure 
of strength as respects tension, and it will be seen that in no case during the whole of 
the experiments was there any appearance of the top yielding to compression. 
