he weight they have to support, being sometimes
within 10 per cent., or less, of the breaking weight. The expansion of
the girders may thrust out the side walls. For instance, in a
warehouse 120 feet x 75 feet x 80 feet, there are three continuous
rows of girders on each floor, with butt joints; the expansion in this
case may be twelve inches. The tie rods to take the strain of the flat
arches must expand and become useless, and the whole of the lateral
strain be thrown on the girders and side walls, perhaps weak enough
already. Again, throwing cold water on the heated iron may cause an
immediate fracture. For these and similar reasons, the firemen are not
permitted to go into warehouses supported by iron, _when once fairly
on fire_.

Cast and wrought-iron have been frequently fused at fires in large
buildings such as warehouses, sugar houses, &c., but according to Mr.
Fairbairn's experiments on cast iron in a heated state, it is not
necessary that the fusing point should be attained to cause it to give
way.[A] He also states, that the loss of strength in cold-blast cast
iron, in a variation of temperature from 26 deg. to 190 deg. = 164 deg. Fahr., is
10 per cent., and in hot-blast at a variation of from 21 deg. to 190 deg. =
169 deg. Fahr., is 15 per cent.; now if the loss of strength advances in
anything like this ratio, the iron will be totally useless as a
support, long before the fusing point is attained.

Much confidence has been placed in wrought-iron tie or tension rods,
to take the lateral strain of the arches, and also in trusses to
support the beams; but it must be evident that the expansion of the
iron from the heat, would render them useless, and under a high
temperature, it would be so great as to unsettle the brickwork, and
accelerate its fall, on any part of the iron-work giving way: again,
the application of cold water to the heated iron, in an endeavour to
extinguish the fire, is almost certain to cause one or more fractures.
The brick-arching is also very liable to fall, especially if only four
and a half inches thick, independently of the weight which may be
placed upon it, for it is not uncommon after a fire in a large
building, to find the mortar almost completely pulverized to the depth
of three inches, or four inches, from the face of the wall. When a
fire occurred under one of the arches of the Blackwall Railway, on the
15th July, 1843, a portion of the lower ring fell down, and also a few
bri