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ments necessitated by the varying properties of these mate- 
rials. A cast-iron beam, for example, should have its bottom 
flanch * six times the area of the top one, or, in practice, at 
least four times, to suit the convenience of casting - . 
On beams of wrought- iron, on the contrary, a different 
arrangement is necessary, the area of the top flanch requiring 
to be nearly double that of the bottom, — to attain a maximum 
power of resistance. In large wrouglit-iron girders for bridges, 
the construction is again modified by the introduction of tubes 
or cells in the upper flanch, as in the Conway and Britannia 
bridges, when the areas should be proportioned in the ratio of 
11 to 12. Again, it should be understood, that the strengths of 
girders of similar proportions increase as the squares of the 
span, whilst their weights increase as the cubes of the span. 
With these preliminary observations, it may be interesting to 
glance cursorily at the history of the application of iron to 
constructive purposes down to the present time, when it is 
most strikingly exemplified in the works in progress at South 
Kensington. 
The exact time when cast-iron came into use appears to be 
very uncertain, although for the casting of cannon and other 
purposes it has been employed from an early date. It was not, 
however, till 1794 that John Bennie applied it successfully to 
millwork, in the machinery of the Albion Mills, Southwark. It 
was employed for bridges as early as 1777, by Mr. Pritchard 
at Colebrook Dale. 
The first notice we have of its application to buildings, was 
the use of cast-iron beams at Messrs. Philips and Lees’s cotton 
mill in Salford, in 1801. These beams, extending from wall to 
wall, were supported in the middle by cast-iron columns and 
carried brick arches to form the floors, which were consequently 
fireproof. For half a century this same principle of construc- 
tion has been successfully followed and applied to nearly the 
whole of the factories of Lancashire and Yorkshire. 
In the use of cast-iron there is, however, a hidden source 
of danger, arising from flaws in the castings, want of pro- 
portion, &c., and several serious and fatal accidents have 
resulted from this cause, as well as from ignorance of the 
properties of the material. Hence we earnestly recommend 
the more extensive application of wrought-iron in every case 
where tensile strains have to be resisted. The breakage of cast- 
iron beams not only involves the destruction of property, but, 
what is far more serious, a great sacrifice of life, of which the 
accident at the Hartley Colliery is a fearful example ; and this 
* The flange or flanch is the projecting ridge at either side of the beam, 
