S

[TeX: $a = \frac{T}{S}$]

or, by substituting the working strengths for the various materials in
the formula, we have for wood,

a = T/2000

Wrought Iron, a = T/1500

Cast Iron, a = T/4500

But, in practice, cast iron is seldom used except to resist
compression.

=Strains of Compression.= Allowing the same letters to denote the
same things as above, we have for

Wood, a = T/1000

Wrought Iron, a = T/12000

Cast Iron, a = T/25000

As this pamphlet has to do with wooden bridges only, nothing will be
said of the proper relative dimensions of cast-iron columns to sustain
the strains to which they may be subjected, but a table of the
strength of columns will be found further on.