be investigated experimentally, to
determine not only the value of Mr. Stephenson's original conception (of
a circular or egg-shaped wrought-iron tube, supported by chains), but
that of any other tubular form of bridge which might present itself in
the prosecution of my researches. The matter was placed unreservedly in
my hands; the entire conduct of the investigation was entrusted to me;
and, as an experimenter, I was to be left free to exercise my own
discretion in the investigation of whatever forms or conditions of the
structure might appear to me best calculated to secure a safe passage
across the Straits." {329a} Mr. Fairbairn then proceeded to construct a
number of experimental models for the purpose of testing the strength of
tubes of different forms. The short period which elapsed, however,
before the bill was in committee, did not admit of much progress being
made with those experiments; but from the evidence in chief given by Mr.
Stephenson on the subject, on the 5th May following, it appears that the
idea which prevailed in his mind was that of a bridge with openings of
450 feet (afterwards increased to 460 feet); with a roadway formed of a
hollow wrought-iron beam, about 25 feet in diameter, presenting a rigid
platform, suspended by chains. At the same time, he expressed the
confident opinion that a tube of wrought iron would possess sufficient
strength and rigidity to support a railway train running inside of it
without the help of the chains.

While the bill was still in progress, Mr. Fairbairn proceeded with his
experiments. He first tested tubes of a cylindrical form, in consequence
of the favourable opinion entertained by Mr. Stephenson of the tubes in
that shape, extending them subsequently to those of an elliptical form.
{329b} He found tubes thus shaped more or less defective, and proceeded
to test those of a rectangular kind. After the bill had received the
royal assent on the 30th June, 1845, the directors of the company, with
great liberality, voted a sum for the purpose of enabling the experiments
to be prosecuted, and upwards of 6000 pounds were thus expended to make
the assurance of their engineer doubly sure. Mr. Fairbairn's tests were
of the most elaborate and eventually conclusive character, bringing to
light many new and important facts of great practical value. The due
proportions and thicknesses of the top, bottom, and sides of the tubes
were arrived at after a vast number of tr