ials; one of the results of the
experiments being the adoption of Mr. Fairbairn's invention of
rectangular hollow cells in the top of the beam for the purpose of giving
it the requisite degree of strength. About the end of August it was
thought desirable to obtain the assistance of a mathematician, who should
prepare a formula by which the strength of a full-sized tube might be
calculated from the results of the experiments made with tubes of smaller
dimensions. Professor Hodgkinson was accordingly called in, and he
proceeded to verify and confirm the experiments which Mr. Fairbairn had
made, and afterwards reduced them to the required formula.

Mr. Stephenson's time was so much engrossed with his extensive
engineering business that he was in a great measure precluded from
devoting himself to the consideration of the practical details. The
results of the experiments were communicated to him from time to time,
and were regarded by him as exceedingly satisfactory. It would appear,
however, that while Mr. Fairbairn urged the rigidity and strength of the
tubes without the aid of chains, Mr. Stephenson had not quite made up his
mind upon the point. Mr. Hodgkinson, also, was strongly inclined to
retain them. Mr. Fairbairn held that it was quite practicable to make
the tubes "sufficiently strong to sustain not only their own weight, but,
in addition to that load, 2000 tons equally distributed over the surface
of the platform,--a load ten times greater than they will ever be called
upon to support."

It was thoroughly characteristic of Mr. Stephenson, and of the caution
with which he proceeded in every step of this great undertaking--probing
every inch of the ground before he set down his foot upon it--that he
should, early in 1856, (_sic_) have appointed his able assistant, Mr.
Edwin Clark, to scrutinise carefully the results of every experiment, and
subject them to a separate and independent analysis before finally
deciding upon the form or dimensions of the structure, or upon any mode
of procedure connected with it. At length Mr. Stephenson became
satisfied that the use of auxiliary chains was unnecessary, and that the
tubular bridge might be made of such strength as to be entirely
self-supporting.

While these important discussions were in progress, measures were taken
to proceed with the masonry of the bridges simultaneously at Conway and
the Menai Straits. The foundation-stone of the Britannia Bridge was laid
o