and no view need yet be taken as to the need for it, which would be resolved by further experiments. A span model was constructed and tested at Fairbairn's Millwall shipyard, and used as a basis for the final design. Stephenson, who had not previously attended any of Fairbairn's experiments, was present at one involving this 'model tube', and consequently was persuaded that auxiliary chains were unnecessary. Although Stephenson had pressed for the tubes to be elliptical in section, Fairbairn's preferred rectangular section was adopted. Fairbairn was responsible both for the cellular construction of the top part of the tubes, and for developing the stiffening of the side panels. Each main span weighed roughly 1,830 tonnes. Construction and use On 10 April 1846, the foundation stone for the Britannia Bridge was laid, marking the official commencement of construction work at the site. The resident engineer for the structure's construction was civil engineer Edwin Clark, who had previously aided Stephenson in performing the complex structural stress calculations involved in its design process. The first major elements of the structure to be built were the side tubes, this work was performed in situ, using wooden platforms to support it. The construction method used for the iron tubes was derived from contemporary shipbuilding practices, being composed of riveted wrought iron plates thick, complete with sheeted sides and cellular roofs and bases. The same technique as used for the Britannia Bridge was also used on the smaller Conwy Railway Bridge, which was built around the same time. On 10 August 1847, the first rivet was driven. Working in parallel to the onsite construction process, the two central tube sections, which weighed apiece, were separately built on the nearby Caernarfon shoreline. Once they had been fully assembled, each of the central tubes was floated, one at a time, into the causeway and directly below the structure. The in-place sections were gradually raised into place using powerful hydraulic cylinders; they were only raised by a few inches at a time, after which supports would be built underneath the section to keep it in place. This aspect of the bridge's construction was novel at the time. Reportedly, the innovative process had been responsible for costing Stephenson several nights of sleep at one stage of the project. The work did not go smoothly; at one point, one of the tubes allegedly came close to being swept out to