each other. It therefore appears that the main strength of a beam consists in its power to resist compression and extension, and that the middle is comparatively useless, so that to obtain the greatest amount of strength, the given quantity of material to be used should be accumulated at the top and bottom, where the strain is greatest; or, in plain terms, the middle of the beam, whether of wood or iron, should be bored out. All iron girders, all beams in houses—in fact all things in domestic or naval architecture that bear weight—are subject to the same law.
A hollow beam of iron having been fixed upon as the form which the projected bridge should take, an extensive series of experiments were undertaken with a view to ascertain the shape capable of sustaining the greatest weight. A rectangular tube, with a height considerably greater than its breadth, and strengthened at the top and bottom, was eventually selected. The genii of the lamp were now set to work, and the quiet folk of North Wales witnessed similar wonders to those which have since astonished the Londoners. The principal tubes were constructed on piles at high-water mark, and were formed of wrought-iron plates riveted together with white-hot iron bolts.
A system of longitudinal tubes or cells gave the required strength to the top and bottom of each fabric, these cells being quite as effectual as solid metal. Every means was taken to make the tubes
