CORRESPONDENCE.

15

made, such decided advantages, that in the following letter Iventured to anticipate, even at this early period, an ultimatecellular form of section for the great bridge itself, and proposedto Mr. Stephenson two ideas, the one of a tube with a series ofsquare cells on the top, as at a, a* (see fig. 9), and the otherwith a number of circular pipes having flat plates riveted totheir tops and bottoms, as shown at A (see fig. 10). The readerwill not fail to observe how much the first of these sketchesresembles the tubes actually constructed for the Conway andBritannia Bridges. The whole of the arrangements for our sub-sequent proceedings were pointed out at the time to Mr. Hodg-kinson, and appeared to meet with his approval.

{Private.)

My dear Sir, Millwall , Sept. 20th, 1845.

I have been uninterruptedly employed on the experiments for the wholeof this week, and for two days I have had the benefit of the presence andassistance of my friend Mr. Hodgkinson. According to his views, aswell as my own, we are progressing satisfactorily, and although we havenot as yet arrived at the strongest form of tube, we are nevertheless ap-proaching that desideratum. You will be aware, on referring to my lastletter, that the great difficulty we had to encounter was a due proportionof the parts, so as to neutralize, or render the two resisting forces ofcompression and extension equal: out of nine experiments on cylindricaltubes, two failed by crushing in at the top, and seven by tearing asunderat the rivet-holes. The latter were however fractured, owing to the close-ness of the rivet-holes and the construction of the tubes, the foremanhaving omitted to cross the joints.

From eleven experiments on rectangular tubes, eight yielded to thecrushing force, and three only were torn asunder by extension.

The elliptical or egg-shaped tubes invariably failed, with only one ex-ception, by compression ; four having been crushed in at the top, andonly one tom asunder at the bottom. Collectively, these appeared toindicate weakness on the upper side of the tube, and a necessity for aehange of form in order to give rigidity and stiffness to that part. Tocounteract the forces of compression, I got two tubes constructed of the* See sketches in letter.