16

CORRESPONDENCE.

annexed forms; one elliptical,with a deep fin a on the top, andthe other rectangular, with asimilar fin at b, as per annexedsectional sketch. These were, ac-cording to the dimensions heremarked, the one 12 by 7 ^ inches,and the other 13 by 8 inches, and18 feet 6 inches between the sup-ports. The plates were y^-th of aninch thick, and the tubes broke orwere crushed respectively withdeadweightsof6867and88121bs.

The defective powers of resistance of all the tubes of this shape, havesuggested a new arrangement and distribution of the metals ; it beingevident from the experiments, that the tube will resolve itself into a hugehollow beam or girder, leaving the two resisting forces of compressionand extension as wide apart as possible. It is further conclusive, thatthe sides must be made comparatively light, and considerable additionalmaterial introduced into the top and bottom of the tube. This will givegreatly increased strength, and a few more experiments will determinewhich of the two shall have the preponderance. It is more than pro-bable that the bridge, in its full size, may take something of the followingsectional shape.

The parts a, a, being two longitudinal plates, Fig. 9.

divided by vertical plates so as to form squares,calculated to resist the crushing strain in thefirst instance, and the lower parts, b, b, also lon-gitudinal plates, well-connected with rivetedjoints, and of considerable thickness to resistthe tensile strain in the second: or it mayresolve itself into something of this form, (seethe next fig.), with a series of tubes extendingthe whole length of the upper side, to resistcompression, and two tubes with strong lon-gitudinal plates, c, c, to resist tension, and pre-vent tearing in that part: the sides in this caseto be made light, in order simply to connect

Fig. 7. Fig 8.