TRACT 5.

ARCH FOR LONDON BRIDGE.

129

“ Getting out and securing the foundation of

the two abutments ’

432,000 cubic feet of granite or other hard stone 86,40020,029 cubic yards of brickwork, at 20s. - - 20,029

“ Questions respecting the Construction of the annexed Plate

and Drawings of a Cast Iron Bridge of a Single Arch, 600feet in the Span, and 65 feet Rise.

“ 1. What parts of the bridge should be considered aswedges, which act on each other by gravity and pressure,and what parts as weight, acting by gravity only, similar tothe walls and other loading, usually erected upon the archesof stone bridges.—Or, does the whole act as one frame ofiron, which can only be destroyed by crushing its parts ?

“ 2. Whether the strength of the arch is affected, and inwhat manner, by the proposed increase of its width towardsthe two extremities, or abutments ; when considered verti-cally and horizontally. And if so, what form should thebridge gradually acquire?

“ 3. In what proportions should the width be distributedfrom the centre to the abutments, to make the arch uniformlystrong ?

“ 4. What pressure will each part of the bridge receive,supposing it divided into any given number of equal sections,the weight of the middle section being given. And on whatparts, and with what force will the whole act upon the abut-ments ?

19,200 cubic feet of timber in tyes, at 3s. 6d.6,500 tons of cast iron, including scaffold

130,000

3,360

and putting up, at 20l.Making roadways and footpaths

2,500

.£262,289”

VOL. I.