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and ingenious manner, with the materials ( 3 )and (4), mentioned in the preceding list, page 314.At each end of a beam, (s), two of those of thenarrower dimensions, ( 4 ), were connected with it,by screw bolts, (5), and in this manner the jointedbeams, D, fig. 6, formed alternately of single anddouble pieces, were easily set up, and prolongedto the full length of the floor of the intendedcommunication when required for use.

Several important objects were accomplishedby this ingenious contrivance. The individualparts were of very convenient length, for beingtransported on carriages—easily put together—and readily adjusted as the work proceeded. Thebeam of the larger dimension in breadth ( 3 ) beingused for the single part, whilst two of half thatdimension ( 4 ) were applied to form the link whichconnected it with the next single beam, gave tothe whole, throughout, a sufficient and nearlyuniform strength. The joints resting upon thecross pieces C C, permitted the beams D D, toconform, by their flexibility, at the points C C,polygonally*, with the curvature of the bridge ;

* The bearers of the road-way of the bridge being placedat the nine angular points, it may be considered as a speciesof funicular polygon, and the various strains at the differentpoints found by the composition and resolution of forces. Itis evident, however, that we cannot far err in supposing the