Thus we perceive that by means of a truss,the strain of any weight may be made to acton other bodies, in certain directions and pro-portions; and consequently, that trusses maybe combined with each other, to form an ex-tensive system of frame-work, in which thebeams are placed in a state of extension orcompression, and act as solid bodies as large asthe spaces inclosed. For if another truss O P D,be applied, the strain B II (G I) arising fromthe weight A D, is sustained by the king-postPB; and the thrust B I, upon the foot B ofthe rafter A B, is resisted by the opposite thrustB It of a third truss FEB.

This principle, originally suggested for woodenbridges in several publications on the subjectof carpentry, and used in the construction ofroofs, domes, centres of arches, &c. has beenbeautifully applied by Major By, of the RoyalEngineers, for a bridge of considerable span, amodel of which may be seen in the Arsenal atWoolwich, and which does great credit to theingenuity of this able officer.

By means of a simple truss, timber of strengthsufficient only for a very small span, appliedsimply, may be used for a much greater width,BI) fig. 13, either in one piece, or scarfed at1): no mortise should be made there; the beams