The late Mr. George Smart, inventor of the ingenious truss called the " bow-andsitring-rafter," in experiments tried to ascertain how far the strength of a beam is increased by confining its ends, so as to prevent them from approaching each other when the centre is heavily loaded, found that a lath which, when simply laid on two points of support, broke with a load of 11 lbs. placed in the middle, would sustain 270 lbs. when the ends were firmly secured by wedges. These experiments led him to the construction of trussed beams of unusual lightness, in wrought-iron as well as in wood. In 1826 he submitted to the Society of Arts a wrought-iron beam adapted for use in bridges, roofs, floors, and other constructions in which stiffness, strength, and lightness are required, for the invention of which he was rewarded with their silver Vulcan medal. This beam or girder is made by welding the ends of an arched bar of wrought-iron to a longer straight bar, and then turning the ends of the straight bar either up or down, as may be most convenient for fixing, according to the particular use to which it is to be applied. Blocks of well-seasoned wood are than inserted at intervals between the arch and the straight bar, to prevent buckling, and the whole is held together by iron 'straps inclosing the blocks and the iron bars. Mr. Smart conceived that wrought-iron beams made in this way would support so much more weight than those of cast-iron of similar dimensions, that they might be made of any given strength for one-half the cost of cast-iron girders ; and he refers to the application of such a beam to sustain a very heavy mass of brickwork over a gateway leading from the Poultry, under circum stances which would have precluded the use of timber. Very light timber beams were made by Mr. Smart upon the same principle; and so strong was his confidence in the application of trussing upon au extensive scale, that he published a design for a foot-bridge of trussed timber to cross the river Thames at Hungerford by a single span. • Cast-iron beams are frequently trussed with wrought-iron a similar manner to those of wood, and are applied to purposes for which great strength is required. The brest-aummers used over large shop-windows, to sustain the front wall of the house, are often made in this way ; and similar girders are much used in the construction of railway bridges. The bridge which supports the London and IlLackwall Railway across the street called the 3linoriee is a remarkable example. It has a clear span of aixty-three feet, and Is supported by six massive trussed beams, weighing about fifteen tons each. When used in floors or roofing, timber may be applied on each side of the iron, supported by lodges formed for the purpose, to render the fitting of the joists and other timbers easy. Smart recommends the use of bridging and ceiling joists formed of wide hoop-iron riveted together, with a slip of poplar between them to hold the flooring or lath-nails ; and he observed that neither fir nor oak will, when thin, receive nails near so well as poplar, without splitting : in addition to which he says that, besides being very durable if kept dry, it has the advantage of being much less combustible.
Trussing has been applied with advantage to the raising of sunken re floors, and the repair of beams which, from sagging or from the ravages of dry-rot, have become dangerous. In the forty-second volume of the 'Transactions of the Society of Arts,' pp. 149-152, is an account of a method of raising a sunken floor, which had been successfully practised by Mr. F. Richman, and for which he was rewarded with the large silver medal of the Society. The floor on which he had operated had sunk in the middle to the extent of five inches, and had become very tremulous, in consequence of the removal of a trussed partition, iu order to throw two rooms into one. The ceiling and cornice of the room below being valuable, it was desirable to apply a remedy from above, which was accomplished by removing the floor, and applying to each aide of the sagged beam a cast-iron arch, of which the span was equal to the length, and the height or spring equal to the depth, of the beam. The ends or abutments of these arches were united by tic-rods of wrought-iron. Four equidistant holes were then cut trans versely through the beam near its lower edge, and in each of these was placed a abort bar, having an eye projecting on each aide of the beam. Through these eyes were passed vertical bars or trussing-bolts, which rose above the to of the beam, and were there connected together by cross-pieces, which rested upon blocks attached to the cast-iron arches. By screwing down nuts upon the upper ends of these bolts, as the arches themselves could not yield, the beam was compelled to rise, and was so restored to its original straightness. The cast-iron arches and their tie-rods were formed of several small pieces, so that they might be very conveniently applied; but for the method of accomplishing this, and for other matters of detail, we must refer to Mr. Itichman's account, which is illustrated with engravings, In the same volume (pp. 153.164) is an account of the means adopted by Mr. Alfred Ainger for supporting the decayed timbers of the roof of the church of St. Mary Aldennary, in the city of London, by means of cast-iron trusses or cradles. This operation was performed under circumstances of unusual difficulty, some of the beams being so much decayed by the dry-rot as to have lost all bearing upon the walls. The ingenious con trivances adopted, which were rewarded by the Society of Arta with their large gold medal, prevented the necessity for destroying a richly ornamented ceiling, the restoration of which would probably have cost 50001.