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R, R, are braced diagonally to each other; andthe beams of the bridge laid longitudinallyon them, as in fig. 25.

For any span or breadth not exceeding 130feet, one string-piece at top, and one at bottom,will be sufficient for each truss; and for spansof 70 or 80 feet, planks inch-and-half thick willbe sufficient; but in every case where fourstring-pieces are used, the bearers for the floorare placed on S, S, fig. 25, the upper of the twolower rows. Light rafters are laid, as shown infig. 1, pi. 12, and covered with shingles andother light material, to keep out rain. The sidetrusses are planked on the outside, for the likepurpose.

Fig. 2, pi. 12, is an elevation of such abridge, one end of which is shown withoutthe side planking, that the whole may beclearly understood.

These bridges have stood for several years,across rivers of very considerable width. Amongthese may be particularly mentioned the fol-lowing—at Nashua in New Hampshire ; New-bury-port, Mass.; Northampton , Mass.; Spring-field, Mass.; another at Providence , Rhode Island ; that near Philadelphia , in New Jersey ;one across the Delaware above Trenton ; one2200 feet long, on stone piers, across the Sus-

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