Chap. XXIII.
1351
lig. 2170.
THIRD PRINCIPLE.
instead of abutting against the pendant timbers, as in the third example, they are laid overeach other, and constitute in effect but one beam.
Fig. 2177.
FOURTH PRINCIPLE.
The fifth principle is a modification of the preceding, and is particularly adapted to a bridgewhere long pieces of timber cannot be obtained : the bridge of St. Clair, over the Rhone,
FIFTH VRINCIPLB.
at Lyons, is so constructed, as it was not found possible to obtain timber of sufficientdepth and breadth.
The sixth principle was adopted at the bridge of the Mulatiereat Lyons, over the Saonc;such were the bridges constructed over the Thames formerly at Kingston and Walton:
Fig. 2179.
SIXTH PRINCIPLE.
there is no advantage gained by shortening the beams, because the angles of junction beingmore obtuse, the strain in the direction of the length is considerably increased.
The seventh principle exhibits a framed rib, but it must be remarked that the weight to
SEVENTH PRINCIPLE.
be applied in the middle must proportion its depth : a number of curved ribs may be soput together that they will resist either extension or compression.
4 r 4