1362

THEORY AND PRACTICE OF ENGINEERING.

Book II.

to which they are attached by iron pins, and the spreading out of which they prevent;the intervals between are filled up by planks, of from 3 to 4 inches in thickness, which

Fig. 2204.

are also nailed; these intervals are generally 6 feet 6 inches, and care has been taken toprolong the pieces, and to place at their extremity a tenon, on which a brace is framed,which consolidates the posts of the parapet, and is prolonged a little further, to preventthe water from remaining in the framework ; the posts are also strengthened by anotherbrace, also framed into the main timbers; they are kept together in the length ofthe bridge by two courses of horizontal rails, one of which, placed on their heads, islaid so as to cap them : within and against the foot of the parapet are planks calledguards, which keep in the pavement and sand. It may be remarked that in this floorthe main timbers are entirely covered either with joists or planks : there is therefore aconstant dampness, and in demolishing ancient bridges, it is constantly observed that theupper parts and the interior of the main timbers are entirely perished, whilst at their

Fig. 2205.

exterior extremities they appear quite sound: this essential defect has been avoided infig. 2205. The platform of boards is carried on joists from 9 to 10 inches square, atabout 3 feet apart: the planks should then be placed lengthwise on the bridge, but thelinings with which they are covered are laid transversely, for the purpose of preventing thefeet of the horses from sliding on the wood : not only has this latter floor the advan-tage of allowing the air to circulate round the principal bearers, but it also permits of theprincipals being removed further apart. Sometimes it is usual to substitute iron balus-trades for the wooden parapets on timber bridges ; they are composed of upright standards,which should traverse the joists of the floor, and the brestsummers of the principal, onwhich they bear by means of shoulders secured by screws ; these uprights are stiffened byconsoles, and their intervals filled up either by rails which traverse them in the middle, orbv irons diagonally placed.

It has been proposed, in order to preserve the floors of wooden bridges, to cover theplanks with plates of lead and copper, and it appears that the greater duration producedby this precaution will compensate for the expense, or the wooden floors might beentirely omitted, and the main timbers covered with a plate of lead, the edges of whichshould be lapped one over the other.

During the middle ages throughout Europe , as nas been already stated, numerous timberbridges were constructed, generally supported by one or two rows of piles, forming apier, united by several horizontal binding-pieces, and strengthened by inclined struts : suchpiers were placed from 15 to 25 feet apart, and protected by sterlings covered with planks,the intervals between the piles being filled in with stone or chalk, and a capping of tim-ber placed on them, on which the girders which carried the platform rested.

Considerable improvements have since been made in constructing wooden bridges, andalmost every country has had its peculiar system: the abutments, and often the piers,are formed of stone or brick : where the streams are narrow, a simple platform laid ontimbers of the requisite scantling for the bearing is sufficient; for an increased widthstruts are added, and trusses of various descriptions.

Foot Bridges of the simplest construction have the beams laid across the openingwithout any struts; for a span of 15 or 16 feet, a depth may be given to the principalbearers of 15 inches and a breadth of 8 inches; these may be laid about 2 feet apart, andcovered with plank : or two piles may be driven, inclining inwards, coupled at thehead by transverse beams mortised and pinned into them; they may be furtherstrengthened by horizontal pieces at different portions of the height; on the transversebeams may be laid as many joists as are required for the width of the bridge, for which, ifof an ordinary kind, two will be sufficient; the parapets may be strutted on to the pro-