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every point by which it may be equally sustained, withoutI throwing an undue strain upon any of the joints or beams, itis well to make the distance between the intrados and extradosas great as can be ; hence the advantage of a double arch ofI timbers, with the intermediate space filled in with judiciously-| arranged braces and diagonal pieces, by which the ribs arej kept in their proper position without materially increasingi the weight.

| In the construction of wooden bridges it must be con-i stantly borne in mind that every piece of timber introduced! should be placed, as far as can be, so as to bear the strainwhich will be thrown upon it in the direction of its length ;for the more it inclines from the direction of the thrust towhich it is opposed, the less resistance it will be capable ofexerting, and its useful effect will decline in the ratio of theradius to the sine of the angle of its inclination. In additionI to this, the tendency to fracture is much greater by a trans-verse strain than by a force acting longitudinally by com-pression through the entire length of the beam.

When it is required to join two or more pieces of timberend to end, so as to make them, together, sufficiently long forany required purpose, the joint made use of in that case isi called a scarf. This is made by cutting away a portion ofthe thickness of the wood in both pieces, so that they may fiti into each other : the scarf is then made complete by boltingj them well together with iron bolts, or with trenails, andI : putting iron straps round the joint where necessary. A scarfI ' may be made by cutting away half the wood at the end of| ! each piece, that so they may partially overlie or lap upon| | each other, or else, in a more advantageous way, by cuttingI ! away the wood in a slanting direction, at the same timemaking indentations or notches in one piece which exactlycoincide with similar projections cut near the end of thepiece to be joined to it; such scarfs are usually from 18j i inches to 3 feet long.

| i Tenon-and-mortise joints are used for uniting timbers atright angles to one another. The thickness is diminished onI both sides at the end of a piece of wood, so as to leave anoblong projection, called a tenon, which is fitted into a spacetermed a mortise, hollowed out of the piece to which it is tobe united, and placed at about a third of the height from thej under side ; a wooden pin is frequently put through the mor-i tise and tenon so as to prevent them from getting out of theirj places. A combination of a tenon-and-mortise joint with aj scarf is sometimes made for joining timbers longitudinally ;j and it is found to add greatly to its strength. Scarfs inI beams likely to undergo a transverse strain must always be: carefully made ; they should be very long and well notched,

| and this tenon-and-mortise joint added, the whole strongly: bolted ; and it is often a good precaution to put a plate ofj iron on the under and upper sides of the scarf,j A dove-tail joint is made by forming a tenon increasing inj width towards the extremity of the wood, and having anarrow neck near the shoulder; this is fitted to a mortisemade in the same shape, on another piece of wood, into whichit is admitted laterally; when the wood is hard, this is a goodkind of joint, but the ends of the inverted wedges are liableto be broken off if their sides diverge too much, and if theyI are made in too straight a direction, they are apt to shrinkand thus become useless. This joint is, however, very suit-] able for ties when well made, and it is used in cases where it| is thought desirable to secure two horizontal pieces of wood' more firmly together, which are joined to one another at an! angle. In that case, a piece of wood is made to act as a tie,by being dove-tailed into both the timbers at some distancefrom the joint.

The ends of struts and braces are usually let into the tim-

bers with which they are connected at the ends by mortise*and-tenon joints; these joints are generally cut at the endperpendicularly to their direction, and in this way resist tostrains which they have to bear more effectually.

A king-post is a principal vertical timber, to which tncentres of bays or girders are attached, and by which means,properly speaking, they should be sustained.

Queen-posts are similar to king-posts, but do not occupyso important a position in a piece of carpentry. A numberof these may be introduced with great advantage ;should be well framed into the top and bottom timbers, eith eby halving or by mortise-and-tenon joints; and iron strap 9should, in most cases, be made use of to make them secure.

The timber, &c., used in the construction of woodebridges is, according to the position it is made to occupy>obliged to undergo four kinds of forces, which tend to ovei :come the cohesion exerted between the particles of which it ^formed. These are, first, and principally, transverse P r ? ssure, or a force acting transversely upon a beam, tendi othereby to destroy the cohesion and continuity of itssuch is the force that the platform of a bridge must be ma ^to resist ; such, also, the roof, the joists, the flooring, amany of the levers, of which it is composed. This, hovvev >is a compound force made up of the two succeeding fy rC ,.j2nd. A compressive or crushing force, which acts long ltunally ; this is called generally into play in the framing <d \ 1bridge, posts, struts, piles, &c. 3rd. A tensile force, in 11there is a tendency to pull the particles of the wood from 0another, and thus cause a total destruction of its parts Jstretching and drawing it to pieces ; this force is exerted ^king-posts, tie-beams, the under parts of the roadway, anthe joists, walings, &c.; and, lastly, there is a twisting » 0 *’ ’by which the particles of bolts, rivets, trenails, gudgeons, «*•are strained or wrenched from eacli other, or are constan lsubjected to a force tending so to disunite and destroy

In order to prevent these forces from having the eflec - oJiinjuring the bridge, great care must be taken in the selecof the proper materials ; in the manner of disposing thet®the bridge, by adopting such an arrangement as will al ‘one force act as a set-off to another by neutralizing i tsrious effects, and, above all, by distributing the pressjudiciously, so as never to let any portion of the structure ^loaded with more than one-third of the weight which 1calculated to bear without breaking. _ . ^as

In the case of a beam supported at its extremities, 1been found that if it be cut on the upper side, ne ? r j l8 rdmiddle, to about one-third of its depth, and a piece m ^wood be inserted, in the form of a wedge, so as j n «

entire beam a slight camber, that its strength wjfi . j a0creased about a sixth of what it was previously. I* 1 ! .£ oC emay be practised with great advantage when the <* t j,e

between the supports is small, because the rigidity

material is so much greater in proportion to the weighsupported, that the deflexion is not injurious; but w^ ^ oJJtspans are great, and the props or piers far remove D teach other, it becomes necessary to truss the beams to p j 3 a 9their falling below the horizontal line. This defies 1 ® a „dthe product of the weight and cube of the length direc t j,einversely as the product of the breadth and the cub ^ , 9depth. A deflexion of about the fiftieth part of an , oV ;dednot very injurious, but, where possible, it should h e P^ eX ioOagainst. It may be well to state here, that the t£) t )jefrom a weight uniformly distributed over a bridge a adeflexion caused by the same weight placed in the c5 is to 8.

In the event of a beam being loaded up to

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