1308

THEORY AND PRACTICE OF ENGINEERING.

Book II.

from their resemblance to forked lightning, have a great variety of forms given them,and are sometimes bolted through, at others strapped round with strong hoops of iron :where bolts are dispensed with, it is perfectly clear that the joint cannot have half thestrength of an entire piece: where the stress is longitudinal, two irons put on each side

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Fig. 2081.

will prevent the scarfthat is merely in-dented from pullingasunder; but sucha provision will notmaintain the con-stant horizontal po-sition of the timber.

When a scarf isforced to its bearingsby the introduction ofkeys or wedges driventight, they sometimesreceive an additionalstrain, and it is oftenfound advisable toomit them, and to

SCARPING.

Fig. 2CS3.

Fig. 2082. SCARFING.

bring the joints to a bearing by some other means before the bolts are inserted : whenkeys are made use of they should be of veryhard wood, having a curled grain, which re-sists the insertion of the fibres opposed to it.

To prevent lateral movement cogging isadopted, in addition to the ordinary method,and a small tenon or cog is left upon a por-tion of the scarf, which enters into a notchprepared in the piece which is to cover it:beams intended to resist cross strains requireto be lengthened more frequently than anyothers, and from the nature of the strain adifferent form of scarf must be made use ofto that which is required for a strain in thedirection of its length: when timber is sub-jected to both strains, the cross strain is thatwhich demands the greatest attention : wherea floor is supported, the scarfing requires to befurther secured by iron bolts made to passthrough a longitudinal piece, laid to cover theunder side of the joint.

Bearing Posts, when used to support thefloors of a magazine or warehouse, are ge-nerally formed exactly square: to obtain theirdimensions, we must refer to what has beenalready stated in reference to the weight theseveral timbers will*bear in the direction of theirlength, or the weight they will carry previousto the compression of their fibres: sometimber will support, whilst that of another quality will suspend the most ; therefore in the

SCARFING.

selection of story-posts, we must pay attention to these peculiarities,generally used for these purposes, in consequence of its horizontal sec-tional area occupying less space than timber of the same strength.

When a tie-beam is mortised through to receive a king or queen-post, and it is necessary to provide for the means of holding it up, thetenon should not be pinned through, as it is not advisable to dependentirely on the pins for the support: the tenon should be cut like ahalf dovetail, or in a sloping direction, on one side, and left straighton the other; the mortise-hole should be so cut that the lower end

Iron, however, is

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