16>8 ■ LECTURE XIV. 



the union with the greatest care on the convex side of the beam, which is 

 stretched by the operation of the force. Where no inconvenience can re- 

 sult from the projection of a piece on one side, it is easy to preserve the 

 strength unimpaired, by splicing or fishing it on the convex side ; and if the 

 depth of the piece added be only half as great as that of the original beam, 

 the strength will be somewhat increased by the operation, supposing the two 

 ends to meet each other without any connexion. Such pieces require, how- 

 ever, to be firmly united, either by pins passing through them, or by blocks 

 or joggles let in to a certain depth, in order to prevent their sliding on each 

 other; and this mode of union is stronger than scarfing them, because it does 

 not diminish the depth. (Plate XIII. Fig. 170, 171.) 



Where the pieces to be connected together are in different directions, the 

 end of one of them is usually reduced in its size, and becomes a tenon, while 

 a mortise is cut in the other for its reception, and the joint is also often se- 

 cured still more firmly by a stiap of iron. If a joist be let into a beam, at its 

 upper edge, and made very tight by wedges, the strength of the beam will 

 not be materially diminished ; but the vicissitudes of moisture and dryness 

 may very much impair the firmness of the union, and the end of the joist 

 may fail in dry weather to afford sufficient resistance to the flexure of the 

 beam: so that in some cases it might be more adviseable to cut the mortise 

 near the middle of the depth of the beam. If two pieces meet obliquely, and 

 one of them exerts a thrust against the other, the simplest mode of opposing 

 this thrust is to bind them together by a strap of iron fixed to the second 

 piece; this strap renders it impossible for the first to advance without having 

 its extremity crushed ; it is also common to make a mortise in the second 

 piece, a part of which serves as an abutment for the first : and for this pur- 

 pose the piece must be continued far enough beyond the abutment to give the 

 projection sufficient force of adhesion, a condition which is the more easily 

 fulfilled when the action of the strap produces a pressure on it. The assist- 

 ance of a strap is still more indispensable where the pieces are perpendicular to 

 each other, and the force tends to draw one of them away from the other: in 

 this case the mortise may be made a little wider at the remoter part, and the 

 end of the tenon may be made to fit it by driving in wedges, in the same 

 manner as Mr. Smeaton united his blocks of stone; but a large mortise would 

 weaken the beam too much, and a strong strap or hoop is usually required for 



