ON ARCHITECTURE AND CARPENTRY. 129 



and they must be as little prominent as possible, partly because the con- 

 tiguous piece must be excavated for their reception, and partly because 

 their strength is diminished when they project more than one sixth of their 

 length. A beam united to another in this manner is said to be scarfed. 

 (Plate XIII. Fig. 169.) 



In order to preserve the strength of a compound beam, intended to re- 

 sist a transverse action in a particular direction, it is necessary to avoid, as 

 much as possible, reducing the depth of the beam in that direction, and to 

 secure 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 result 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 somewkat increased by the operation, supposing 

 the two ends to meet each other without any connection. Such pieces re- 

 quire, however, 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 secured still more firmly by a strap 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 mois- 

 ture 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 ad- 

 visable 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 toge- 

 ther 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 purpose 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 assistance 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 

 ^additional security. Such a strap ought always to be as straight as 

 possible, so as to act only in the direction of the force to be resisted : it 

 has been too customary to accommodate the strap to the form of the beams, or 

 to make it deviate in other ways from a right line : but wherever a strap 



