130 LECTURE XIV. 



is bent in any direction, to a distance from a right line equal only to its 

 depth in that direction, its strength is so reduced, as not to exceed one 

 seventh of what it would have been if it had remained straight. (Plate XIII. 

 Fig. 172.. .174.) 



It is equally necessary in all other cases which occur in carpentry, to 

 avoid as much as possible a transverse strain, the disadvantage of which is 

 obvious from the great inferiority of the strength of any substance, resisting 

 a transverse force, to its primitive cohesive or repulsive strength. For 

 similar reasons it is proper to avoid employing a very open angle at a point 

 where a load is supported, the great obliquity of the two pieces forming the 

 angle requiring them to exert a great force in order to oppose a much 

 smaller one. Allowance must also be made for the contraction of the 

 timber, and care must be taken that it do not so alter the arrangement of 

 the parts, as to bring a disproportionate strain on a point not calculated to 

 support it. If the two pieces forming an obtuse angle consisted, either 

 wholly or partly, of wood cut across the grain, and the piece joining their 

 extremities were cut in the usual manner, the oblique pieces would contract 

 considerably more as they became drier, and the angle would become more 

 obtuse, so that the strain, produced by a given weight, would be greater 

 than in the original state of the triangle. Sometimes the work is liable to 

 be deranged by the operation of a lateral force, which may have appeared 

 too trifling to produce any considerable effect, but which may still destroy 

 the greater part of the strength, by causing the resistances to deviate from 

 the plane of the forces which they are intended to oppose. 



The framing of a roof is one of the most common and most important 

 subjects for the employment of the theory of carpentry. If the rafters 

 were simply to abut on the walls, they would force them outwards ; a tie 

 beam is therefore necessary, to counteract the thrust. In order to enable 

 the tie beam to support a weight, a king post is suspended from the rafters ; 

 and frequently braces are again erected from the bottom of the king post, 

 to support the middle of the rafters. Sometimes a flat or less inclined 

 portion is placed in the middle, forming a kirb or mansard roof, somewhat 

 resembling an arch ; this form has the advantage, when it is properly propor- 

 tioned, of lessening the transverse strain on the rafters by making them 

 shorter ; but this purpose is answered equally well by the addition of the 

 braces which have been already mentioned. A kirb roof affords, however, a 

 greater space within, than a plain roof of the same height, and produces also 

 somewhat less strain on the tie beam or on the abutments : the tie beam may 

 be suspended from it by a king post and two queen posts, descending perpen- 

 dicularly from the joints ; and the place of the king post may be supplied 

 by a cross beam uniting the heads of the queen posts and keeping them at 

 a proper distance ; this beam may also be suspended by a shorter king- 

 post from the summit. Such a roof appears to be more advantageous 

 than it has been commonly supposed. (Plate XIII. Fig. 175. ..177.) 



The angle of inclination of a roof to the horizon usually varies in 

 different climates : in Italy the height is generally less than one fourtli of 

 the breadth ; in England it was formerly three fourths, but it now com- 

 monly approaches much more to the Italian proportion. In northern 



