DESCRIPTION OF PLATES. xv 



PLATE XIII. 



Fig. 166. Joints for a tie beam. The joints at A and B cannot be more than 

 half as strong as the entire beam, supposing the adhesion, produced by the pressure 

 of the bolts, as strong as could be required. The joint at C is called a dovetail 

 joint ; its strength is a little less than that of A and B, but the adhesion is more 

 easily secured, since a force tending to separate the beams must tighten the joint. 

 P. 128. 



Fig. 167. Joints for a tie beam. The joint A, if sufficiently tight, may possess 



of the strength of the beam. The joint B may be as strong as the beam, if the 



adhesion were great enough, but it would be difficult to apply sufficient pressure to 



create such an adhesion, and if the beam were subject to be much shaken, the joint 



would be a very bad one. P. 128. 



Fig. 168. A good joint for a tie beam ; the adhesion being secured by a slight 

 diminution of the strength. P. 128. 



Fig. 169. A, a simple scarfed joint, which may be tightened by a wedge at the 

 centre ; it is not strong. B, a scarfed joint, which is much stronger. P. 129. 



Fig. 170. A joint for a beam supporting a weight by its transverse strength. 

 The junction might be made, if it were necessary, by means of a third piece, of 

 which the limits are marked by the dotted line. The strength is but little diminished 

 by the joint. P. 129. 



Fig. 171. A beam supporting a weight by its transverse strength, joined to 

 another by means of a third piece of half the depth, spliced or fished on below the 

 beam, and secured by pins, and by blocks or joggles. The strength is a little 

 greater than that of the original beam. The dotted lines show the proportion in 

 which the strata are extended or compressed, the lower part of the original beam 

 remaining in its natural state, without sustaining any pressure, as far as one fourth 

 of the depth, and a little further. P. 129. 



Fig. 172. A joint for a beam pressing obliquely against another. The dotted 

 lines show the form of the tenon, which may occupy a considerable part of the 

 breadth of the beam. The upper strap, A, is in the most usual situation, but the 

 lower one, B, appears to afford greater strength, as it presses the beams more closely 

 together, yet without any danger of crippling them ; besides the advantage of having 

 a firmer hold of the lower beam. P. 130. 



Fig. 173. A joint for a horizontal beam suspended from a vertical one : the end 

 of the tenon being dilated by wedges, and the whole secured by a strong strap. 

 The tenon ought not to be wide, since it diminishes the strength of the horizontal 

 beam. P. 130. 



Fig. 174. The straps, bent so as to deviate from the right lines joining their 

 extremities in the degree that is here represented, have their strength reduced to 

 about one seventh of that which they would have if straight. Thus, A B is only 

 one seventh as strong as C D, supposing the substance inflexible. P. 130. 



Fig. 175. The simplest form of a roof. A B, A C, are the rafters, and B C the 

 tie beam ; the weight of each half being represented by A B, or A C, the thrust in 

 the direction of the rafters will be A D, and the horizontal thrust each way B D or 

 CD. It is obvious that A D will be least when B A C is a right angle. P. 130. 



Fig. 176. A common roof, with braces. AB is the king post, and B C, BD 

 the braces. P. 130. 



Fig. 177. A kirb or mansard roof, the rafters of which hold each other in equili- 

 brium. A B and C D are queen posts helping to support the tie beam. The piece 

 A C acts as a strut, in supporting the pressure occasioned by the weight of the tie 

 beam. The heads of the queen posts are not much thickened, in order to avoid the 

 change arising from the unequal contraction of the wood. P. 130. 



