770 



PLATE XIII. 



Fig. 166. Joints for a tie Insam. The joints at A and 

 n cannot be more than half as strong as the entire 

 beam, supposing the ^adhesion, produced by the pres- 

 sure of the bolts, as strong as could be required. The 

 joint at C is calied a dovetail joint; its strength is a 

 little less than chat of A and B, but the adhesion is 

 nvorc easily secured, since a force tending to separate 

 the beams must tighten the joint. P. 167. 



Fig. 167. Joints for a lie beam. The joint A, if 

 sufliciently liglit, may possess t of the strength of tl)e 

 beam. The joint B might be as strong as the beam, if, 

 the adhesion were great enough, but it would be diffi- 

 cult to apply sufficient pressure to create such an ad- 

 hesion, and if the beam were subject to be much 

 sliakcn, the joint would be a very bad one. P. 167. 



Fig. 168. A good joint for a tic beam; the adhe. 

 •ion being secured by a slight diminution of the 

 strength. P. 167. 



Fig. 16?. A, a simple scarfed joint, which may be 

 tightened by a wedge at the centre; it is not strong. 

 B, a scarfed joint wliich is much stronger. P. 167. 



Fig. 170. A joint for a beam supporting a weight 

 by its transverse strength. Thejunction 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. 168. 



Fig. 171. A beam supporting a weight by its trans- 

 verse strength, joined to anotlier 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 jog- 

 gles. The strength is a little greater than that of the 

 original beam. The dotted lines show the proportion 

 in which the strata ate extended or compressed, the 

 lower part of the original beam remaining in its natu- 

 ral state, without sustaining any pressure, as far as one 

 fourth of the depth, and a little further. P. 158. 



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 uppers trap, A, is in the 

 most usual situatiou, 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. 169. 



Fig. 173. A joint for a horizontal beam suspended 

 from a vertical one: the end of tlie tenon being di- 

 lated by wedges, and the whole secured by a strong 

 strap. The tenon ought not to be wide, since it dimi- 

 nishes the strength of the horizontal beam. P. 169. 



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 seventli as strong 

 as C D, supposing the substance inflexible. P. 169. 

 Fig. 175. The simplest form of a roof AB, AC, are 

 the rafters, and B C the tie beam; the weight of each 

 half being i-eprcsented by AB, or A C, the thrust in the 

 direction of the rafters will be A D, and the horizontal 

 thrust each way BD or C D. It is obvious that A D 

 will be least when B AC is a right angle. P. 170. 



Fig. 176. A common roof, with braces. A B is the 

 king post, and B C, B D the braces. P. 170. 



Fig. 177. A kirb or mansard roof, the rafters of 

 which hold each other in equilibrium. A B and C D 

 ai-e 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 

 cf the wood. P. 170. 



