ROO 



ROO 



thrust thus acting on the walls is 

 proportional to the length of a line, 

 d, e, drawn at right angles to the raf- 

 ter, intersecting a vertical line drawn 

 from the apex, which it is manifest 

 must increase as the roof becomes 

 flatter. To counteract the thrust above 

 mentioned, nothing more is necessa- 

 ry than to tie together the feet of the 

 rafters, as in the following diagram 

 (Fig. 2), in which A B is the tie in 

 f,v 2. question, and thence 



is called a lic-bcam. If 

 ^B the extent be not very 



great, the rafters may 

 be kept from spreading by a minor 

 tie, as at a b, called a collar. Beyond 

 certain lengths or spans, however, it 

 will occur to the reader that a tie- 

 beam will itself have a tendency to 

 bend, or sag, as the workmen call it, 

 in the middle ; and from this circum- 

 stance a fresh contrivance becomes 

 necessary, which will be seen in the 

 annexed diagram {Fig. 3), marked r 

 Fig. 3. d : tins is called a king- 



^^sf'^fs%^ ''°*'' '^^^ niore properly, 

 f^^„!^^^ king-piece, inasmuch as 

 i i it does not perform the 

 office of a post, but rather of a tie, 

 for it ties up the beam to prevent its 

 bending. If the rafters be so long as 

 to be liable to bend, two pieces, a, a, 

 called sinils, are introduced, which, 

 having their footing against the sides 

 of the king-post, act as posts to sup- 

 port or striU up the rafters at their 

 weakest point The piece of framing 

 thus contrived is altogether called a 

 truss. It is obvious that by means of 

 the upper joints of the struts we ob- 

 tain more points of support {Fig. 4), 

 Fig. 4. or, rather, suspen- 



^-;*i]j5!=;5;^^^_^ sion ; and that but 

 --^''^JMidJ^^^ for the compres- 

 3 il sibility of the tim- 



ber, there would be no limit to the 

 space which a roof might be made 

 to cover. This compressibility takes 

 place at those points where the fibres 

 of the wood are pressed at right an- 

 gles, or nearly so, with their direc- 

 tion ; and many ways are adopted for 

 avoiding this inconvenience. There 

 is a species of roof, dependant in con- 

 struction on the principles we have 



^^A 



^^ 



just described, whicii we i^hall here 

 briefly notice, and whereof the fol- 

 lowing is a diagram {Fig. 5). This 

 roof has three Fig. 5. 



points of support, 

 A, B, A; the posts 

 A A, A A are call- ^ 

 ed quccn-posts ; the collar, A B A, is 

 here a slrainiug piece, instead of a 

 tie, as it was in the example of ties 

 first noticed, its operation being ex- 

 actly the reverse of a tie. The curb 

 or mansard roof is one in which a 

 story is obtained, as may be seen in 

 the annexed diagram (Fig. 6). Its 

 principles are the same 

 as those already men- 

 tioned, and do not here 

 require farther notice. 

 In the execution of ' 

 roofs the expense of trussing every 

 pair of rafters would be unneces- 

 sary, and the practice would also load 

 the walls with a far greater weight 

 than would be expedient ; it is there- 

 fore the custom to place these prin- 

 cipal parts of a roof at certain inter- 

 vals, which, however, should never 

 exceed ten feet. The rafters which 

 are actually trussed are called princi- 

 pal raficrs ; and by the intervention 

 o( the pzirlinc. A, in the diagram (i^/o-. 

 7), are made to Fig.'. 



bear the smaller 

 or common raf- 

 ters, which are 

 notched down on 

 it. These common rafters are re- 

 ceived by, or pitch upon a plate, B, 

 called a polc-plalc ; and the principal 

 rafters, which fall on the tie-beam, are 

 ultimately borne by the wall-plate, C. 

 When beams in either roofs or floors 

 are so long that they cannot be pro- 

 cured in one piece, two p, g 

 pieces, to form the requi- . — - j^ — , 



yaA Ir^n/TtK o*./^ r.^n^fn.1 tn t ^ k ' 



to- 



red length, are scarfed to- 

 gether by indenting them 1 — ' ^ 

 at their joints, and bolting them 

 gether, of which practice two modes 

 are here subjoined {Fig. 8)." 



R()(JK. Corciis frugilegus. The 

 gregarious crow, almost domesticated 

 in England in rookeries. They live, 

 for the most part, on insects. The 

 young are sometimes eaten. 



667 



