534, 



CARPENTRY. 



CXXVII. 



Kg. 1- 



Fiff. 2. 



Construe- thickness of both halves, having a broad part in the 

 l ' ve middle, in order that the braces may abut the whole 

 ^ dimensions of their section ; or otherwise the two 

 *"V" abutments are made in the form of an inverted wedge 

 at the bottom, and rises cylindrically to the top, 

 where they are screwed and nutted. These modes 

 may be either constructed with one king-bolt in the 

 middle, or with two truss-bolts placed each at one- 

 third of the length from each end. When there are 

 two bolts, they include a straining piece in the mid- 

 dle. The two braces may either be constructed of 

 oak, cast or wrought iron : the latter material is, 

 however, very seldom employed. As wood contracts 

 less in length than most metals, oak is better for the 

 purpose than cast iron ; but then the parts of the 

 core must be so much stronger. As to the bolts, 

 wrought iron is indispensable. It is obvious that 

 the higher the girder the parts will receive the less 

 stress, and consequently there will be less risk of 

 giving way by heavy weights, or long bearings. 



Fig. 1. is the section of a girder, with two braces 

 and king-bolt. 



Fig. 2. is the section of a girder, with a straining 

 piece, two braces, and two truss-bolts, of the most ap- 

 proved form, and certainly the best. No. 1. repre- 

 sents the girder laid open, in order to show the core ; 

 No. 2. shows the two halves bolted together ; No. 3. 

 shows the edge of a washer ; No. 4. the face of the 

 same ; No. 5. represents the side of the butmental 

 bolts in the transverse direction of the girder ; No. 6. 

 the side of the same in the longitudinal direction ; 

 No. 7. represents the transverse direction of the truss- 

 bolts or king-bolt ; and No. 8. the longitudinal di- 

 rection of the same. 



Fig. 3. The section of a girder, calculated from 

 its rise to sustain very heavy weights. If the tie 

 beam is very strong, the abutments may be wedged ; 

 but then it must be observed, that the wedge ought 

 to be very long, that the laper may be very small, so 

 as to have no inclination to rise, ajid then the excess of 

 length may be cut off afterwards ; however, the bolts 

 represented at No. 5. and No. 6. Fig. 2. are to be 

 preferred. 



Fig. 4v 'represents the end of a girder, with part 

 of the bending joists on each side of it. No. 1. shews 

 the part of the bending joist fitted in, and the man- 

 ner of cutting the mortice and tenon. The tenon 

 thus formed, with the sloping shoulder with the 

 small projection below the tenon, is said to be tusk- 

 ed ; and the mortice cut to receive the tusking, is 

 said to be housed. 



Fig. 5. represents the method of cocking beams 

 upon wall-plates, according to the best method, which 

 precludes every possibility of the beam being drawn 

 from, or pushed farther over the wall-plate, with- 

 out tearing the rising part away from the wall- plate, 

 or splitting the piece away between the notch and 

 the end, on the under side of the beam. The prac- 

 tice in former times was by a dovetail, the planes 

 of the joint being perpendicular and parallel" to the 

 horizontal sides of the beam, two of the said' planes 

 being inclined to the vei tical sides, at an equal an- 

 gle to each adjacent side, and forming the dovetail 

 at the bottom, or on the horizontal sections at the 

 end of the- beam, so as to be wider at the extre- 

 :i>ity than at the shoulder. If the timber were with- 



Jiig 



out shrinking, and the joint closely fitted together, Construc- 

 this would be an excellent method ; but as we are tive 

 certain that the timber does shrink, a small de- Carpec 

 gree of shrinking will allow a very great draught. "* 

 No. 1. represents part of the longitudinal direction 

 of the beam, and section of the wall-plate ; No. 2. 

 represents part of the longitudinal direction of the 

 beam turned up- side-down, to shew the notching. 



Wall-plates are pieces of timber laid on the walls, Wall- 

 in order to distribute the pressure of the roof equally plates; 

 upon, and to bind the walls together. The wall- 

 plates are sometimes called raising plates. 



Trusses are strong frames of carpentry resolved Trussed 

 into two, or a series of triangles, so as to make the 

 truss act as a solid .body, and thereby support cer- 

 tain weights, each at a given immoveable point, the 

 truss itself being suspended from two such immove- 

 able points. The trusses of roofs are constructed 

 generally of a triangular form, and disposed equi- 

 distantly on the wall-plates, in parallel vertical planes 

 at right angles to the walls. The top of the op- 

 posite walls are the two points of suspension, and the 

 weights supported by the truss at the immoveable 

 points are horizontal pieces of timber running trans- 

 versely to the planes of the trusses ; the horizontal 

 pieces of timber support other equidistant pieces pa- 

 rallel to the upper sides of the trusses ; and these 

 last timbers support the covering, or the covering 

 and timber work, to which the covering is fixed. 

 In a truss, some pieces of timber are in a state of 

 tension, and some are in a state of compression ; but 

 a piece of timber, which is neither extended nor com- 

 pressed, is useless. If a quadrilateral figure, or frame, 

 be so constructed, that each two adjoining timbers 

 be made moveable round a point at their intersection, 

 it may be put into an infinite number of forms, be- 

 cause the whole frame will be revolvable about the 

 angles ; but if any one of the angles be immoveable, 

 the whole quadrilateral frame will be immoveable. 

 Two pieces of timber forming an angle, and revol- 

 vable round a point at their intersection, may be 

 made immoveable by fastening each end of a bar to 

 each leg, or by taking any two points in the bar, 

 and fastening each point to each leg. Now, if a 

 force be applied at any of the three angular points, 

 the triangular frame will be immoveable ; but one of 

 the two legs which form the angle will be in a state 

 of tension, and the other in a state of compression, 

 provided that the direction of the force applied does 

 not fall within the angle produced ; but if the line 

 of direction of the force applied fall within the angle 

 of the triangle, then both legs are either in a state of 

 tension, or in a state of compression, according as the 

 force applied is pulling or pressing. If one of the 

 sides of the triangle be lengthened without the boun- 

 dary, and a force be applied transversely to the part 

 so lengthened in the plane of the triangle, this force 

 will bend the side of the triangle, which is in the 

 straight line with the side to which the force is ap- 

 plied ; therefore, suppose again a quadrangle or qua- 

 drilateral revolvable about the angles, and a bar be 

 fixed to any two sides forming an angle, viz. a point 

 in the bar to a point in one of the legs, and another 

 point in the bar to a point in the other leg ; and sup- 

 pose the two points not to be in two of the angles, 

 or one of the points to be in the side, at some 



