CARPENTRY. 



Contrnc- tance from either end, the quadrilateral will he divi- 

 nve ,i,.j into two parts, one will always be a triangle ; 

 tlu-n, if the figure be supported at twro of its angles, 

 aiui a force be applied in the plane of the figup- to 

 the angle opposite, the angle which hat the bar fixed 

 to its legs, so that the direction of the force thus ap- 

 plied does not tend to the fixed point, which is the 

 farther extremity of the one leg where the force is 

 applied to the- angular point at the other extremity ; 

 all the sides of this figure will be bent, and the bar 

 thus fixed will occasion transverse strains to the 

 sides : But if the bar be fixed to two opposite angles, 

 and if the frame be held immoveable at one of the 

 angular points where the bar is fixed, and also at 

 one of the other angles at the extremity of one of 

 the legs of the said angle, and a force in any direc- 

 tion in the plane of the figure be applied to the 

 angle where the frame is unsupported, and where the 

 bar is not fixed, the frame will be by this means ren- 

 dered immoveable, and the force by this disposition 

 will not occasion any transverse strain on the sides 

 of the frame. Suppose the frame to be pentagonal, 

 and a bar fixed in like manner to two angles, at the 

 ends of two adjoining sides, these two adjoining sides 

 and the bar will form a triangular compartment in 

 the figure. If the frame be suspended by two of the 

 angles of the triangle, the three remaining sides will 

 be moveable, at the extremities of the bar, round the 

 remaining angles ; but if another bar be fixed to any 

 one of the three angles of the triangle at one end, and 

 to one of the angles of the other three sides, to form 

 another triangle, three of the sides of the pentagonal 

 frame will be made immoveable, and the two remain- 

 ing sides will be 80 likewise. In like manner, of 

 whatever number of sides the frame consists, by first 

 forming a triangle of two of the sides, and fixing a 

 second bar from any angle of the triangle to one of the 

 other angles of the figure, at the remote ends of two 

 adjoining sides of the frame, there will be formed ano- 

 ther immoveable side, and another immoveable point at 

 the next angle of the frame will be given ; if from this 

 fixed point, or any of the other three points, which 

 are the angles of the triangle, the end of a third bar be 

 fixed, and the other end of the bar to one of the re- 

 maining angles of the frame, so as to form a triangle 

 with the second bar, and one of the adjoining move- 

 able sides of the frame, or a triangle, with one of the 

 fixed sides of the frame and the adjoining moveable 

 side, and by proceeding in this manner successively, 

 until all the sides are fixed, the frame will be made 

 immoveable ; so that if any two angles of this frame 

 be supported, and a force or forces be applied at one 

 or each of the angles in the plane of the figure, the 

 whole figure will be immoveable. Frames of a tri- 

 angular form, which have to resist only a simple 

 force, or support one weight, are most simple and 

 best constructed of three sides, the frame being sus- 

 pended from two angles, and the force or weight to 

 the other. A triangular frame, supporting only one 

 weight, has no occasion for any subdivisions to com. 

 part the external space, provided the compressed 

 timber or timbers were inflexible, so as to support 

 their own weight without bending, and the tensile 

 timbers incapable of extension. Though a frame 

 should have to support several weights, the external 

 /igure may be of any form whatever, provided that 



the points from which the weights arc hung, and the 

 two points from which the frame is suspended, be- all 

 -iy supported, by comparting the figure 

 with timber divisions, and thereby forming a succes- 

 sion of adjoining triangles, of which each two conti- 

 guous ones have a common side ; that is, when tw 

 the angles of each of the adjacent triangles are coinci- 

 dent. It may be proper to observe here, that thougk 

 it may not be at all times eligible to divide a fr. 

 so that all the compartments will be triangles, yet 

 the succession must not by any means be discontinued, 

 by the intervention of quadrilateral or polygonal fi- 

 gures ; but these compartments may adjoin without 

 injury to the truss. The triangle is the most simple 

 of all rectilineal figures ; it is also easier constructed, 

 and better adapted to the discharge of rain or mois- 

 ture in a roof, than any other figure ; but in adopting; 

 it for large buildings, as several weights must be 

 supported, and as there is only one point from which 

 this weight can be suspended, it becomes necessary 

 to take other equidistant points in the sides, in or- 

 der to support the covering equally. These point* 

 may be made stationary by the former means, of di- 

 viding the interior space into a succession of triangu- 

 lar compartments. But if the two upper sides of the 

 frame be of equal lengths, and equally inclined to 

 the horizon, the opposite points may be made to 

 counteract each other, without a concatenation ot 

 triangles, by introducing timbers from point to point 

 parallel to the horizon : in this the compartments will 

 be trapezoids, except the upper one, which will be 

 a triangle. These beams may be supported by verti- 

 cal bolts passing transversely through them from the 

 points where the weights are supported, and the bolts 

 may be nutted below the beams. This mode of se- 

 curing the points of support depends entirely upon 

 the dottrine of equilibrium ; and thus a very little 

 difference from the equality of forces might easily 

 occasion a change of figure, to which the other me- 

 thod, by a series of triangles, is not liable. The se- 

 curing of the points of support by beams is not con- 

 fined to triangular frames ; but may be applied to 

 roofs having two or several rafters upon each side, so 

 that their lengths and inclinations are equal, and their 

 junctions on the same bevel. The beauty of every 

 truss is to dispose the timbers in positions as direct 

 to each other as possible. Oblique directions require 

 timbers of large scantlings, and exert prodigious 

 thrusts on the abutments, so as to compress the jog* 

 gle pieces, and render the truss in danger of sagging. 

 Trusses are variously constructed, according to the 

 width of the building, the contour of the roof, and 

 the circumstances of walling below. 



Tie, is any piece of timber in a state of tension, 

 viz. a piece of timber which is acted upon by two 

 forces which have a tendency to a more remote dis- 

 tance. 



Straining-piece, is any piece of timber compressed 

 by two opposite forces, which have a tendency to a 

 nearer approach ; hence ties and straining pieces are 

 affected in contrary directions. A straining piece 

 requires an inflexible piece of timber ; but a chain, or 

 rope, or small bar of iron, will answer the purpose of 

 a tie, provided that it is incapable of any farther ex- 

 tension than being drawn into a straight line. 



Tie beam, the timber of the foot of the truss cm 



