ON ARCHITECTURE AND CARPENTRY. 16«> 



additional security. Such a strap ought always to be as straight as possible, 

 so as to act only in the direction of the force to be resisted: it has been too 

 customary to accommodate the strap to the form of the beams, or to make it 

 deviate in other ways from a right line: but wherever a strap is bent in any di- 

 rection, to a distance from a right line equal only to its depth in that direction, 

 its strength is so reduced, as not to exceed one seventh of what it would have 

 been, if it had remained straight. (Plate XtV. Fig. 172 . . 174.) 



It is equally necessary in all other cases which occur in carpentry, to avoid 

 as much as possible a transverse strain, the disadvantage of which is obvious 

 from the great inferiority of the strength of any substance, resisting a trans- 

 verse force, to its primitive cohesive or rcpulsive strength.. For similar 

 reasons, it is proper to avoid employing a very open angle at a point where a 

 load is supported, the great obliquity of the two pieces forming the angle re- 

 quiring them to exert a great force in order to oppose a much smaller one. 

 Allowance must also be made for the contraction of the timber, and care 

 must be taken that it do not so alter the arrangement of the parts, as to bring 

 a disproportionate strain on a point not calculated to support it. If the two 

 pieces forming an obtuse angle consisted, either wholly or partly, of woodcut 

 across the grain, and the piece joining their extremities were cut in the usual 

 manner, the oblique pieces would contract considerably more as they became 

 drier, and the angle would become more obtuse, so that the strain, produced 

 by a given weight, would be greater than in the original state of the triangle. 

 Sometimes the work is liable to be deranged by the operation of a lateral force, 

 which may have appeared too trifling to produce any considerable effect, bat 

 which may still destroy the greater part of the strength, by causing the re- 

 sistances to deviate from the plane of the forces which they are intended to 

 oppose. 



The framing of a roof is one of the most common and most important sub- 

 jects for the employment of the theory of carpentry. If the rafters were 

 simply to abut on the walls, they would force them outwards; a tie beam is 

 therefore necessary, to counteract the thrust. In order to enable the tie 

 beam to support a weight, a king post is suspended from the rafters; and 

 frequently braces arc again erected from the bottom of the king post, to sup- 



VOL. I. z 



