48 TECHNICAL PRol'KKTIKS OF WOOD. 



"Wood shows the least strenj^ftli of all in resisting' shearing, 

 at'cordiug; to Fischer it is ouly 44 atmospheres. Beechwood is 

 said to possess the j,'reatest streu^^th of this description amouf^ 

 European woods. 



6. Traiisririic Stvcmjth. 



(a) General Account. — Transverse strength is the most import- 

 ant of all, and is requisite to resist the fracture by bending of 

 beams, joists, rafters, ladder steps, axle-trees, &c. It measures 

 the resistance which wood offers to breakage by a force acting at 

 right angles to its grain, and is the result of a combination of 

 a resistance to tension and to crushing. The transverse 

 strength of certain woods may be double, and even more than 

 double, that of others. The demands on the transverse strength 

 of wood for building purposes are most economically met by 

 placing the timber so that the greater transverse dimension is 

 in the plane of bending. When great transverse strength is 

 required, iron or steel may be substituted for wood. 



As regards the factors which affect the greater or less trans- 

 verse strength of wood, the following are the most interesting, 

 the results being taken from Bauschinger's and Tetmajer's 

 experiments. In the first place, it is clear that thorough sound- 

 ness of timber is an essential condition of its strength. 



(b} Anatomical Structure, — Straight-grained wood, free from 

 any defect, possesses comparatively high transverse strength ; 

 knotty wood, and wood containing resinous concretions, occluded 

 wounds, or other defects, is weak, and wood with wavy or twisted 

 fibre, according to Nordlinger, often possesses only two-thirds 

 of the transverse strength of straight-grained wood, otherwise 

 similar to it. 



(c) Specific Gravity. — The specific gravity of a wood is also of 

 great importance in determining its transverse strength, but 

 only in comparing woods of one species. Wood which is highly 

 resinous is brittle, and hence that of the black pine is deficient in 

 transverse strength as compared with that of many other conifers. 



(d) Locality. — As regards the influence of the locality in which 

 the tree has been grown, Tetmajer's experiments show that the 

 woods of silver- fir, spruce and larch grown on cold aspects are 



