THE STRENGTH OF TIMBER. 587' 



ually shrink in the breadth; while the next plank, shown in Fig. 0, 

 must have been derived from near the centre or heart of the tree, and 

 it will not shrink in the breadth but in thickness, with the full dimen- 

 sion in the middle, but tapering to the edges. 



The foregoing remarks apply more especially to the stronger ex- 

 ogenous woods, such as beech, oak, and the stronger firs. The softer 

 woods, such as yellow Canadian pine, are governed by the same law ; 

 but, in virtue of their softness, another law comes into force, which to 

 some degree affects their behavior, as the contracting power of the tu- 

 bular wood has sufficient strength to crush the softer medullary rays 

 to some extent, and hence the primary law T is so far modified. But 

 even with the softer woods, such as are commonly used in the con- 

 struction of houses, if the law is carefully observed, the greater part 

 of the evils of shrinking would be obviated. Hence, also, it is that 

 when a round block, as a mast, is formed out of a tree, it retains its 

 roundness because it contracts uniformly or nearly so, whereas, if a 

 round spar is formed out of a quartering of the same tree it will be- 

 come an oval, or otherwise contorted toward that shape. 



It would , not be in accordance with the object to enumerate 

 all the woods that are employed in the arts, therefore a few only 

 are selected, or such as are employed for purposes where strength 

 is the primary object, viz., ash, beech, elm, fir, hornbeam, mahogany, 

 oak, and teak. 



Ash is a coarse wood, but possessed of considerable strength, and 

 is distinguished for its great toughness and elasticity, and is usually 

 employed where severe shocks and wrenches have to be encountered, 

 such as for agricultural implements, the felloes and spokes of wheels, 

 and the shafts of carriages, for hammer-shafts, and for spring purposes 

 generally wherever wood is employed for that purpose. 



From its great flexibility it is seldom employed where rigidity is a 

 desideratum. The combination of strength with flexibility is the char- 

 acteristic of ash, and when the wood is from a young tree, or a tree 

 not too old, it is an invaluable wood in many respects ; but as the tree 

 becomes older, the change to brittleness sets in and soon renders it less 

 valuable. It is also remarkable for its endurance when kept dry, but 

 when exposed to damp or to wet it rapidly decays. The numerical 

 value of its properties varies considerably, but in general terms it 

 may be stated that, as compared with oak, good ash has frequently a 

 still greater tenacity and likewise a greater degree of toughness, but, 

 from its flexibility, especially when young, it has considerably less 

 stiffness, which unfits it for many purposes. 



Beech has frequently considerable strength, and is chiefly distin- 

 guished for its uniformity, its smoothness of surface, and closeness of 

 grain. It likewise possesses no little beauty, and takes a good polish, 

 more especially when its silver grain is skilfully exposed. "When well 

 seasoned and not too old, it is frequently used for the cogs of mill-gear- 



