48 



TIMBER. 



4. Soft woods requiring less than 1,600 pounds per square inch to 

 produce an indentation of one-twentieth inch: The greater mass of 

 coniferous wood; pine, spruce, fir, hemlock, cedar, cypress, and red- 

 wood; poplar, tulip, basswood, butternut, chestnut, buckeye, and 

 catalpa. 



CLEAVABILITY. 



When an ax is struck into a piece of wood as shown in fig. 33 the 

 cleft projects beyond the blade of the ax and the process is not one 

 of cutting, but of tension across the grain. The ax presses on a 

 lever, a b, while the surface in which the transverse tension takes place 

 is reduced almost to a line across the stick at b. If the wood is very 

 elastic, the cleft runs far ahead of the ax, the lever arm a b is long, and 

 the resistance to splitting proportionately small. Elasticity, therefore, 

 helps splitting, while great shearing strength, a good measure for 



transverse tension and hardness hinder it. 



Wood splits naturally along two normal planes, 

 the most readily along the radius, because the 

 arrangement of fibers and pith rays is radial, and 

 next along the tangent, or with the annual rings, 

 because the softer spring wood forms continuous 

 planes in this direction. Cleavage along the 

 radius, however, is from 50 to 100 x>er cent easier, 

 and only in case of cross grain, etc., the cleavage 

 along the ring becomes the easier. In the wood 

 of conifers, wood fibers and pith rays are very reg- 

 ular, the former in perfect radial series or rows, 

 and cleavage is, therefore, very easy in this direc- 

 tion. The same is brought about in the oak by the 

 very high pith rays, but where they are thick and low, as in sycamore, 

 and generally in the butt cuts and about knots, they impede cleavage 

 by causing a greater irregularity in the course of the wood fibers. The 

 greater the contrast of spring and summer wood, the easier the cleav- 

 age tangentially or in the direction of the rings. This is especially 

 marked in conifers and also in woods like oak, ash, and elm, where the 

 spring wood appears as a continuous series of large pores. Very slow 

 growth influences tangential cleavage, narrow-ringed oak breaks out 

 and splits less regularly even in a radial direction ; in conifers, however, 

 this difference scarcely exists. Weight of wood affects the cleavage 

 but little; in heavy wood the entrance of the ax, to be sure, is resisted 

 with more force, but the greater elasticity of the wood, on the other 

 hand, counterbalances this resistance. Irregularities in the course of 

 the fibers, whether spiral growth, crossgrain, or in form of knots, all 

 aid in resisting cleavage. Knotty bolts are split more easily from 

 the upper end, since the cleft then runs around the knots (see p. 23). 

 Moisture softens the wood and reduces lateral adhesion, and therefore 

 wood splits more easily when green than when dry. 



Fig. 33.— Cleavage. 



