ECONOMIC WOODS OF THE UNITED STATES 49 



of fibre, and condition. A knot on the upper side is compressed, 

 while one on the lower side is subjected to tension. The knot, 

 especially (as is often the case) if there is a season check in' it, 

 offers little resistance to tensile stress. Small knots, however, 

 may be so located hi a beam as actually to increase its strength 

 by tending to prevent longitudinal shearing. Knots in a board 

 or plank are least injurious when they extend through it at right 

 angles to its broadest surface. Knots apparently have little 

 effect on the stiffness of timber. 



"At the junction of limb and stem the fibers on the upper and 

 lower sides of the limb behave differently. On the lower side 

 they run from the stem into the limb, forming an uninterrupted 

 strand or tissue and a perfect union. On the upper side the fibers 

 bend aside, are not continuous into the limb, and hence' the 

 connection is imperfect. 



"Owing to the arrangement of the fibers, the cleft made in 

 the splitting never runs into the knot if started on the side above 

 the limb, but is apt to enter the knot if started below, a fact well 

 understood in woodcraft." * 



Sound knots are as hard as, and usually considerably harder 

 than, the wood surrounding them. In coniferous woods they are 

 commonly highly resinous, and in finished lumber are apt, on that 

 account, to fail to retain paint or varnish. When such trees 

 decay the knots remain sound and are prized for fuel. In 

 grading lumber and structural timber, knots are classified accord- 

 ing to their character (sound, loose, encased), size (pin, standard, 

 large), and direction of fibre (round, spike). 



References 

 Roth, Fimbert: Timber, Bui. 10, U. S. Div. For., 1859, pp. 23, 41, 44 



48, 49. 

 Gline, McGarvey, and Knapp, J. B.: Properties and Uses of Douglas Fir, 



Bui. 88, U. S. Forest Service, 1911, pp. 32-37. 



DENSITY AND WEIGHT 



Density of wood varies widely in different species, in different 

 individuals, and even in different portions of the same tree. 

 The specific gravity f of wood substance is about 1.6; hence the 



* Roth, loc. cit., p. 23. 



t By specific gravity is meant the ratio of the weight of thoroughly dried 

 4 



