292 
TIMBEE 
logs, and it in useless specifying, as is sometimes done, that 
the ordinary building timber is " to be free from knots "; 
loose knots are certainly to be avoided, but one seldom 
finds these in sound pine timber. 
Very large knots, especially if at the edges of timber 
which has to stand heavy strains, such as beams or 
columns, are a cause of considerable risk ; it may safely be 
said that in the majority of cases where timber is tested as 
beams or columns it fails at knots. 
Fig. 49 will show the reason for objection to a knot on 
the underside of a beam carrying a load, where it is 
much more serious than if on the top side. In bending, 
Fig. 49.— Showing risk of knots on underside of loaded beams. 
the fibres on the underside of the beam are subjected to 
tension, the fibres of the wood around the knot like those 
in a piece of cross-grained timber ofier but little resistance 
to tension, and the presence of a knot in this position has 
the same effect as putting a saw-cut on the underside of the 
beam. 
A knot is the place where a twig or branch has projected 
Jrom the parent stem ; some timbers are to be got in con- 
siderable length with few knots, such as Oregon, Californian 
redwood, Tasmanian blue gum, kauri, and others, the 
reason being that these trees do not bear branches within a 
considerable distance of the ground, whereas European 
trees, pines and fiis used in building, especially the fir& 
