TIMBER 283 



the spring growth predominates, making the wood somewhat weak. 

 If, on the contrary, the number of rings per inch be many, a slow 

 growth is indicated and there is a greater amount of the dense, strong 

 summer and fall wood. The number and character of the annual 

 rings may thus give some idea of the strength of a piece of timber. 



235. Heartwood and sapwood. The heartwood of a tree may be 

 considered a lifeless conical core, which is increased each year by the 

 addition of a portion of the outer sapwood. Both the sapwood and 

 heartwood contain small tubes that extend from the roots of the tree 

 t<> the branches. These tubes in the sapwood carry water charged 

 with nourishment to the branches and growing parts of the tree. In 

 the heartwood the tubes no longer act as conveyors, although they 

 still contain moisture. The heartwood is the mature wood and is 

 more valuable for structural purposes. 



236. Effect of moisture. It is well known that green wood is not 

 as strong as the same wood when seasoned, which indicates that the 

 effect of moisture in timber is to lessen its strength. A live tree as 



;ii(ls in the forest contains a great deal of moisture. When it 

 has been cut, sawed, and dried, most of this moisture has evaporated, 

 but considerable still remains, and however well seasoned timber may 

 be, it will still contain some moisture. 



In making tests of timber, therefore, it is necessary to determine 

 the percentage of moisture in order that the results may be compared 

 with the results of other tests. This is determined by cutting a 

 small piece from the uninjured portion of the test piece and weighing 

 before and after thorough drying. The difference in weight divided 

 l>y the dry weight and multiplied by 100 gives the percentage of 

 moisture. 



237. Strength of timber. The strength of timber depends upon the 

 amount of heartwood or sapwood, knots (sound or loose), wind shakes 

 and checks, cracks, or any defect that breaks the continuity of the 



In general, the strength of timber is indicated by its weight, 

 tin- heaviest timbers being the strongest. Timber is strongest along the 

 both in tension and compression, as will be seen in what follows. 

 It lias been found that values obtained for the strength of timber 

 l.v testing small, carefully selected test pieces are much higher than 

 obtained by testing large commercial timbers. This is what 



