THE STRENGTH OF TIMBER. 583 



It will only be necessary here to speak of the powers of resistance 

 of a few among the many kinds of wood now employed in the me- 

 chanical arts. The greater number of the varieties of wood owe their 

 commercial value to special characteristics, such as beauty of grain 

 and capability of being polished the description of which does not 

 fall within the scope of the present article. 



As a general rule, we may judge of the hardness of a wood by its 

 specific gravity, if it is in its natural state. But the density may be 

 increased by artificial compression, and this increase of density is gen- 

 erally accompanied by increase of strength. Some varieties of wood, 

 as, for instance, lignianvitce, are so dense that they sink in water, 

 while some of the softer woods have not half the density of that fluid. 

 The presence of gum or resin in any wood adds both to its strength 

 and durability. Many woods will last a long time if kept constantly 

 under water, but scarcely any wood is very durable when allowed to 

 become wet and dry alternately. 



The strength of a piece of timber depends upon the part of the tree 

 from which it is taken. Up to a certain age, the heart of the tree is 

 the best ; after that period, it begins to fail gradually. The worst 

 part of a tree is the sap-wood, which is next the bark. It is softer 

 than the other parts of the wood, and is liable to premature decay. 

 The deleterious component of the sap-wood is absorbed, if the tree is 

 allowed to grow for a longer period, and in time the old sap-wood be- 

 comes proper timber-fibre similar to heart-wood. Hence, the goodness 

 of a tree, for timber purposes, depends on the age at which the tree 

 was cut down. When young, the heart-wood is the best ; at maturity, 

 with the exception of the sap-wood, the trunk is equally good through- 

 out ; and, when the tree is allowed to grow too long, the heart-wood is 

 the first to show symptoms of weakness, and deteriorates gradually. 



The best timber is secured by felling the tree at the age of matu- 

 rity, which depends on its nature as well as on the soil and climate. 

 The ash, beech, elm, and fir, are generally considered at their best 

 when of 70 or 80 years' growth, and the oak is seldom at its best in 

 less time than 100 years, but much depends on surrounding circum- 

 stances. As a rule, trees should not be cut before arriving at maturity, 

 because there is then too much sap-wood, and the durability of the tim- 

 ber is much inferior to that of trees felled after they have arrived at 

 their full development. 



The strength of many woods is nearly doubled by the process of 

 seasoning, hence it is very thriftless to use timber in a green state, as 

 it is not only weak, but is exposed to continual change of bulk, form, 

 and stability. After timber is cut, and before it is properly seasoned, 

 the outside is found to crack and to split more than the inside of the 

 mass, because it is more exposed to the desiccating effect of the sur- 

 rounding atmosphere, but, as the outside dries, the air gradually finds 

 its way to the interior. If timber is cut up by the saw when green, 



