18 TEOHNIOAL PROPERTIES OF WOOD. 



firewood. In what degree the wood of any individual of a given 

 species is fissile can be easily recognised on the standing tree itself, 

 which, to split well, ought to have a long, clean, straight, full and 

 symmetrical bole, and a not too thick bark containing wide, long 

 and straight cracks that have a tendency to extend upwards. The 

 soil and locality also furnish indications. In the case of a felled 

 tree, besides having the points already enumerated, we can also 

 examine a small ribbon of the wood taken off with a plane. A 

 crack, however small, through the centre on the transverse section 

 is a certain proof of easy fissility. Woodmen have often to put up 

 with the disagreeable experience of seeing a tree split up and 

 fall before it is sufficiently cut through. Species whose indivi- 

 duals play this unpleasant trick are always easy to split. 



As a beginning, and subject to numerous additions and correc- 

 tions, we may establish the following classes for India according to 

 aptitude for fission : — • 



1. Extremely fissile — bamboos, canes. 



2. Easily fissile — teak, Anogeissus latifolia, deodar, tun, the firs. 



3. Pretty fissile — mango, Pterocarpus Marsupiwn. 



4. Difficult to split — sal, babul, Terminalia tonientosa. 



5. Very difficult to split — Terminalia belerica, JBoswellia serrata. 



Section VI. — Strength. 



By the term strength is understood the degree of resistance 

 which a given kind of wood offers — (i), to being broken across the 

 grain {transverse strength), or (ii), to crushing, or (iii), to being 

 torn asunder by a shearing force, or (iv), to being twisted. In 

 discussing the strength of woods the mathematical side of the 

 question will not be touched upon, belonging, as it does, to the 

 subject of practical mechanics. 



I. Transverse strength. — For our purpose the resistance which 

 wood offers against a transverse strain stands in the first place, 

 for it is principally this resistance which has to be considered in 

 all timber for roofing, scaffolding, floors, carriage building, ladders, 

 &c. In a general way it may be said that the heavier tlie wood, 

 the greater the transverse strength. But this general rule, al- 

 though nearly always true for specimens of one and the same spe- 

 cies, is subject to modification according to the structure of the 

 wood and the cohesiveness of the fibres. Length and straightness of 

 fibre and uniformity of texture contribute to transverse strength. 

 Moreover, whatever increases elasticity and flexibility, increases also 

 transverse strength. Great abundance of resin, especially in a dry 

 condition, is a cause of weakness. In one and the same tree, pro- 



