12 TBOHJIOAL PHOPKRTIES OF WOOD. 



Sdl, siiudri, iron-wood, khair, sandal-wood, Terminalia tommtosa. 



III. Heavy. — Average weight of cubic foot = over 50 and up to 



60 lbs. 

 Sissu, black-wood, satin-wood, babul, box, Pterocarpus Marsupium 

 and indicus (Padouk). 



IV. Moderately heavy. — Average weight of cubic foot = ove?' 40 and 



lip to 50 lbs. 

 Schima Wallichii, mango, Hopea odorata, Anthocephalus Cadam- 

 ba, walnut, haldu, teak, mahogany, Lagerstromia Flos-Regince, 

 jack. 



V. Light. — Jvei-age weight of cxibic foot = over 30 and up to 40 lbs. 

 Michelia Champaca and excelsa, toon, Gmelina arborea, Pinus 



longifolia and excelsa, deodar, pop'ar, willows. 



VI. Very light. — Average weight of cubic foot = 30 lbs. and under. 

 Siinal, Sterculias, Ailantus excelsa. 



Section III. — Haedness. 



In respect of any wood we may say that its hardness is the re- 

 sistance it offers to penetration by another body. 



It is evident that this resistance is an entirely relative term, 

 and will be different not only according to the shape and nature of 

 the jeaetrating body (whether it is a point or edge, or a blunt pro- 

 jection, and so on), and the manner in which it is forced against 

 the wood (whether by impact or by mere constant pressure, &c.), 

 but also according to the direction in which, with reference to the 

 grain of the wood, the penetrating body is moved. The direction 

 may be (i), parallel to the fibres, or (ii), at right angles to them, or 

 (iii), oblique to them. In the first case the penetrating body may 

 be applied along, or at right angles to, the medullary plates. In 

 all three cases the resistance will be different according as the body 

 is forced into the wood on a longitudinal or on a transverse section. 



Whatever the direction in which penetration is attempted into 

 the wood, the resistance will depend on five several circumstances 

 as follows : — 



(a). IVie structure of the wood. — In the first place, hardness will 

 depend on the coherence with each other of the component elements 

 of the wood ; and for the same degree of cohesion, the closer to- 

 gether the fibres are, that is to say, the denser, or, in other words, 

 the heavier the wood is, the harder will it be. Lastly, anastomosis 

 and a wavy course of the fibres increase hardness, while shortness 

 of the fibres diminishes it. 



(6). Toughness of the fibres. — Tough fibres yield under pres- 

 sure without breaking ; they merely undergo a certain amount 



