MECHANICAL PROPERTIES. 275 



d inches. The toughest and most flexible stool-shoots are those 

 of Osiers, Willows, Ash, Hazel, Birch, Chestnut, Lime, Aspen, 

 Oak, and Elm ; while the toughest and most flexible timber is 

 Ash, Birch, Willow, Poplar, Kohinia, Hornbeam, and Elm. 

 Beech becomes tough and flexible when steamed. 



3. FISSIBILITY is the ease with which wood can be split by 

 driving in a wedge parallel to the run of its fibres, and is im- 

 portant in cooperage, match-making, splitting rails, &c. Eissi- 

 bility is greatest in long and straight-fibred woods, and least 

 where the fibres are short and strongly lignified ; and the drier 

 and more elastic the wood, the easier it is to split (except in 

 Willow and Poplar, in which a wedge holds better when the 

 wood is moist). The greater the fissibility, the more difficult 

 it is to obtain a fine smooth surface by planing. As regards 

 fissibility our woods are classifiable as follows : 



Easy to split : Chestnut, Eobinia, Alder, Lime, and Conifers 



(unless very resinous). 

 Medium: Oak, Ash, Beech. 

 Difficult to split : Hornbeam, Elm, Maple and Sycamore, 



Birch, Willow, Poplar. 



4. HARDNESS is the resistance offered to the penetration of 

 another body, and is usually characteristic of heavy woods, the 

 hardness increasing with the strength and cohesiveness of 

 the woody fibres. The degree of hardness manifested varies 

 with the kind of tools used in wood-working (axe, knife, 

 plane, saw, rasp, nail, &c.), as some work parallel to the run 

 of the fibres and others at right angles ; but the following is a 

 rough general average : 



Hard: Robinia, Maple and Sycamore, Hornbeam, peduncu- 

 late Oak. 



Moderately hard: Ash, sessile Oak, Chestnut, Elm, Beech, 

 and Birch ; also resinous narrow-ringed Larch and Pine. 



Soft : Conifers ; Horse-Chestnut, Alder, Willow, Poplar, 

 Lime. 



