42 OF WOOD IN GENERAL. 



latitudes are, when dry, as heavy as water. 1 Most of the woods 

 in grade 6 of the above table grow within the tropics. 



Hardness of WOOd. Though in testing woods for engineering 

 purposes various resistances, such as stiffness or elasticity and 

 compressibility, have to be ascertained, hardness, or resistance to 

 indentation, is often estimated roughly. It may be expressed 

 with precision by the number of kilograms required to sink a 

 punch one centimetre square to the depth of 1*27 millimetres 

 (^5- of an inch) perpendicularly to the fibres of the wood, or by 

 the number of pounds per square inch to produce such an indent- 

 ation. Here too we may, perhaps, group all woods roughly into 

 six grades : 



1 . Hardest, such as the Iron-wood of India, Mfaua firrea, which 

 turns the edge of almost any tool, and Lignum-vitae (Gudiacum\ 

 which requires 793 kilograms to produce the standard indentation. 



2. Very hard, requiring more than 3200 Ibs. per square inch, 

 such as Hickory and good Oak and Elm. 



3. Hard, requiring from 2400 lbs.-3200 Ibs., such as Ash, 

 Walnut, Beech, Holly and Sycamore. 



4. Medium, requiring from 1600 lbs.-2400 Ibs., such as Douglas 

 Spruce. 



5. Soft, requiring less than 1600 Ibs., such as the majority of 

 coniferous woods, Pine, Spruce, Cedar, Poplar, Linden and 

 Chestnut. 



6. Very soft, such as the so called Cotton-tree of India (Bombax 

 malabdricum), which is so soft that a pin can be readily driven 

 into it with the fingers. 



Hardness and density or weight to a great extent vary together. 

 They also increase from the base of a stem up to its first branch, 

 and decrease from that point upward. 



Colour of wood. The colour of the heartwood affords in 

 many cases a useful aid in identification, while mere differences of 

 tint are often indicative of quality or soundness. The black 

 duramen of the Persimmon (Diospyros virginidna), of other species 



1 This is true when the contained air is not eliminated. For more pre- 

 cise estimates see Chapter VII. 



