WEIGHTS OF WOODS 35 



places of decimals. A more useful form of statement, however, 

 is perhaps the weight of a standard cube, either that of a cubic 

 foot in pounds or that of a cubic decimetre in grams.^ Thus, 

 while water weighs 62-321 lbs. per cubic foot, timbers range from 

 13 to 85 lbs. per cubic foot. 



They may be grouped from this point of view in the following 

 six grades: 



Approximate weiglit 



of 1 cubic foot Specific' 



in pounds. gravity 



1. Very light, not exceeding 24 -26— '4 



Spruce, Willow and most Poplar. 



2. Light, - 24-30 •4— '5 



Northern Pine, Hemlock Spruce, Linden, Chestnut. 



3. Medium, 30—36 -5— -6 



Pitch-pine, Douglas Spruce, Sycamore. 



4. Heavy, --...-.. 36—42 -6— '7 



Most Birch, Beech and "Walnut. 



5. Very heavy, - . 42—48 •7__-s 



Hornbeam, Hickory and good Ash and Elm. 



6. Heaviest, - - - - ^ - _ - Above 48 Above 'S 



Some Oak, most Teak, Mahogany, Jarrah, Mora and Greenheart. 



Whilst such kiln-dried weights as those employed here range 

 from a specific gravity of -26 in Ficus aurea, or 13 lbs. to the 

 cubic foot in Erythrina suherosa, to 1-3 in Black Iron-wood (Go7i- 

 ddlia ferrea), or 85 lbs. to the cubic foot, as in Anjan {Hardwickia 

 Undta), none of the native woods of temperate latitudes are, when 

 dry, as heavy as water.^ 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 stifiness 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 

 {■^■^ of an inch) perpendicularly to the fibres of the wood, or by 

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

 tion. Here too we may, perhaps, group ah woods roughly into 

 six grades : 



1. Hardest, such as the Iron-wood of India, Misua ferrea, which 

 turns the edge of almost any tool, and Lignum- vitse {Gudiacum), 

 which requir3S 793 kilograms to produce the standard indentation. 



^ To facilitate the conversion of one measure into the other it may he noted that 

 1 cubic foot=nearly 28 J (28*315) cubic decimetres, and 1 pound avoirdupois =453^ 

 (453-592) grams. 



2 This is true when the contained air is not eliminated. For more precise esti- 

 mates see Chap sr YII. 



3—2 



