50 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



malabaricum is so soft that a pin may be easily driven 

 into it, whereas Mesua ferrea is so hard that it turns 

 the edge of almost any tool ; between these extremes 

 we find all degrees of hardness, and it is the moderately 

 hard woods which are so useful for general purposes, 

 e.g. teak and oak. The dry weight of a timber is 

 usually not far out of proportion to its hardness, and 

 characters can sometimes be derived from a sort of 

 rough scale of weight the weight of a cubic foot or 

 metre, or some other unit being chosen for com- 

 parison. Thus, the wood of Erythrina suberosa may 

 weigh as little as 13 Ibs. the cubic foot, while that of 

 Hardwickia binata may reach 84 to 85 Ibs., and all 

 degrees of heaviness are found in different timbers 

 between such extremes. At the same time more in- 

 formation is needed as to the relative weights of equal 

 volumes of wood, as we have already seen how falla- 

 cious ordinary rough-and-ready weighings may be, 

 made, as they usually are, without any guarantee that 

 each specimen was dried to the same extent : on 

 the whole, we- may, perhaps, call any timber light 

 which, when air dry, weighs less than 30 Ibs. per 

 cubic foot, and moderately heavy if it reaches 40 

 to 50 Ibs. ; anything over 60 Ibs. is decidedly heavy. 

 The " closeness " or " porosity " of different timbers 

 bears an obvious relationship to their hardness and 



