28 TIMBER AND SOME OF ITS DISEASES, [chap. 



will gradually increase in weight again as it imbibes 

 moisture. 



Now it happens that the weight of a piece of timber, 

 compared with that of an equal volume of some 

 standard substance — in other words, the so-called 

 specific weight — is of very great importance, because 

 several other properties of wood stand in relation with 

 it, e.g. the hardness, durability, value as fuel, tendency 

 to shrink, &c. Fresh-cut timber in very many cases 

 contains on an average about 45 to 50 per cent, of its 

 weight of water, and if " seasoned " in the ordinary 

 way this is reduced to about 15 to 20 per cent; but 

 the fresh timber also contains air, as may easily be 

 shown by warming one end of a piece of fresh wood at 

 the fire or in hot water and watching the bubbles driven 

 out, and the seasoned timber contains less water and 

 more air in proportion, so that we see how many 

 sources of error are possible in the usual weighings of 

 timber. At the same time, many comparative weigh- 

 ings of equal volumes of well-seasoned timber do yield 

 results which are of rough practical use. 



The fact is that the so-called " specific weight *' of 

 timber, as usually given, is not the specific gravity of 

 the wood-substance, but of ihzXplus entangled air and 

 water. It is interesting to note that, although we 

 associate the property of floating with wood, timber 



