DECAY. 103 



Mr. John Knowles, who made a particular study of the means 

 most generally employed in seasoninjT timber, has given an account 

 of a series of experiments undertaken in the arsenals of Deptford 

 and Woolwich, to determine the rate of drying and ultimate degree 

 of dryness attained by timber variously treated — unprepared and 

 prepared by previous submersion in water. The pieces of timber 

 were placed vertically, now in the position they had occupied in 

 growing, now in that opposed to this ; and it was found that, circum- 

 stances the same, they dried more quickly in the former than n the 

 latter. The general results of these experiments were as follows : 

 1st. That the pieces of timber were best seasoned by being kept 

 about thirty months in the air, but in the shade and protected from 

 wet. 2d. That they lost more of their original weight after six 

 months' alternate immersions and dryings, than by being kept under 

 water for six months and then dried. Ship-builders are generally 

 agreed that it is not expedient to make use of timber until three 

 years after it is cut.* 



Duhamel advises strongly, that in ship-building all timber from 

 trees already on the decline should be rigorously rejected ; and this 

 the rather, that the most careful examination often fails at first to 

 perceive any alteration in the heart-wood of such trees, although it 

 never fails to show itself by and by at a sufficient interval after the 

 felling. This is undoubtedly a precept which it would be well to 

 bear constantly in mind ; but timber does not always carry within 

 itself the germs of its speedy decay ; and that which has been sea- 

 soned with the most scrupulous care, and was originally of the best 

 quality, does not escape the rot when it is placed under unfavorable 

 circumstances, any more than that which was of inferior worth and 

 less carefully treated. 



Wood appears to perish or decay through three principal a d ap- 

 preciable causes, which all require similar conditions to con d into 

 play, viz., stagnant air, sufficient warmth, and moisture. L;ke the 

 generality of organic substances, wood, when moistened in contact 

 with the oxygen of the air, and under the inlluence of a sufficiently 

 high temperature, undergoes decomposition of a kind which has been 

 compared to a slow combustion, upon which we shall find occasion 

 to say more by and by. It is with a view to escape this kind of de- 

 cay as much as possible that timber is never, or ought never, to be 

 employed in the construction of ships and buildings until it has been 

 thoroughly seasoned. 



Besides this first cause of decay, which may be prevented in a 

 great measure by using certain precautions, wood has still two re- 

 doubtable enemies, insects and certain plants of the family of the 

 cryptogamiaj. In one case, the wood perishes because it is fed upon 

 by certain animals which live and grow at its expense ; in the other 

 it decays because it serves as the soil to one crop of fungus after 

 another which luxuriate on its surface, while their roots penetrate 

 deeply into its interior. There is nothing in either accident which 



* Dupln, Ann. de Cliimic, t. i\Ti. p. 277. 



