348 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



wood changes superficially, and takes a deeper colour; it 

 oxidizes or undergoes combustion, but the action is exceedingly 

 slow. 



When the wood is placed in a warm moist situation, \vi 

 there is little or no ventilation, fermentation of the sap ta 

 place dry rot is the result. 



When wood is very wet, or is kept very wet, in a situation 

 where there is little or no ventilation, wet rot is the result. 



The characteristics of the two kinds of rot are found to differ 

 materially. 



Dry rot produces a brownish spot in the direction of the 

 fibres of the wood, which penetrates gradually to the heart. 

 The tissue appears swollen by moisture ; it remains compact but 

 is very easily cut, and then presents a smooth surface. 



When wood is attacked by wet rot it develops a fungus 

 of determinate character, and appears at parts the least exposed 

 to the light. 



Preserving Timber. The mechanical means usually adopted 

 for preserving timber used in marine works from the attacks 

 of the worm consist in covering the whole surface, between 

 high-water mark and the bottom, with flat-headed or scupper- 

 nails, which may be of iron, copper, or zinc, about 1 to 1 J inch 

 in diameter, and driven so close together that the heads touch 

 but do not overlap. This method is expensive both on account 

 of materials employed and the time required in the operation. 



A second plan is to cover the timber with zinc or copper 

 sheets, the timber being first tarred and then covered with 

 brown paper tacked on, and again tarred in the same way that 

 ships' bottoms are treated before being covered with the copper 

 or zinc sheathing. Either of these methods answer the pin | 

 well when applied to straight piles or timbers where no under- 

 water connections are required; but for framed struct 

 neither can be considered altogether suitable, inasmuch a> 

 the slightest defect in the covering will leave room for the 

 entry of the worm, and owing to the numerous cuttings nnl 

 bolt-holes that must necessarily occur for framing and fasten i; 

 such defects are practically unavoidable. 



Metal sheathing or close scupper-nailing to piles should 

 be carried down at least one foot below the ground line and up 

 to extreme high-water mark. 



At Flushing, copper sheathing is used for the protection of 



