346 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



begins to prey upon the wood as soon as it is put into the 

 water. 1 



The Xycophago, a small bivalve mollusc, is also very destruc- 

 tive, entering the wood when young, and growing to maturity 

 inside. 2 



Two theories have been advanced to explain the cutting 

 powers of these creatures, one chemical and the other mechanical; 

 but traces of acid solvents are only found in some calcareous 

 borers, and they also occur in animals which do not bore. On 

 the other hand, silicious cutters have been found on some borers, 

 such as the Teredo? 



The rate at which the worm will destroy timber will 

 depend greatly upon the locality, and whether there are 

 alternations or admixtures of fresh water. In cold water, the 

 destruction will not be so rapid as in warm latitudes. At 

 Greenock, a pile 13 inches square was eaten through in seven 

 years. At Portsmouth, some of the stage-piles used during 

 the extension of the dockyard were destroyed in about five 

 years. 



In Malta Harbour, isolated, unprotected stage-piles of white 

 " Triest " timber 13 to 14 inches square were reduced, in between 

 three and four years, to less than one-fourth the original dimen- 

 sions in fact, destroyed for all practical purposes by the 

 Ckelura terebrans, while the rate of destruction from the same 

 pest, over the area of close-piled dams, was observed to be over 

 one inch per annum. 



Mr. Stevenson found that the Memel timber used at the 

 Bell Rock Lighthouse was destroyed by the Limnoria terebrans 

 at the rate of about one inch per annum. 4 



In the Gulf of Mexico, it is recorded that the marine 

 worms cut off an unprepared pile in eight months. 5 



At Port Darwin, South Australia, Mr. James found that 

 chafing-pieces of kerry wood, a West Australian timber, were 

 soon attacked, and after about three years completely destroyed 

 at the lower ends by the Teredo, the ravages of which extended 

 to about halfway between high- and low-water mark. 6 



Of the timber usually employed in marine works, greenhearfc, 



1 Hurst's " Carpentry," 2nd. ed., p. 377. 



2 Professor Mclntosh, in Engineering, vol. xxxviii. p. 436. 



3 Ibid., p. 436. 4 Stevenson's " Bell Rock Lighthouse." 



Engineering, vol. 1. p. 607. 6 M.r.LC.K, vol. ciii. p. 338. 



