95 



wliere the bark IkuI liecn broken off in tin- liandlinu. In two years the Limnoria attack 

 in one specimen was H^ inches deep, antl some Ban kin galleries M) inches long and 

 ^'g inch in diameter had developed. 



Fig. 36. Pik- from .\^Sll^i.ltlMl < )i| (O. \\ h.irf, I'cirt Costa, (lii\iii wii 

 removed in 1921. Borers atUickeil where Ixirk proteelion was daiiKiyei 



I k on, presumably in 1910, 



It cannot be definitely stated that bark covered piles have never given good 

 results, but the evidence which has come to the attention of the Committee indicates 

 that the additional life to be expected by lea\ing the bark on is uncertain and not over 

 one or two years. 



METALLIC rRUTECTIOXS FOR WUOUEN PILES 

 Scupper N.viling 



An ancient method of protecting exposed wood is that of driving fiat headed nails 

 into the wood so as to practically cover the surface with metal. In some cases the 

 nails are spaced so that the heads cover only a portion of the surface — possibly 50 per 

 cent — it being expected that the formation of rust will sufficiently cover the exposed 

 wood, and that the nails in the interior will retard the borer action. 



In San F~rancisco Bay the borers have been found to attack rust impregnated 

 wood around bolts and nails; the interior borers simply change their course to miss the 

 metal obstruction ; with repeated heavy infestation the wood is thoroughly honey- 

 combed, and the Limnoria completes the destruction, if it does not begin it. The 

 Santa Fe Railway and the Spreckels Company used this type of protection in structures 

 in San Diego Bay built in 1881-3 and 1898 respectively. Some of the piles are still in 

 place and serviceable. They have since discarded it for other types of piling; but the 

 superintendent originally in charge of the experiment for the Santa Fe Company 

 says it was only because of the prohibitive rise in the cost of the boy labor employed 

 in the nailing. The method has undoubtedly some value, but this is not sufficient to 

 be compared with that of other available protections, and the method has now no 

 importance except as a matter of record. 



Sheet Met.xl 

 Metal sheathing has been used since ancient times to cover wood exposed to 

 marine l)orers, particularly ship bottoms. Iron, zinc, yellow metal and copper have 

 been used for this purpose. Of these, the first is comparatively unsuccessful because 

 of rapid corrosion ; zinc and yellow metal are \ery expensive and corrode in a short 

 time; copper is likewise expensi\e, but its rate of corrosion is less than that of the 

 others and it has gi\en good service in many cases. 



