154 THE SE AS 



substance. Scientific investigations into the most suitable 

 means of affording this type of protection have been 

 conducted both in this country and in the United States 

 for some years past, by the Sea Action Committee of the In- 

 stitution of Civil Engineers in the former and by a Committee 

 on Marine Piling Investigations in the latter. After ex- 

 haustive tests of a great variety of substances, various 

 organic compounds containing arsenic, some of which 

 were used as poison gases during the war, have been found 

 the most efficacious, and important experiments are now 

 being carried out with these substances, by exposing in 

 the sea rafts containing timber treated in various ways, 

 and finding the degrees of protection afforded by the various 

 poisons (Plate 57). In all cases it is necessary to impregnate 

 them into the wood in solution in creosote, otherwise they 

 wash out and the wood is left fully exposed to infection. 

 The creosote alone, if the timber is well impregnated with 

 it, often provides good protection for a long period. The 

 wood has to be thoroughly dried before impregnation, 

 the latter process being carried out either by the aid of a 

 vacuum which draws the creosote through or else by 

 boiling with the impregnating fluid in an open tank. Up 

 to date these experiments are having good results so far 

 as the Shipworm is concerned but are not so efficacious with 

 the Gribble, which appears to be less susceptible to poisons. 

 This is probably to be explained by the fact that only fully 

 grown Gribbles attack new timber and these will not be 

 so easily poisoned as the minute larval creatures which are 

 the carriers of infection in the case of the Shipworm. The 

 age-old problem of the protection of timber in the sea re- 

 mains, though progress is slowly being made, still unsolved. 

 The rock borers do not do a fraction of the damage 

 of the wood borers and are a minor problem. Mechanical 

 borers, such as the Piddock, may do a good deal of damage 



