258 Sir U. Davy on the Preservation of Metals, ^c. [ApRit, 



what I can learn, however, the chance of a certain degree of 

 fouhiess, in consequence of the application of the^'full pro- 

 portion of protecting metal, will not prevent ship owners 

 from employing this proportion, as the saving of copper 

 is a very great object; and as long as the copper is sound, no 

 danger is to be apprehended from worms. 



It ought to be kept in mind that the larger a ship, the more 

 the experiment is influenced by the imperfect conducting power 

 of the sea water, and consequently the proportion of protecting 

 metal may be larger without being in excess. 



I have mentioned these circumstances because they apply to 

 ships already coppered, and because I have heard that a Liver- 

 pool ship, of which it was doubtful whether the copper was in a 

 state such as would enable her to make another voyage to India 

 with security, has, by the application of protectors of ■^, made 

 this voyage,* without apparently any wear of her sheeting ; and 

 that she is now preparing with the same protectors to make 

 another voyage. 



In cases when ships are to be newly sheathed, the experi- 

 ments which have been detailed in the preceding pages render 

 it likely, that the most advantageous way of applying protection 

 will be under, and not over the copper: the electrical circuit 

 being made in the sea water passing through the places of junc- 

 tion in the sheets ; and in this way every sheet of copper may be 

 provided with nails of iron or zinc, for protecting them to any 

 extent required. By driving the nail into the wood through 

 paper wetted with brine above the tarred paper, or felt, or any 

 other substance that may be employed, the incipient action will 

 be diminished ; and there is this great advantage, that a consi- 

 derable part of the metal will, if the protectors are placed in the 

 centre of the sheet, be deposited and redissolved ; so there is 

 reason to believe that small masses of metal will act for a great 

 length of time. Zinc, in consequence of its forming little or no 

 insoluble compound in brine or sea water, will be preferable to 

 iron for this purpose ; and whether this metal or iron be used, 

 the waste will be much less than if the metal was exposed on the 

 outside : and all difficulties with respect to a proper situation in 

 this last case are avoided. 



The copper used for sheathing should be the purest that can 

 be obtained ; and in being applied to the ship, its surface should 

 be preserved as smooth and equable as possible : and the nails 

 used for fastening should likewise be of pure copper ; and a 

 little difference in their thickness and shape will easily compen- 

 sate for their want of hardness. 



In vessels employed for steam navigation, the protecting 



• The Dorothy. 



