346 Sir Humphry Davy*s further researches y ^c. 
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 
compensate for their want of hardness. 
In vessels employed for steam navigation the protecting 
metal can scarcely be in excess ;* as the rapid motion of these 
ships prevents the chance of any adhesions ; and the wear of 
the copper by proper protection is diminished more than 
two- thirds. 
• I have mentioned in the two last communications on this subject some appli- 
cation of the principle ; many others will occur. In submarine constructions — to 
protect wood, as in piles, from the action of worms, sheathing of copper defended 
by iron in excess may be used j when the calcareous matter deposited will gradu- 
ally form a coating of the character and firmness of hard stone. 
I 
