344 Sir Humphry Davy's further researches on 
be concluded, that they adhered either to the oxide of iron, 
or the calcareous deposits occasioned by the excess of nega- 
tive electricity. 
In the navy the proportion adopted has been only ^-3^ of ‘ 
cast iron, at least for vessels in actual service, and when the 
object is more cleanness than the preservation of the copper. 
It is very difficult to point out the circumstances which 
have rendered results, such as these mentioned with respect 
to Liverpool traders, so different under apparently the same 
circumstances, i. e. why ships should exhibit no adhesions 
or barnacles after two voyages, whilst on another ship, with 
the same quantity of protection, they should be found after a 
single voyage.* This may probably depend upon one ship 
having remained at rest in harbour longer than another, or 
having been becalmed for a short time in shallow seas, where 
ova of shell fish, or young shell fish existed ; or upon oxide 
of iron being formed, and not washed off, in consequence of 
calm weather, and which consolidating, was not afterwards 
separated in the voyage. From what I can learn, however, 
the chance of a certain degree of foulness, in consequence of 
the application of the full proportion 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 conduct- 
ing power of the sea water, and consequently the proportion 
of protecting metal may be larger without being in excess. 
* The quality of the copper may be another cause. 
