ON THE ACTION OF AIR AND WATER UPON IRON. 27 
~The Commissioners of Woods and Forests, in their *‘ Report of 1812, on 
Timber for the Navy,’ estimated the average duration of a ship at fourteen 
_years, while other authorities take it at twelve and a half years. Frigates, 
when built of American red pine, seldom lasted longer than five years, and 
the Ocean, Foudroyant, St. Domingo, Rodney, Ajax, and Albion, new ships, 
all fell to pieces from dry rot in about four years. 
382. Were it the fact, therefore, that unprotected iron vessels corroded 
equally throughout every part immersed, we could easily calculate, by the aid 
of our preceding researches, the durability of a ship of given scantling, and 
predetermine, under such and such conditions, at what time her hull would 
have become dangerously thin, and might rest with the assurance that for this 
period the iron ship was the best and safest that could be put upon the waters ; 
but unfortunately we have found that corrosion does not take place with per- 
fect uniformity, as has been already pointed out; and hence, without pro- 
tection, ships of iron must be always liable to the dangerous consequences of 
local corrosion and consequent thinning down of iron at particular spots, 
until at some unforeseen moment, possibly of least preparation and greatest 
external peril, a decayed plate is burst through and the vessel fills. The 
facility of introduction of water-tight bulk heads in iron ships greatly reduces 
the danger of such an accident, but it must always be attended with danger 
and loss of property, and occurring where it is most likely to happen, namely, 
in the engine compartment of an iron steamer in bad weather, would be almost 
eertain to involve the loss of the ship. 
The more sanguine advocates of iron ship-building have, in their anxiety to 
prove their durability to be such as to render protection needless, appealed to 
the existence of iron canal-boats of forty years of age or more, and to some 
of the earliest built iron vessels which have been occasionally in salt water. 
Most of the vessels alluded to however have been principally in fresh water, 
and on referring to Table XV. it will be obvious how vast a difference there 
is in durability of a ship of any given sort of iron, exposed to the action of sea 
and of fresh water. Thus, suppose a vessel of Low Moor plates; in one cen- 
tury the depth of corrosion would be— Inch. 
In clear'seawater -.  .)s 2 -  « « OZI5 
In foul sea water. . 2. .°. « « - 07404 
In clear fresh water only - . . . . 0°035 
In other words, while the ship, if originally of half-inch plates, would be almost 
destroyed in foul sea water, it would not have lost one-tenth of its scantling in 
clear fresh water in the same time. These cases therefore prove nothing to 
_ the point. 
383. It therefore seems to me that protection against local corrosion and 
“fouling” are essential to the safety and perfection of iron ships, and are 
alone wanting to render our future iron ships as much safer and more en- 
during than those of timber, as the steam ship of today is safer and more 
enduring than the sailing vessel of two centuries ago. 
384. The mechanical methods which have been proposed for removing 
foulness from the bottoms of iron ships, namely by scraping with a large 
_ wooden frame drawn under the hull by suitable rope tackle, appears quite 
incapable of removing more than the mere exterior fringe as it were of the 
“foulness,” or some of the larger animals when once become adherent. The 
force with which both animals and plants adhere to the coat of calcareous and 
rusty matter on an iron plate is very great, and no instrument sufficiently sharp, 
and pressed hard enough to the ship’s hull, could probably be successfully used, 
unless of iron, and this would be liable not only to injure the surface, but be- 
come constantly caught against small projections of the ship’s bottom. 
