1843.] on the effects of Sea-rvater on Iron. 1085 



2nd. There is reason to believe the corrosive power of Sea-water 

 varies indifferent localities of this Harbour and the Red Sea ; it appears 

 to be far greater than in the intermediate ocean, while on the Western 

 Coast of South Africa, it far exceeds either. The corrosion sustained 

 by the " Berenice's" paddle-wheels, during the short time she was off 

 the Cape of Good Hope, was apparently greater in extent than during 

 the whole time she has since been in India, and the Iron Boat taken 

 out to the Niger by Mr. Laird, is said to have suffered much from corro- 

 sion. In the Red Sea a small description of barnacle forms on iron, 

 beneath which corrosion is very rapid, destroying the outer coat of the 

 metal ; it becomes honey-combed, the same effect in a degree in Bombay 

 Harbour also, a weed grows upon it, destroying any paint or varnish 

 with which it may be coated ; the best preservative has been found in 

 a composition of red lead and linseed oil; but for Steam Vessels' 

 bottoms, mineral pitch and rosin melted in spirits of turpentine. 



3rd. It may be worthy of remark, that the " Hugh Lindsay" has 

 still the identical paddle-arm segments she started with in 1829. 



4th. Time has not yet been given to form any very correct data of 

 the durability of the Iron Boats, and in any opinion founded on analogy 

 with the paddle-wheels of Sea-going Steam Vessels, various elements 

 which we are as yet little acquainted with, must be taken into ac- 

 count. Thus the galvanic action of a stream of water passing 

 along the copper, has been known to cut through heavy paddle- 

 beam stays in an incredible short time, as also the arm and segments ; 

 the quality of the iron may have an influence, as also the smoothness 

 of surface in the plates composing the bottom of an Iron Vessel. 



5th. Mooring chains are observed to be perfectly sound when laying 

 in the mud, partially corroded when suspended, but very soon destroy- 

 ed near the surface and in contact with the copper. 



6th. Time and observation can alone throw light upon this little- 

 understood and interesting theory. I have, &c. 



(Signed) , R. Oliver, Capt. R. N. 

 To C. B. Greenlaw, Esq. Secretary Marine Board, Calcutta. 



(True Copy.) 



(Signed) C. B. Greenlaw, Secretary. 

 Fort William, Marine Superintendent's 

 Office, the 23d September, 1843. 



