142 HARBOUR CONSTRUCTION. 



We are thus, I think, justified in assuming that the plates, 

 angles, etc., of the Gambia were of their full original thickness 

 when the wreck occurred. 



In the measurements above given, it should be remembered 

 that the corrosive action was going on simultaneously on both 

 sides of the iron. This would not usually occur in practice, 

 because hollow piles would be or, at all events, ought to be 

 protected from corrosion inside by filling them with cement and 

 sand, or other suitable material. Under-water bracings would 

 probably suffer most, and should therefore be made of extra 

 strength. 



In the wreck of the Gambia the rate of decomposition 

 appears to have been about one inch in thickness in 320 years, 

 assuming one surface only to have been exposed. This is about 

 seven times as great as that indicated by Mr. Andrews' experiments. 



A good many o the Gambia's plates were removed and used 

 in the construction of plant. Their strength did not seem to 

 have been impaired in the least degree by the exposure to which 

 they had been subjected. 



I mention this because it is well known that the strength of 

 cast iron is greatly reduced by long exposure to the action of 

 sea- water. In fact, the iron is chemically changed, and becomes 

 a kind of plumbago. 



Mr. Thomas Stevenson, in his work on " Harbours " (p. 47), 

 says : 



" In castings at the Bell Rock lighthouse the loss by corrosion 

 has been at the rate of an inch in a century. One of the bars, which 

 was free from air-holes, had its specific gravity reduced to 5*63, and 

 its transverse strength from 7409 Ibs. to 4797 Ibs., and yet presented 

 no external appearance of decay. Another apparently sound specimen 

 was reduced in strength from 4068 Ibs. to 2352 Ibs., having lost 

 nearly half its strength in fifty years." 



Wrought-iron double-headed rails, weighing about 84 Ibs. 

 per yard, were used as bracings in the old breakwater at Port 

 Elizabeth (Cape Colony). These were placed within the tidal 

 range, and were quite unprotected by paint or otherwise. I 

 had no opportunity of ascertaining their actual original section, 

 but after a lapse of about twenty- three years they seemed to have 

 lost very little by corrosion, notwithstanding that they were 

 thickly coated with rust. 



