Iron and Steel in the Construction of Ships and Bridges. 263 



protected by a coating of oxide, which in ordinary cases soon 

 forms upon it. The pecuniary loss caused by the rapid corro- 

 sion of iron ships has given rise to very serious anxiety both 

 to our own and to foreign governments. This loss is so enor- 

 mous that about twenty years ago the government of this 

 country determined to abandon the use of iron ships of war. 

 The evil is of the greater magnitude, as iron ships cost nearly 

 four times as much as those of timber formerly in use in our navy. 



Few persons have any idea of the rapidity with which iron 

 ships are eaten away under the action of s*ea water. Large 

 holes have been produced in two or three years, plates and 

 rivets have been destroyed over a large surface, so as to put 

 even the safety of the ship in peril. The vast number of the 

 patents taken out for paints and other protective compounds, 

 shows the seriousness of the difficulty, and at the same time 

 proves the futility of the efforts which have been made to 

 grapple with it. 



The rapid destruction of the ship is not the only serious 

 evil that accompanies the use of iron in shipbuilding. Iron 

 ships are peculiarly liable to fouling, and this to an extent 

 which is not only a cause of great delay, but often of imminent 

 danger. When copper sheathing is used with wooden ships, 

 the slight galvanic action which attends its slow solution in the 

 sea water completely prevents the adhesion of marine plants 

 and animals. This trifling corrosion formerly gave rise to con- 

 siderable annoyance on account of the loss it entailed^ and 

 efforts were made, not without success, to prevent it. The 

 sagacity of Sir Humphrey Davy suggested to him as a pro- 

 tecting agent the use of a metal at once more strongly electro- 

 negative and less costly. Such a metal was found in zinc, 

 which, in suffering the electric action, would save the copper. 

 But the removal of such action from the sheathing led to an 

 amount of fouling which soon caused all attempts to preserve 

 the metal to be given up, and it was even considered very 

 fortunate that at the expense of the copper, fouling could be 

 prevented. 



A sheathing of copper having been found so advantageous 

 with wooden ships, it might be supposed that it would be 

 equally useful with those of iron. But, instead of the copper 

 protecting the iron, the iron would but too effectually protect 

 the copper, and an enormous galvanic battery being produced, 

 the destruction of the vessel would proceed with fatal rapidity. 

 Were it possible entirely to prevent contact between the 

 iron and the copper, the destruction of the former might 

 be averted, but all attempts in that direction have been vain, 

 so many circumstances are liable constantly to occur which 

 render every precaution that may be taken unavailable. 



