Iron and Steel in the Construction of Ships and Bridges. 265 



There is no reason why only the outside skin of an iron 

 ship should be carefully protected. It is true that this portion 

 is corroded, if unprotected, with great rapidity, but every part, 

 especially if exposed to the action of bilge or other water, 

 rapidly decays. This general destruction might be, at least in 

 a great degree, prevented by a judicious application of zinc, 

 which when dissolved, or otherwise rendered ineffective, might 

 be easily replaced. Experiments recently made at Portsmouth 

 show that when the zinc sheathing and the iron of the ship are 

 in perfect metallic contact, the zinc remains clean, and free 

 from a coating of any kind. The interposition of felt, or any 

 other imperfectly conducting material, as might be anticipated, 

 has been found injurious. Complete contact of the metals is 

 no more than sufficient to secure just enough galvanic action 

 to preserve the one metal and keep the other clean. 



The advantage of protecting iron ships from corrosion, 

 is evident from the immense numbers of them annually con- 

 structed in the United Kingdom. The tonnage of the iron 

 ships built on only six of our large rivers, during last year, 

 amounted to nearly half a millon. of tons. The value of all 

 those at present in use must, therefore, be enormous ; and their 

 preservation from a decay which has been found alarmingly 

 rapid must be a question of the highest importance. The 

 application of zinc to the protection of iron, especially after its 

 having been employed long since for the protection of copper, 

 is a proceeding so obvious that we can only wonder it has 

 not been long ago, and universally, adopted with iron ships • 

 its use in that way being rather the application of a well- 

 known principle, than a novel discovery in science. And, 

 indeed, as Sir Edward Belcher observed at the recent meeting 

 of the British Association at Nottingham, its application to this 

 purpose is not new. 



Among the important advantages which accompany the use 

 of zinc as a protection for iron ships, not the least is the 

 absence of the necessity for insulation. If copper or " yellow 

 metal " is employed as sheathing, it must be perfectly removed 

 from conducting communication with the iron : or it will, so far 

 from protecting the iron, cause its destruction to be far more 

 rapid; since the least contact between the metals will not 

 only transfer the chemical action to the iron, but render it far 

 more intense than it would have been had the iron been merely 

 left to its fate. The effect on iron of contact with copper is 

 very great. The accidental dropping of a copper coin into the 

 bilge-water, so as to come in contact with the iron of a ship, 

 has more than once given rise to the formation of large and 

 dangerous holes. In the attempts made to use copper sheath- 

 ing with iron ships various means have been vainly used to 



