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



Neither can the iron be protected by paint, or any other 

 similar composition. All those compounds hitherto tried have 

 failed, the corrosion, commencing in points, and spreading 

 under the paint in every direction. Indeed, in proportion as 

 they are successful in preserving the iron, they are mischievous 

 in giving rise to fouling, large quantities of barnacles and 

 weeds accumulating rapidly on the protected portions. The 

 retardation thus caused is a source of serious additional ex- 

 pense. A comparatively small deposit would give rise to a loss 

 of speed to the extent of one knot an hour, and the additional 

 cost of fuel, seamen's wages, etc., consequent on this, is a mat- 

 ter of very simple calculation. Thus, supposing, for example, 

 the average speed of the vessel to be ten knots an hour, the 

 expense would be increased by one-tenth. 



Fouling can be averted only by a clean metallic surface, on 

 which electricity is constantly developed. It is vain to attempt 

 the destruction of marine plants and animals by poisonous 

 substances, the only way to prevent their adhesion is to present 

 to them such a surface as will afford them no means of adhe- 

 sion. This is best effected by the slow removal of the surface 

 itself, on account of gradual corrosion, and the washing off of 

 the resulting compound. 



We may, therefore, conclude, that with iron, as with wooden 

 ships, some metal must be sacrificed, and the only question 

 that remains for solution is what that metal shall be. It is 

 fortunate that in considering the important subject of the 

 electro -chemical preservation of iron ships, we are not confined 

 to the use of copper, on the contrary, a far less expensive 

 and a thoroughly effective substitute for it is in our hands. 

 If iron perishes in presence of copper, because it is electro- 

 negative with reference to the copper, to preserve the iron as 

 completely as the iron would preserve the copper, we have only 

 to place in contact with it a metal which bears the same rela- 

 tion to it that it bears to copper. Such a metal is zinc, and 

 it is the better suited to the purpose since its conductivity and 

 other properties only just permit that amount of corrosion 

 which is enough to secure a surface sufficiently clean to pre- 

 vent fouling. 



The efficiency of zinc, as a sheathing for iron ships has 

 already been tested, and the experience of several months at a 

 time has shown that its surface remains perfectly clean, under 

 the ordinary conditions of immersion in the sea. The necessity 

 for the substitution of zinc for copper as sheathing is fortu- 

 nately not accompanied by increased expense, but rather the 

 contrary. The cost of zinc is, indeed, only about one-third 

 that of copper, but as sheathing it costs very nearly the same as 

 copper. 



