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



secure its insulation. Thus plugs of ebonite were inserted in 

 the iron skin, and over the latter was spread, first sheets of 

 felt, and then sheets of india-rubber. But unforeseen and acci- 

 dental circumstances have rendered every such attempt to pre- 

 serve insulation unavailing. 



It may be asked, Why, if zinc is so much cheaper than 

 copper, and at the same time so effective, it did not supersede 

 copper as a sheathing even for wooden ships. The reason is 

 very satisfactory : it would not have prevented fouling, since 

 the condition which is secured by its contact with iron — namely, 

 galvanic action — would not have been sufficiently secured by its 

 contact with wood. 



There is one rather serious difficulty that attends the use of 

 zinc as sheathing for iron ships — its attachment. The drilling 

 of numerous holes in the iron skin would be, not only expen- 

 sive, but very objectionable for other reasons. This, however, 

 has been obviated, and by very simple means. The edges of 

 the plates forming the skin of the vessel are kept asunder by 

 a slight interval, which is filled up with compressed teak, and 

 into this are driven the zinc nails which are used to attach the 

 sheathing, and which are rendered more secure by their points 

 being, where they come in contact with the iron rib, turned 

 back by it into the teak. The sheets of zinc can thus be nailed 

 to the iron skin as easily as if it were wood, and the insertion 

 of the nails in the teak serves to wedge it more tightly in the 

 joint. In forming the latter, it is obvious that the edges of 

 the iron plates may be left as they come from the forge, dress- 

 ing of any kind being superfluous — a circumstance which 

 considerably lessens the expense. 



Iron supplies to the engineer a most excellent material for 

 the construction of bridges. Not only is it cheaper for the 

 purpose than stone or brick, and more durable than wood, but 

 it allows the construction of bridges of immense span. It may 

 be in the form of cast iron, wrought iron, or steel. Cast iron 

 answers well enough within certain spans, but the soundness 

 of the casting can never be entirely depended on. In the form 

 of voussoirs or ribs, engineers have ventured to employ it in 

 spans of 240 feet. Malleable iron is most generally used, and 

 invariably with very large spans — almost always in plates 

 varying from less than a quarter to little more than five-eights 

 of an inch in thickness. Looking to the rapidity with which 

 iron is corroded, even in the atmosphere, how short is the 

 period during which these plates will retain any amount of 

 strength, without a degree of care which no one thinks of 

 bestowing upon them. The danger from this source is not 

 imaginary. The best constructed and most carefully preserved 

 of our iron bridges are, as it were, melting away perceptibly. 



