Iron and Steel in the Construction of 6%ips and Bridges. 261 



plate of steel, which, at first is only a quarter of an inch thick. 

 In the one case, the strength of the plate is diminished only to 

 the extent of one-eighth : in the other, with the same amount of 

 oxidation, to the extent of one-fourth. In the former case the 

 plate would stand, when in the latter it would be sure to give 

 way. Again, it might be supposed that steel would answer, 

 best where violent concussions are to be withstood, and that, 

 therefore, it would be an excellent material for armour plates. 

 The reverse is, however, true. Steel is strong only when ex- 

 posed to a force that is slowly applied. A cannon-ball which 

 would cause only indentations in iron, or, at the worst, merely 

 pass through it, would dash a plate of steel to pieces, dealing 

 destruction, most probably, to all those in its vicinity. 



The construction of ships and bridges of iron, and espe- 

 cially of the former, supposes the power of producing iron bars 

 and plates. As the mode of forming these, except on a very 

 limited scale, and in a very imperfect way, was unknown until 

 near the close of the last century, iron ships and bridges must 

 necessarily be of very modern invention. There has been by 

 consequence very little experience regarding them : it is, there- 

 fore, the more necessary that we should bring to bear on so 

 important a subject, everything we know regarding it. 



In comparing the fitness of iron as a material for ships with 

 that of wood, it is requisite to know their relative strength. 

 The tensile breaking strain of iron greatly exceeds that of the 

 very best wood. That of malleable iron is to that of dry Eng- 

 lish oak as about four to one ; and, taking into account the 

 weakness caused in the case of plates by riveting, as two and 

 three-tenths to one. If, however, the riveting diminishes the 

 strength of the iron, on the one hand, it increases the strength 

 of the ship, as a mass, on the other ; since, by means of it, the 

 plates are united firmly in every direction. Considering the 

 injuries to which ships are liable, it is not enough to bear in 

 mind mere tensile strength. A compressing strain is often a 

 great source of danger to them. The resistance of malleable 

 iron to such a strain is to that of dry English oak, as seven to 

 one. On the whole, however carefully constructed, a ship of 

 timber is greatly inferior in strength to one properly made of 

 iron. 



Iron may be used either partially or exclusively, in the con- 

 struction of ships. Each mode of proceeding has its peculiar 

 advantages and disadvantages. Thus an iron vessel, timber- 

 sheathed, and copper-fastened is greatly superior to one of 

 iron, as far as fouling is concerned, and it is considerably 

 lighter. But these advantages would be at an end, if a 

 means were discovered of preventing the adhesion of animals 

 or vegetables to the bottom of iron ships, and if steel were to 



