350 THE ADDRESSES, LECTURES, ETC., OF 



boat capable of taking railway trains would be a desideratum 

 justified by the ever-increasing intercommunication between this 

 and Continental countries. 



The public inconvenience arising through the obstruction to 

 traffic by a sheet of water, is well illustrated by the circumstance 

 that both the estuaries of the Severn and of the Mersey are being 

 undermined in order to connect the railway systems on the two 

 sides, and that the Frith of Forth is about to be spanned by a 

 bridge exceeding in grandeur anything as yet attempted by the 

 engineer. The roadway of this bridge will stand 150 feet above 

 high-water mark, and its two principal spans will measure a third 

 of a statute mile each. Messrs. Fowler and Baker, the engineers 

 to whom this great work has been entrusted, could hardly accom- 

 plish their task without having recourse to steel for their material 

 of construction, nor need the steel used be of the extra mild quality 

 particularly applicable for naval structures to withstand collision, 

 for, when such extreme toughness is not required, steel of very 

 homogeneous quality can be produced, bearing a tensile strain 

 fully double that of iron. 



The tensile strength of steel, as is well known, is the result of 

 an admixture of carbon with the iron, varying between ^th and 

 2 per cent., and the nature of this combination of carbon with iron 

 is a matter of great interest both from a theoretical and practical 

 point of view. It could not be a chemical compound which would 

 necessitate a definite proportion, nor could a mere dissolution of 

 the one in the other exercise such remarkable influence upon the 

 strength and hardness of the resulting metal. A recent investiga- 

 tion by Mr. Abel has thrown considerable light upon this question. 

 A definite carbide of iron is formed, it appears, soluble at high 

 temperatures in iron, but separating upon cooling the steel gradu- 

 ally, and influencing only to a moderate degree the physical pro- 

 perties of the metal as a whole. In cooling rapidly there is no" 

 time for the carbide to separate from the iron, and the metal is 

 thus rendered both hard and brittle. Cooling the metal gradually 

 under the influence of great compressive force, appears to have a 

 similar effect to rapid cooling in preventing the separation of the 

 carbide from the metal, with this difference, that the effect is more 

 equal throughout the mass, and that more uniform temper is likely 

 to result. 



