PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 901 



Engineer), resulting from its own -weight, while increasing the 

 span to 1,000 feet, it avouUI fall with its own Aveight alone. Just 

 see what a gain, if some material of great strength could be sub- 

 stituted for Avrought iron : and we have it. So soon as the arts 

 and sciences demand a forward march, some one is ready to take 

 the lead. In the want expressed above, Mr. Bessemer has nol)ly 

 come to the front, and presented the profession with a method of 

 making steel in large masses, and applying it to all principles and 

 details of construction. This invention of Mr. Bessemer is slowly 

 instituting a perfect revolution in the mechauic arts, and steel 

 HOW bids fair to usurp the place of wrought iron in a great 

 degree. Certainly in all cases where strength combined with 

 light weight is demanded, and often a necessity. By its judicious 

 use we are enabled to accomplish results that have hitherto been 

 deemed,if not impossible, at least problematical! It seems surprising 

 to me that so few have given attention to its use in girders and 

 trusses of all descriptions, and I can account for it in no other 

 wa} than the general disposition of professional men to have some 

 one else lead in an innovation, and demonstrate a success that can- 

 not but be foreseen from well established facts and experiments., 

 Thus far there have been but two'steel bridijes built : the tirst, 

 the one built by a Mr. Worthington over the Sankey canal, 

 England ; the second w^ill greet the visitor to the World's exposi-. 

 tion to be held this spring at Paris. The former is a swino-bridjxe 

 in four girders, of iifty-six feet in length, and weighs hnt Jive- 

 eigldhs of what a wroiight-iron bridge of the same pattern would 

 Aveigh. The latter is a tressled arch bridge, of eight^'-two feet 

 span, and was submitted to enormous tests, which were satisfac- 

 tory in the highest degree. 



A proposition was broached some two or three years since to 

 span the straits of Messina with a single span of 3,300 feet, of 

 course ou the suspension system. These and many other conside- 

 rations led me to adopt steel as the material to be employed in the 

 plan suggested, but none have had a greater weight with me than 

 the study of Mr. David Kirkaldy's exhaustive experiments, made 

 at the works of Messrs. Robt. Napier & Son, Glasgow, and extend- 

 ing over a period of nearly four years. No expense was spared in 

 making these experiments under all conditions, and they are to be 

 regarded as eminently trustworthy, and a positive guide to the 

 profession in designing works of both iron and steel. His numer- 

 ous experiments upon forged steel of the best quality, showed a 



