40 ANNUAL OF SCIENTIFIC DISCOVERY. 



strength and toughness is obtained by a certain mixture of pure 

 iron with carbon and cinder, thoroughly worked and incorporated. 

 When the fibrous and laminar aggregation becomes so dense as to 

 be lit for the manufacture of steel, then are by this very process suf- 

 ficient impurities expelled, and the greatest degree of cohesion is 

 obtained. Hence strong steel can only be made of strong iron, no 

 matter what chemicals may be administered during the process. 



Keeping the above process before our mind, we may now under- 

 stand why even the best fibrous wrought iron, when exposed to long- 

 continued vibration under tension, or to torsion, bending or twisting, 

 must inevitably become brittle, because the iron threads and lamina 

 become loosened in their cinder envelopes. But the cohesion between 

 the iron and its cinder once destroyed, and its strength is gone. 

 Now whether cohesion is the result of magnetic attraction (according 

 to Faraday) or otherwise, this process appears to be purely mechan- 

 ical. But let the explanation, which is here offered, be correct or 

 not, the fact remains that fibrous iron, and all kinds of iron and steel, 

 will be rendered brittle by vibration and tension, or by bending and 

 twisting, without undergoing any mysterious change in its molecular 

 arrangement. 



It is only within the last one hundred years that wrought iron has 

 become a necessity on public and private works. Large structures 

 entirely composed of iron are of a still more recent date. Long ex- 

 perience on a large scale is therefore wanting. But, as far as it goes, 

 the opinion is fully sustained, that good iron, not overtaxed by tension 

 and vibration, and otherwise preserved, will prove one of the most 

 durable building materials at our disposal. 



The Menai Chain Suspension Bridge has now stood about thirty- 

 six years, and is still considered a safe work, although it has, for the 

 want of stiffness, on several occasions suffered severely from gales. 

 The old wire Suspension Bridge at Friburg, in Switzerland, has 

 been in use about twenty-seven years, but it does not possess enough 

 of strength and stiffness to guarantee its safety much longer in its 

 present state. 



It should be remembered that there are many suspension bridges in 

 this country, as well as in Europe, built without any regard to stiff- 

 ness, and are therefore constantly subjected to vibration, which must 

 greatly limit their durability. 



The cables of the Niagara Bridge, on the other hand, are free from 

 vibration, consequently will last as long as the nature of good wrought 

 iron will permit, when subjected to a moderate tension, not exceeding 

 one-fifth of its ultimate strength. This durability I am unwilling to 

 estimate at less than several hundred years. 



EFFECTS OF VIBRATORY ACTION AND LONG-CONTINUED 

 CHANGES OF LOAD UPON WROUGHT-IRON BRIDGES AND 

 GIRDERS. 



Mr. Fairbairn, in presenting a paper with the above title to the 

 last (1861) meeting of the British Association, said that the subject 

 was one of great importance as affecting the construction of tubular 



