MECHANICS AND USEFUL ARTS. 33 



DURABILITY OF WROUGHT IRON WHEN USED FOR STRUC- 

 TURAL PURPOSES. 



Mr. John A. Roebling, the engineer of the Railway Suspension 

 Bridge spanning the great gorge below the Falls of Niagara, in a 

 recent report to the directors of the bridge company, on the condi- 

 tion of the bridge, after five years' continuous use, thus discusses the 

 important subject of the durability of wrought iron, when used for 

 structural purposes. His remarks will be found by our readers to em- 

 body a great amount of useful and interesting information. Ed. 



The fact is well known, that wrought iron, under certain condi- 

 tions, will undergo certain radical changes. And so will all kinds 

 of matter, material existence being but a theatre of change, break- 

 ing down, of reduction, and of reconstruction of the elements of mat- 

 ter. And as all human fabrics, being but material constructions, will 

 have to succumb to the same inexorable law, we cannot expect that 

 the Niagara Bridge will form an exception. 



Two kinds of changes are known, which will affect the strength of 

 iron and other metals. The one is wrought by the chemical process 

 of oxidation, and can be guarded against effectually, and is so guard- 

 ed in the Niagara Bridge. All iron and wire, within reach are kept 

 well painted, and thus preserved against rust. The anchor-chains 

 and their connections with the cables, inside of the anchor-masonry 

 and in the rock below, after three coats of paint, are protected by 

 the cement grout, which forms a solid envelope, excluding air and 

 moisture. 



But, aside from the mechanical protection thus afforded, I depend 

 principally upon the well-known chemical action of calcareous ce- 

 ments in contact with iron. Oxygen has a greater affinity for lime 

 than for iron. So long, therefore, as the cement will combine with 

 oxygen, or, in other words, has not become completely crystallized, 

 which is a very slow process inside of heavy masonry, the iron will be 

 protected. The cement, not exposed to the air, when setting slowly, 

 has a tendency rather to expand than to contract : but suppose there 

 should be cracks around the anchor-bars large enough to. admit 

 air and moisture, water will then find its way through those 

 cracks, but, on reaching the iron, will be more or less impregnated 

 with cement, and thus add another protecting coat. The chemical 

 principle, which I have explained here, I apply daily in my factory 

 for the preservation of wire against dampness. I have also carried 

 on direct experiments for a number of years, which have convinced 

 me of the preserving property of calcareous cements in damp situa- 

 tions. 



On examining recently the anchor-bars of the Monongahela Sus- 

 pension Bridge at Pittsburgh, built sixteen years ago, I found them 

 perfectly preserved, as far as the cement, in which they are embed- 

 ded, was removed. 



But iron, under certain conditions, will undergo another change, 

 which is not so well understood, and is indeed, as yet, a partial mys- 

 tery. And this fact has been seized upon as an invincible argu- 

 ment against iron bridges generally, and against the Niagara Bridge 



