Iron and Steel in the Construction of Ships and Bridges. 267 



Very recently more than forty tons of rust were removed from 

 the Menai tubular bridge ; but, large as this quantity was, it 

 does not represent anything like the entire corrosion which has 

 taken place in this bridge during the few years it has been in 

 existence, since it consisted only of the rust which had formed 

 on the exterior. How many additional tons would the 

 interior and inaccessible portions have furnished, places where 

 corrosion may be going on with an unsuspected but most dan- 

 gerous rapidity. A very small extent of surface deeply cor- 

 roded would suffice to endanger the stability of the largest 

 constructions of iron, and might at any moment give rise to 

 its sudden destruction. 



The effect of air and moisture upon iron bridges must be 

 greatly increased when, as is but too often the case, not only 

 no effective means are taken to preserve them, but they are 

 wantonly exposed to the most injurious influences. How long can 

 they reasonably be expected to last, when moist clay and gravel 

 are kept constantly heaped up against and in immediate contact 

 with them, or when the pillars on which they, at least partially, 

 rest are immersed in salt or brackish water. Experiments 

 have shown that cast iron is rapidly changed, by the action of 

 salt water, into a friable mass, closely resembling plumbago in 

 both texture and weakness. 



Independently of the danger which must arise to the public 

 from the rapid decay of so many iron bridges, their renewal 

 within a comparatively very short period, which, under the cir- 

 cumstances, will be unavoidable, must hereafter entail a most 

 serious expense and inconvenience on railway companies, and 

 indeed, indirectly at least, on the public. This consideration 

 should lead to the adoption of a more rational treatment of 

 iron bridges, and even to some attempt at a retardation of that 

 decay which seems almost inseparable from them. Protec- 

 tion from moisture, with the exception of that contained in 

 the air, is, in many cases at least, possible. But our electro- 

 chemical knowledge might, perhaps, if properly employed, 

 supply us with the means of imparting to iron constructions 

 a degree of permanency almost without limit. Why do we 

 not avail ourselves of the principle which is now being applied 

 to the protection of iron ships — the transfer of the corrosive 

 action to zinc. The cost of the metal which would thus be 

 consumed, especially taking into account the ease with which 

 it could be replaced, would bear no proportion to the expense 

 and trouble which the frequent renewal or extensive repair of 

 so many iron bridges must necessarily entail. 



The subject of iron as a material for ships and bridges is 

 one of the most important that can be considered. There is 

 not one whom it does not, in some way or another, concern. 



