MECHANICS AND USKFUL ARTS. 47 



day, we may conclude that, as a low average, 200 trains pass 

 daily over the majority of our metropolitan and suburban railway 

 bridges, and, as a maximum, the hardest-worked member of the 

 bridge tribe possibly undergoes as many as 300 alternate changes 

 of active and passive conditions from sunrise to sunset. Adapting 

 this calculation to our theory, we may estimate the life of the 

 hardest-worked railway girder to extend over a period, in round 

 numbers, of 100 3'ears, under ordinary circumstances. 



A difference will obviously present itself respecting the ultimate 

 durability of cast and wrought iron girders individually. When 

 the former fail they fail completely ; there is no repairing a frac- 

 tured cast-iron beam, whatever shape it may possess ; it is only 

 fit for the cupola or the puddling furnace. The same circum- 

 stances do not attend the dissolution of wrought-iron girders, pro- 

 vided they are well watched and the "first symptoms" attended 

 to. The Menai Bridge, for example, might be replaced piece- 

 meal, accordingly as every plate, angle iron, or other portion of 

 it becomes deteriorated to an extent sufficient to imperil the safety 

 of the structure. In this sense a wrought-iron bridge is practi- 

 cally indestructible, since it admits of any and every degree of 

 partial repair, and, after the lapse of its first hundred years of life, 

 may be completely' rejuvenated and commence a fresh career. 

 Lattice bridges — those constructed upon the open-web system — 

 in general afford special facilities for this process of gradual recon- 

 struction, since a bar can be taken out and replaced without in any 

 manner jeopardizing the safety of the remainder. The external 

 effects, or visible appearance of the influence of time, must noi be 

 confounded with that invisible and inexplicable action that is in- 

 cessantly in progress in connection with the molecular composi- 

 tion of the material. For similar reasons that the wrought-iron 

 girder, as a structure, can be preserved by successive reparation 

 from the results of visible corrosion and decay, so is it also inde- 

 pendent, in some degree, of any atomic alteration, un]j!ss we 

 imagine the whole girder to be equally affected, and to iracture 

 precipitately like one of cast iron. It has always been a puzzle 

 to engineers to satisfactorily account for the sudden fracture of 

 cast iron, whether in the form of girders, axles, or engine-t aams, 

 under a much smaller strain than what they had previously oorne 

 with impunity for a long period of time. 



Whatever the exact nature of the change may be, or the rate at 

 which it progresses, until the cohesive power of the matenal is 

 injured, it is impossible to assert; but we are nevertheless certain 

 that the continual repetition of severe strains on a girder must 

 ultimately impair its powers of resistance. In a word, then^ upon 

 this hypothesis, every cast-iron girder is doomed to break at some 

 time or another, and, what is worse, break suddenly, the precipi- 

 tation of the passing load into the gulf beneath being the first 

 sign of danger. — Engineer. 



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