TUBULAR GIEDEK BRIDGES. 193 



weight, and that of the roadway, the experiment generally 

 proceeds as follows : — 



First — To ascertain the increased deflection due to the 

 heaviest load, as a dead weight placed upon the bridge. 



Secondly — The amount of vibration produced by the 

 passage of the same load at different velocities. 



Thirdly — The amount of deflection due to the rolling 

 load, and the variations, if any, when the trains are retarded 

 or accelerated ; and, 



Lastly — The principle of construction is taken into con- 

 sideration, and the excess of strength which a bridge should 

 have over the greatest load, in order to declare it safe for 

 general trafiic. 



On the first, second, and third, there appears to be little, 

 if any, difiference of opinion ; but on the latter, the greatest 

 and most opposite views are entertained. Some contend- 

 ing for three, and others for four, five, and six times the 

 greatest load; whilst others again, more timid than the 

 rest, insist upon eight or ten times the greatest load in order 

 to be safe. Such appear to be the present views entertained 

 by the profession, and such they will continue to be, unless 

 decided by some high authority, from which there is no 

 appeal, as to what should be the resisting powers of a bridge. 



I make no doubt we have now in existence several 

 bridges which, to all appearance, are duly performing the 

 important functions of supporting heavily loaded trains 

 within the nan*ow limits of probably half the weight that 

 would lead to destruction ; and others again are of such 

 enormous strength as to bid defiance, for ages to come, to 

 the heaviest load that can by possibiHty assail them. 



Such I believe to be the present state of a considerable 

 number of our railway structures, and such are the widely 

 spread notions which have taken possession of some of our 

 railway engineers. Under these discrepancies, it becomes 

 a question of deep importance as to what should be the 



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