The Waves Of An Iron Bridge

A train always exerts greater strain on an iron bridge when going quickly than when going slowly, but the difference in the straiu depends on the structure of the bridge and is much more in some cases than in others. When the train goes over the bridge, it causes a wave to travel along the structure, owing to the elasticity of the iron. That part of the bridge just in front of the train is raised a little, and the part under the train is lowered, so that each part of the bridge is successively raised and lowered as the train goes over it, and the more quickly the train travels the more sudden this will be, and consequently the more violent. The strain produced will vary with the square of the velocity of the wave, so that the train wi)J exert four times the strain when it doubles its velocity, and nine times when it triples its locity.-