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STRENGTH OF RAILS.

the upper being quite straight, the line, Ibounding the under side in the form of section,usually considered the strongest, is that of twosemi-parabolas, the vertex being the point ,,where the force acts; but, in the case of a tweight rolled along the Rail-road, the pressureis occasionally acting throughout the wholelength. On that account, perhaps, the linebounding the under side should be that whichpresents the greatest strength, when the beamis supported at each end, and loaded equallythroughout the whole length, which is a semi- |ellipse.

The want of a practical treatise on thestrength of cast-iron, and on the various formsof section, suitable for beams, acted upon indifferent ways, was, until lately, very muchfelt. Mr. Tredgold has, in his treatise on thestrength of cast-iron, supplied that want, andgiven numerous practical examples on thesubjects. The reader, who wishes to informhimself further on the subjeet, will find somevery useful information in that work.