STRENGTH OK RA1I.S.

163

Tn comparing the strength of the differentrails with each other, we find a great varia-tion, not only between the different kinds ofmetal, but also in rails cast from the samemetal. The only constant and regular lawappears to be, that the weight, or specificgravity of rails, formed of a mixture of differentkinds of metal, is uniformly greater than ofone description of metal separately, and alsothat such a mixture makes the rails invariablystronger. This is a very useful discovery, andenables the founder, by mixing different metalsin the proper proportions, to form a rail muchstronger with the same weight of metal, thancould otherwise be done by casting them ofany particular kind of metal alone.

The depth of the middle section of the cast-iron rail renders them very rigid, and thedeflexion is comparatively trifling before frac-ture. The recorded weights are those whichproduced fracture. In loading the rails, theweights should of course be much less thanthat which breaks the rail; inequalities of theroad, or occasional obstacles occurring uponthe surfaces of the rails will sometimes producejerks or shocks to the wheels of the carriages,and the re-action will transfer those to the ra 1,and cause blows which, from the brittleness ofthe material, will be very liable to produce

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