the upper being quite straight, the line, bounding the under side in the form of section, usually considered the strongest, is that of two semi-parabolas, the vertex being the point where the force acts; but, in the case of a weight rolled along the Rail-road, the pressure is occasionally acting throughout the whole length. On that account, perhaps, the line bounding the under side should be that which presents the greatest strength, when the beam is supported at each end, and loaded equally throughout the whole length, which is a semiellipse.

The want of a practical treatise on the strength of cast-iron, and on the various forms of section, suitable for beams, acted upon in different ways, was, until lately, very much felt. Mr. TREDGOLD has, in his treatise on the strength of cast-iron, supplied that want, and given numerous practical examples on the subjects. The reader, who wishes to inform himself further on the subject, will find some very useful information in that work.

Account of some Experiments made at Walker Foundery, near Newcastle-upon-Tyne, the property of Messrs. LOSH, WILSON, and BELL, on the strength of cast-iron rails.

The rails were all cast from the same pattern, the difference in weight being accidental. Section similar to Fig. I. and III., Plate II., two inches and a quarter broad on the upper side, and tapering away to one inch and a half in the middle, and again swelling out at the bottom into the square c,b, each side of which is seven-eighths of an inch. Extreme depth, in the centre, six inches, gradually decreasing towards the ends, or points of supports, in a parabolic form, to four inches. In the experiments the rails were fastened in the usual manner to the chairs, which were fixed upon beams of wood.

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In comparing the strength of the different rails with each other, we find a great variation, not only between the different kinds of metal, but also in rails cast from the same metal. The only constant and regular law appears to be, that the weight, or specific gravity of rails, formed of a mixture of different kinds of metal, is uniformly greater than of one description of metal separately, and also that such a mixture makes the rails invariably stronger. This is a very useful discovery, and enables the founder, by mixing different metals in the proper proportions, to form a rail much stronger with the same weight of metal, than could otherwise be done by casting them of any particular kind of metal alone.

The depth of the middle section of the castiron rail renders them very rigid, and the deflexion is comparatively trifling before fracture. The recorded weights are those which produced fracture. In loading the rails, the weights should of course be much less than that which breaks the rail; inequalities of the road, or occasional obstacles occurring upon the surfaces of the rails will sometimes produce jerks or shocks to the wheels of the carriages, and the re-action will transfer those to the rail, and cause blows which, from the brittleness of the material, will be very liable to produce

fracture. In the form of carriages, too, placed upon four wheels, the weight upon any one of these wheels is far from being regular. The frame of the carriage is necessarily made square, and the sides quite parallel to each other, and are kept permanently so by the sides being firmly bolted or fastened together. The bearing section of the wheels is, therefore, perfectly square and parallel when the road is not similarly square and parallel, (and which, in practice, is seldom or never the case,) the weight of the carriage will be frequently, if not always resting upon three wheels only; and, in many instances, when a change in the parallelism of the rails takes place, upon two, on opposite sides of the carriage, the transition of the weight from one wheel to another will, therefore, produce a continual succession of blows or shocks to the rails, which will be productive of considerable injury, and occasion frequent breakage.

From these causes, and others which it is not necessary to mention, we find that it is not practicable to subject the rails to a greater load than is considerably within the limit of their absolute strength. In the preceding experiments, the least weight of the rails formed of a mixture of metals is seven tons, and of the unmixed rails five tons; and the rails were of the