The experience obtained year after year in the wear of rails under 190 heavy traffic, led to continued improvements both in the method of rolling and in the selection of the iron to form the rail-pile; one description of iron was found more suitable for the head, or running surface, and another for the vertical web; but, even with the best machinery and most carefully assorted materials, high-class wrought-iron rails were liable to lamination, and long thin strips of iron became detached from the upper, or wearing, surface. The rail was composed of many layers of iron, and it was not always possible to ensure that they were all thoroughly welded, or incorporated together. As early as 1854 a few experimental solid steel rails were laid down on some of the principal railways, and gave excellent results as to evenness of wear and durability, but their cost of manufacture rendered their extended use almost prohibitory. Compound rails of steel and wrought-iron, as in Fig. 260, were also tried on several railways, but the practical results were not such as to lead to a very extended adoption. In preparing the _pile_ for a compound rail, suitable wrought-iron bars were placed to form the lower member or flange, the web, and part of the head, and a slab of steel was placed on the top to form the upper portion of head, or wearing surface of the rail. It was intended that in the process of rolling these distinct layers were to be incorporated together, to form the section shown in Fig. 260. Doubtless many good wearing rails were manufactured on this system, but the inherent difference of the two materials, steel and iron, rendered it very difficult to ensure such uniform incorporation as would withstand the constant pounding under heavy, fast traffic. It was not until some years later that the process of the Bessemer Converter was discovered and perfected, by means of which steel can be produced in large quantities far more rapidly and at much less cost than by any other method hitherto adopted. The introduction of this process for making steel caused a complete revolution in the material for rails. Steel which had previously been excluded on account of its cost, could now be supplied at a moderate price, and, from its compact and homogeneous character, promised a very much longer wearing life than the best wrought-iron rails that had ever been rolled. Experience has shown that these promises have been fully verified; wrought-iron rails are things of the past, steel rails have taken their place, and can now be purchased 191 at a less price per ton than the iron rails of twenty years ago.