It was soon observed that sand and fine particles of gravel from the ballast worked their way into the lower part of the recess, and became so compact as to prevent the wooden wedges working further down to increase their grip on the rail. Even when the recess was kept free and clear of sand, the enormous pressure exerted by the wooden wedges broke the iron at A, although an extra thickness was given to that 217 part of the section. The cast-iron was exposed to the greatest strain at the point where it was the least capable of offering resistance. Much ingenuity was displayed in many of the patterns brought forward, but in dealing with a hard unyielding material like cast-iron, it is difficult, if not impossible, to impart any soft, elastic effect; and the different systems of cast-iron sleepers failed to become popular on our home railways, on account of the noise and vibration when trains passed over them. Another objection was the great multiplicity of parts required in many of the types, and the constant and severe strain produced on the fastenings on the passing of every wheel. The bolts might be made tight at first, but the incessant shaking would work them loose, the threads became stripped, and the rails ceased to be held in a proper and secure position. The cast-iron sleeper road was considered unsuitable for the heavy and fast traffic of our home lines, and was ultimately all taken up and replaced with wooden transverse sleepers. At the same time, there is no doubt that cast-iron sleepers have been of great value in India and tropical climates, where timber sleepers were not only scarce, but perish very rapidly. Very large numbers of them have been laid down abroad of patterns very similar to those shown in Figs. 309, 310, and 311, and have done good service for many years. They are not affected by rain or heat, but, unfortunately, being castings, are liable to considerable annual loss from breakage. Improvements in plate-rolling machinery, and in appliances for bending and stamping wrought-iron, have materially assisted in developing the introduction of wrought-iron and steel sleepers. Cast-iron and wrought-iron are, in the abstract, hard and non-elastic as compared with wood; but whereas cast-iron can only be made into fixed, unyielding shapes, wrought-iron and steel can be worked into forms that possess a certain spring-like effect, which not only enables them almost entirely to resist fracture, but also imparts a measure of elasticity to the permanent way. The simplest form of wrought-iron sleeper would be a plain, flat plate, to which the chair, or rail-bracket, would be attached; but as this form would have bearing surface only, without any lateral hold on the ballast to keep the rails to line, it could not be adopted.