Not a few important improvements in the art of construction have been effected by using old principles in anew way ; and it does not appear that those who thus divert the ideas of others into novel channels, deserve less credit than more original inventors. Success is, after all, the popular test of inventive skill; and as an invention fails or succeeds, so will the voice of public opinion award praise and wealth on the - one hand, or .oblivion on the other, to the producer. A short time since an apparently novel, and certainly ingenious application of an old principle to a new purpose was brought under our notice in Sheffield. Whether the idea involved is or is not absolutely new, we shall not pretend to decide. Certain it is, that if as successful as it promises to be, the invention will effect a considerable advance in the manufacture of rails, and therefore we have no hesitation in bringing it prominently before our readers. For very many years attempts have been made,from time to time, to produce a rail which shall have a hard table and a comparatively soft and ductile web and foot; such a condition would obviously best be complied with by a rail, the table of which would be of hard steel, while the web and foot would be of iron. Nearly all these attempts have resulted in failure. Dodd's rails, the upper tables of which were converted by a species of cementing or case-hardening process, have not become popular; either because the process of converting was uncertain in its results,or the cost was greater than the result was worth. Steel-topped rails, made by welding the steel top "jo an iron bottom failed, because, under heavy work, the steel invariably peeled away from the iron, unless the weld were carried into the web ; and even then only puddled steel, little harder than some varieties of iron, could be used. No one, so far as we are aware, has attempted to weld cast steel to an iron web by hammering or rolling. The cost, including wasters, would be enormous, and the difficulty of securing a perfectly sound weld over miles of bars insuperable. It follows that rails, as now made and generally used, are all iron or all steel, or of the compound type used by Mr. Ashcroft on the Charing Cross line, in which a steel top and web are secured between wrought iron angle flitches by cross bolts. We have recently examined rails with cast steel tops made at Sheffield by, as we have said, a new application of an old process,which bid fair to solve a difficult problem. Too few of these rails have been made to enable us to pronounce the process a complete success ; but bearing in mind the very imperfect nature of the experimental appliances by which they were produced, the results have been very satisfactory ; and as new furnaces and plant are being put down to test the principle thoroughly, we shall soon be in a position to pronounce a positive verdict on the subject, one way or the other. The process of manufacture is excessively simple and may be explained in a very few words. An immense number of cutting blades, for shearing iron, slicing tobacco, carpenters' planes and chisels, wood-turning tools, etc., are made every year in Sheffield, in which a very moderate quantity of cast steel, of the best quality, is secured to. anything rather than a moderate quantity of, it may be indifferent, iron. Popularly, it is thought that the steel is united to the iron by welding under the hammer; but this is contrary to fact. The cost would be too great, and the weld might or might not be good. A far more elegant system is adopted. Let us suppose that a heavy steeling for a pair of shears is required. In producing this, an ordinary steel ingot mold is taken, and set up on end in the casting house. The mold is made of iron, rectangular in section, and in halves, secured together by bands and keys. For convenience, we give here a plan of the mold, looking down it from the top. A pile of scrap iron is heated and forged under the hammer. Its weight may be anything, from 30 Tb. or 40 lb. to 2 cwt. or 3 cwt., and in section its shape is shown in the cut next below. This pile or bloom is about the same length as the mold. A short time before the steel pots are ready to be drawn, the pile is heated, in a proper furnace, to a bright red heat. It is then brought quickly to the casting house and placed in the mold. The whole, then, is in cross section, as in the third cut. Melted cast steeel, in proper quantity,is then poured into the vacant space, and the result is that the steel unites so soundly to the iron, the surface of which it partially fuses, that it is difficult to tell, on making a cross section, where the iron begins and the steel leaves off, to the one sixteenth of an inch. The ingot may then be reheated and worked into any required form by rolling or hammering, the steel always reducing in a given ratio with the iron. We have seen combination ingots, consisting of some 4 cwt. of iron and 1 cwt. of steel, thus made with perfect success. This is the principle which has been applied by Mr. E. Gray, of the Moscow Works, Sheffield, to the mannfacture of steel-topped rails. He places within an ingot mold, of the required size, a heated pile of iron, A, and he fills up the vacant space, B, with fluid cast steel. From personal inspection of numerous samples, we have ascertained that the union of the two metals is perfect. No subsequent rolling or hammering will separate them. In converting this ingot into a rail, it is passed through rolls in the usual way, but care must be taken to drive the mill as though it were working altogether on steel. If the pile is highly heated, and the rolls are run quickly, the steel will behave precisely as cast steel always behaves under such conditions ; it cracks and splits, and breaks up along the edge of the table. If the pile is moderately heated, and the rolls run slowly, and with an easy draft, the steel works perfectly, ! and a rail results,which, judging from inspection, leaves noth ing to be desired. We need hardly add that, should the process be as successful as the inventor in our opinion reasonably believes, a rail will shortly be introduced to the public which will be superior to any other now in the market, pos-sessing,as it will, that combination of hardness and toughness most desirable and most difficult of attainment. The process, it will be seen, was applied to other purposes than the manufacture of rails nearly thirty years back. It is, of course, possible that difficulties may arise, which even the practical steel worker and Mr. Gray has been working steel all his life cannot foresee, which will defeat the success of his process. But it is not easy to understand in what they will consist, and we are, upon the whole, justified* we think, in regarding Mr. Gray's invention as one full of promise, and likely to lead to very important results. The Migineer.