Steel has been made for centuries, but until a few years ago, the process was slow and costly, and the tools with which the men worked were really treasures. In those days a pocket-knife was a thing of great value. The railroads used iron rails but these soon wore out. If it had been suggested that steel be used a protest would have been made on the grounds that steel is too expensive. Trains had to be shortened; coaches and locomotives built of light material because iron rails and bridges could not stand the strain. As land in the cities became more valuable and taller buildings were needed, stone and brick not proving adaptable and too expensive, the Bessemer process, which manufactured steel cheaply and in great quantities, came to meet a long-felt need. Iron was plentiful but the process of converting it into steel had not been mastered. The great difficulty in manufacturing steel is to get just the right proportion of carbon mixed with the iron. The Bessemer system takes all the carbon out and then puts back into it the quantity that is needed. Tons of molten iron are run into an immense pear-shaped vessel called a converter. Blasts of air are forced in from below. These unite with the carbon and the impurities such as sulphur and silicon are destroyed. There is a roar and clatter and a terrific din. A great bolt of red flame shoots forth many feet from the mouth of the converter. Its color changes from red to yellow and then to white. When the flame becomes white the workers know that the carbon and other impurities are all gone; and this is the signal for the blast of air to be turned off. Then a quantity of special iron ore in melted form, containing the right amount of carbon to convert the whole into steel of the desired degree of hardness, is poured into the purified molten iron in the converter. This huge converter is perfectly poised upon pivots so that it can be moved with very little effort. The molten steel at the next stage is poured from the converter into square molds and the blocks resulting from it are called blooms. These are then started through the mill, passed under and between rollers of different shapes and kinds, and drawn out into plates, rails, or beams.