After that operation has been going on for a time certain valves are operated and the gas and air then come in at the other end of the hearth, travelling through it in the opposite direction. And the air comes through the chamber which has the hot bricks in it, bringing back into the furnace a large quantity of that heat which otherwise would have gone up the chimney but which the bricks intercepted. Thus all day long does this reversal take place at intervals, the fresh air all the time picking up and bringing back some of the heat which just previously had escaped towards but not into the chimney. This arrangement enables the process to compete, so far as economy is concerned, with the Bessemer process. At intervals the steel is tapped off from the furnace and run into ingot-moulds, the same as with the other process. On the whole it is regarded as producing a slightly better steel, the operation being under slightly better control. However the steel is made the ingots are reheated and either hammered under a powerful steam hammer or pressed in an enormous hydraulic press. This greatly improves the quality. The steel can then be rolled into plates, bars or whatever form may be required. The finer qualities of steel such as are used for making sharp tools are made in quite another way. Instead of being made from crude iron by taking out the carbon, the materials are the finest qualities of wrought iron and charcoal which are mixed together in the correct quantities and melted in a crucible. This cast steel is very hard, so that it will carry a very fine, sharp edge. It is also capable of being tempered by heating and cooling, so that the exact degrees of hardness and toughness can be attained. Of recent years a special quality of steel for tools called "high-speed" steel has been produced, mainly by the addition to ordinary cast steel of a small percentage of tungsten. The advantage of this is that, within certain limits, this does not soften with heat, and it is, I can assure you, a great invention in war-time, when a nation is straining every nerve to turn out guns and shells as fast as possible. For all these things need to be turned in lathes and if you have ever watched a metal-turning lathe at work you will have noticed that the tool which actually takes a shaving off the article being turned tends to get hot. For this reason lathes are usually fitted with pumps which pump cold soap-suds on to the tool as it works. What you see there is the energy employed in shaving the metal being turned into heat in the tool. If left uncooled by the water it would soon be red-hot. And the faster the machine works the hotter will the tool get.