Now with the old steel a very little heat will suffice to make it soft, when its cutting power is lost. So with the old steel, no matter how much cooling water you might use, there was a distinct limit to the speed of the lathe and the speed at which the work was finished, for if that speed were once exceeded a stop became necessary to regrind the tool or to put in a fresh one. But with high-speed steel that limit is much higher, for it can get almost red-hot before it loses its hardness and consequently machines can be run and jobs finished at a speed which would have been out of the question only a few years ago. If one belligerent knew how to make high-speed steel while the other did not the former would have an enormous advantage in war-time. Speaking generally, steel such as is used for tools is called hard steel, while that made by the Bessemer and Siemens-Martin processes is called mild steel. Leaving out of account for the moment fancy steels such as that just described, where other metals are added to the mixture, the essential difference between all the varieties of steel is simply a slight difference in the percentage of carbon. This is so remarkable that it is worth while to tabulate these percentages again. Cast iron has from 2 to 5 per cent. Steel from one-fifth to one per cent. Wrought iron less than one-fifth per cent. Mild steel, which has least carbon of all the varieties of steel and in this respect is therefore nearest to wrought iron, is used for the same purposes as wrought iron, such as shipbuilding, bridges and roofs, tanks, gas-holders, etc. When the Admiralty want a specially fast ship such as a torpedo-boat destroyer with a hull as light as possible consistent with strength they have it made of steel with a slightly larger percentage of carbon so that the steel is stronger and the vessel's frame can be made lighter. The steel for shells, too, needs to be of a certain strength to give the best results, so the percentage of carbon is adjusted accordingly. For guns themselves, again, special properties are needed, and so not only is the carbon regulated to a nicety but other things such as nickel and chromium are added. Altogether, steel is one of the most marvellous substances known, certainly the most marvellous metal. Copper is just copper and no more, zinc is just zinc, and the same with lead, but iron (which really includes steel) can be adapted to so many purposes, can be endowed at will with so many different properties, that without doubt iron, common, plentiful iron, is the king of all the metals. CHAPTER VIII MORE ABOUT GUNS As has been remarked elsewhere, some of the guns used by the soldiers in land warfare are very different from those used in the navy. The latter, being carried on the ships to which they belong, can be of those proportions which best suit their purpose. Consequently they are usually very long compared with their diameter.