Now a falling object, if only it has such weight that the resistance of the air is negligible, falls according to a well-understood law, which law it obeys with the utmost accuracy. Therefore the distance fallen by the weight between the passage of the shell through two points gives a very accurate record of the time taken to travel from one to the other. Of course several such frames can be used if desired in the same way. But to return to the gun itself. It is not merely one piece of metal but several tubes beautifully fitted one inside another. Moreover, in the British gun at all events, between two of the tubes there is a space filled with "wire." This wire is perhaps better described as steel tape, and is of the finest material for the purpose, flexible and tremendously strong. It is wound round and round one of the tubes until there are many miles of it on a single gun. It is wound tightly, too, by means of special machinery. The purpose of the wire is to resist cracking. The solid steel tubes may crack, and, as is the way with all cracks, these will tend to grow longer and longer. The many turns of wire, however, will not crack. Even if a few turns should break, the damage will not spread, and the gun can probably go on as if nothing had happened. The material of which these guns are made is nickel chrome gun steel. Steel is ordinarily an alloy of iron and carbon, but this metal also contains traces of nickel and chromium, which make it specially suitable for its special purpose. Each of the tubes of which the gun is formed start as an ingot, a mere lump of metal, but roughly shaped. The requisite mixture is obtained in a furnace and the molten metal is run out into a mould. The ingot is heated again and pressed under enormous hydraulic presses until it is approximately the shape required. This pressing not only produces the desired shape, it also improves the quality of the metal. The rough forging is then bored out, to make it into a tube. One is inclined to wonder why the ingot is not cast hollow to commence with, and so save the labour of boring it all out later. The explanation of this is that certain impurities are always present in the metal and these always gather together in the part which sets last. Now in a solid block or ingot it is clear that the centre is the part which will set last, and hence that is the part where the impurities will congregate. Then, when the centre part is all bored out the impurities are entirely removed. The tube is shaped externally by being turned in a lathe. The innermost tube is not simply smooth. There is a spiral groove, called the "rifling," running round and round, screw fashion, inside it. The purpose of this is to give the shell a spinning action which causes it to keep point foremost throughout its flight. But for this the shell would tend to turn over and over, resulting in uncertain and inaccurate flight.