The torches also vary in size from the little 8-ounce "jeweller's" torch, that the scientist used, to nineteen to twenty inches long and a weight of two and a quarter pounds. The average size, however, is twelve inches long with a weight of one pound. The welding torch is made up of two brass tubes, one for the acetylene and the other for the oxygen, connected at the two ends. At the nozzle end there is a sharp turn in the piping so that the tip is very nearly at right angles to the main pipes. At the handle end, are the connections for the rubber tubes that lead to the gas tanks, and the little valves by which the operator can control the flow of gas. The pipes carrying the gases to the tip are the same size the whole length, but at one end are enclosed in a larger tube, which serves as a handle. Now that we have seen the general construction of the oxy-acetylene torches, we will assume that the tanks, which look like large soda-water reservoirs, are filled with pure oxygen and acetylene gas, and transported to some convenient point in a railroad repair shop where great forges are spurting flames, and one can hardly hear the talk of a man beside him for the roar of the hammers and the compressed air riveters. Assume that some large expensive steel part of a locomotive has been broken and must be repaired quickly so that the engine can go out on the road to help haul an accumulation of freight. In the old days an engine would have to be taken apart, a new part turned out at the steel mill, shipped to the shops, and the locomotive put together again. Nowadays it is only necessary to take enough of the machinery apart for the workmen to get at the broken parts. After cutting off the edges to be welded so that they make a small V, and supporting them within the fraction of an inch apart in the exact position and shape that they are to be repaired, the workman selects a rod of steel or iron, to use in somewhat the same way the tinker uses a strip of solder when he wants to repair a break in a kettle with solder and soldering iron. The selection of this filling rod, or wire, is all-important, for the skilful and successful iron worker uses a piece of metal that will fuse well with the parts to be repaired, at about the same temperature at which they themselves will fuse. Mild steel or Norway iron which is 90 per cent. pure is frequently used, but there are no hard and fast rules because every master mechanic has his own ideas about such things, and would not take the word of any manufacturing company.