About the same plan is followed out on all kinds of welding except pipes and rails. Locomotives can be repaired without taking the complicated machinery apart just by working around until the crucible can be so hung, and the pouring gate so arranged that the metal can be poured into the place designed for it. The chief difference lies in the size of the weld to be made and the consequent amount of thermit to be used. Welds have been made where as much as 2,000 pounds of thermit--enough to make 1,000 pounds of steel--have been run into a mould. In these very big welds a certain percentage of steel "punchings," or small pieces of steel, and a little pure manganese are used to give the additional hardness to the weld. Without going into details as to the manner in which the principle of the thermit process is applied on rails or pipes, it will be enough to say that in welding rails three different systems are used. The first is done by building the mould around the ends of the two rails to be welded together and letting the thermit steel run in and completely surround the rails and the space between them. This gives one continuous rail just as far as the welding is carried on, and one through which the electric current of an electric road can pass without any trouble at all. It is plain, then, why this system is used so much on third rails of electric roads. The trouble with it is that the swelling on the top and inside of the rails must be machined down to present a smooth running surface to the wheels. The next system, which is now almost out of date, is one in which two moulds are used so that the thermit does not come up over the running surface of the rails. This relieves the engineers of the necessity of machining the welded joints. The third system is a mixture of the joining by plates and the thermit process. This is called the "Clark joint," after the name of Chief Engineer Charles H. Clark of the Cleveland, (Ohio) Electric Company, who formulated the plan. The rails are joined with plates and bolted, or riveted together in the old way, but a thermit weld is made at the base of the rail, welding the bases of the two rails together and to the plate.