The multiple tool system is also used in milling machines in which a number of milling cutters either of the face or the end type come into play simultaneously upon a piece of work set in a suitable jig, and cut the piece with absolute precision, so that all castings will have faces accurately spaced apart and cut to exactly the same level. It is by such methods as these that we are able to produce such large quantities of machinery at remarkably low cost. One of the most notable examples of such work was the development of the Liberty engine during the World War. This engine did not differ in principle from others built in Europe or in this country, but its design was carefully adapted to permit of interchangeable manufacture. No careful finish was used except where indispensable. Special jigs, tools, and fixtures were prepared. Ingenious wrinkles of American manufacture were introduced. All this consumed time, and great was the irritation of the general public. Under ordinary conditions, it would have taken years to have developed the Liberty engine to the manufacturing stage, but under the urgent stress of war, the whole work of design and preparation for manufacture was crowded into a few short months, and then Liberty engines began to be manufactured on a stupendous scale. CUTTING WITH RED-HOT TOOLS One of the most remarkable advances in machine tools was due to the studies of Fred W. Taylor. He entered a large steel plant in 1880 and was immediately struck with the enormous waste of effort on the part of the men in the plant. There was at that time considerable dissatisfaction among the workmen, and when Taylor endeavored to speed up work he was faced by the incontrovertible argument that he had no idea how much work a certain machine ought to turn out. There was nothing for him to do but either back down or study machine tools and discover their maximum capacity. This led him to investigate the matter of cutting speeds. For years he spent all of his spare time studying this subject, timing machines and experimenting with different types of cutting tools. He estimated that in the twenty-six years of his investigation he converted 800,000 pounds of steel into chips. What he wished to discover was the best depth of cut, the best speed of cutting, and the best speed at which the tool should be fed into the work. He soon discovered that, contrary to prevailing opinion, the round-nosed tool was better than the diamond-pointed tool, that the coarse slow-cutting speed was better than a fine cut at high speed. He discovered that the best method of lubricating the tools was to keep them bathed in a heavy stream of water, supersaturated with carbonate of soda, so as to prevent the metal from rusting. The best tool steel of that day was known as a self-hardening steel. Manufacturers of the cutting steels had warned Taylor that he must not use water on these tools. Taylor, however, was not satisfied to take the word of others,