By 1871 the work of several inventors had produced a press that printed an endless sheet of paper on both sides and folded it automatically. In the same year Ingersoll invented his compressed air rock drill. In 1872, Lyall invented his positive-motion weaving loom, and Clerk Maxwell propounded his electro-magnetic theory of light. According to this theory, luminous and electric disturbances are the same in kind, the same medium transmits both, and light is an electro-magnetic phenomenon. This was a most important invention in the field of physical science, and is now accepted by the majority of scientists. It is not so applicable to the needs of men at the present moment as the weaving loom; but in the future, it may be more so. In the same year, Westinghouse invented an improvement on his original air-brake that made it automatic under some conditions, and in the following year Janney invented the automatic car-coupler. Both of these were brilliant inventions, though not nearly so brilliant as Clerk Maxwell's. They were immeasurably more important, however, from the standpoint of material contributions to the Machine. One result was that the inventors were immeasurably more rewarded in a material way than was that great mathematical physicist, Clerk Maxwell. In the same year of Our Lord, 1873, Willis invented his platinotype photographic process, in which finely divided platinum forms an image virtually permanent, and Edison invented his duplex telegraph. This was the first of those wonderful inventions that made Edison famous; and it embodied possibly as brilliant an idea as he ever conceived. The principle was exceedingly simple, and consisted merely in using currents that increased in strength as the key was pressed to actuate an ordinary electro-magnet for one message, and using currents whose direction was reversed when the key was pressed, to actuate a polarized relay for another message. By combining this scheme with one long before proposed, of putting the receiving instruments across the arms of a Wheatstone Bridge, the entire system could be duplicated, and two messages sent at the same time in each direction. This, of course, constituted quadruplex telegraphy.