Gray, while using the principles involved in compounding a point, seems to have discarded the ways of transmitting magnetic impulses of varying strength commonly in use. His method he calls the "step-by-step" principle, and it is a striking example of what patience and ingenuity may accomplish in the management of what is reputedly the most elusive and difficult of the powers of nature. The machine was some six years in being brought into practical form, and was perfected only after a long series of experiments. In its operation it deals with infinitesimal measurements and quantities. The first attempts were on the "variable current" system, which was later discarded for the "step-by-step" plan mentioned. In writing an ordinary lead pencil may be used. From the point of this two silk cords are extended diagonally, their directions being at right angles to each other, and the ends of these cords enter openings made for them in the cast iron case of the instrument on each side of the small desk on which the writing is done. Inside the case each cord is wound on a small drum which is mounted on a vertical shaft. Now if the pencil-point is moved straight upward or downward it is manifest that both shafts will move alike. If the movement is oblique in any direction, one of the shafts will turn more than the other, and the degree of all these turnings of each shaft in reference to the other will be precisely governed by the direction in which the pencil-point is moved. [Illustration: DIAGRAM OF MECHANICAL TELAUTOGRAPH. BOW-DRILL ARRANGEMENT.]