In order to understand how a variation in currents of this kind can be applied in general for producing a rotary motion in the two directions, it will only be necessary to refer to Figs. 3 and 4. The conductors, L1, L2, and L3 communicate with the bobbins of three electromagnets, E1, E2, and E3, whose poles are bent at right angles to the circumference of the wheel, R. There is never but one pole opposite a tooth. The distance between two consecutive poles must be equal to a multiple of the pitch increased (Fig. 3) or diminished (Fig. 4) by one-third thereof. It will be seen upon a simple inspection of the figures that R will revolve in the direction of the hands of a watch when the currents follow the order L1, L2, L3, etc., in the case shown in Fig. 3, while in the case shown in Fig. 4 the rotary motion will be in the contrary direction for this same order of currents. But, in both cases, and this is the important point, the direction of rotation changes when the order in the succession of currents; is inverted. Fig. 6 gives a perspective view of the registering apparatus, and Fig. 5 represents it in diagram. It will be at once seen that, the toothed wheel, r, is reduced to its simplest expression, since it consists of two teeth only. The electro-magnets are arranged at an angle of 120 deg., and for a change of current the wheel, r, describes an angle of 60 deg., that is to say, a sixth of a circumference. The motion of r is transmitted, by means of the pinion, d, and the wheel, e, to the wheel, T. For a one-meter variation in level the wheel, T, makes one complete revolution. It is divided into 100 equal parts, and each arc therefore corresponds to a difference of one centimeter in the level, and carries, engraved in projection, the corresponding number. As a consequence, there is upon the entire circumference a series of numbers from to 99. The axle upon which the wheel, T, is keyed is prolonged, on the side opposite e, by a threaded part, a, which actuates a stylet, g. This latter is held above by a rod, I, which is connected with a fork movable around a vertical axis, shown in Fig. 6. The rectilinear motion of g is 5 mm. for a variation of one meter in level. Its total travel is consequently 40 mm. The sheet of paper upon which the indications are taken, and which is shown of actual size in Fig. 7, winds around the drum, P, and receives its motion from the cylinder, W. This sheet is covered throughout its length with fine prepared paper that permits of taking the imprints by impression. [Illustration: FIG. 5] [Illustration: FIG. 6--RECEIVER OF SIEMENS AND HALSKE'S MARIGRAPH.] [Illustration: FIG. 7]