Having slightly diverged from the original lines by describing a system which is at present not introduced into Europe, a few remarks on the subject of electric motors may not be inappropriate, as they are almost universally worked in the United States, from the installation which supplies electric light. There is a considerable profit to the electric company if electric power is taken in the district, the wires conveying the lighting current are thus economically employed during the day. In the diagram, Fig. 15, which represents a district at Boston, the curve on the right principally represents the demand for power which takes place between the hours of 8 A.M. and 3 P.M. A circular was addressed to all the leading electric companies in America a short time ago, asking if they supplied power as well as light, also for what purposes it was used. Answers were received from 56 companies, who stated that the motors were employed for:—driving ventilator fans, collar-and-cuff machines, printing-presses, various apparatus in repair-shops, sewing-machines, coffee-mills, gun-shop tools, sausage-machines, elevators, lathes, pumps, saws, ice-cream freezers, organ-bellows, and washing-machines. The size of motors varied from one-eighth to 15 horse-power; 26 companies have supplied motors from arc light circuits, 14 from arc and incandescent, and 16 from incandescent circuits alone. The motors are principally owned by the subscribers, and are charged for at a rate varying from £3 to £15 per horse-power per month. The motor business is still in its infancy, but is cited to show how Electric Power can supplant the steam-engine, especially for those purposes in which the power required is small and complete control is desirable. [Illustration: FIG. 15.] CLASS II. THE EDISON PARALLEL SYSTEM, WITH CONTINUOUS CURRENT. It will be found, on examining Appendix II., that in European stations by far the larger number of lamps are maintained from installations employing the Edison system; the Ferranti plan of using transformers comes next, closely followed by Goulard and Zippernowsky; the distribution with secondary batteries follows, and the high-tension multiple series comes last. [Illustration: FIG. 16.] The Edison system has frequently been discussed, in connection with small installations, but in magnitude the stations in Berlin and in Milan exceed anything that has been started here with continuous current. Before describing the central electric light station at the former city, it may be well to recall to mind that the Edison plan is the combination of a number of machines which pump electricity into a network of feeders, mains, and conductors, the lamps being placed in parallel circuit, as shown at L _l_, Fig. 16, and maintained at a constant potential of 110 volts. [Illustration: FIG. 17.]