The rectangles, I and I A, show the street illumination, and are drawn to suit scale A; half an hour after sunset all the lamps are turned on, and the work reaches its maximum suddenly, and continues the same until 12 o’clock, when, according to the municipal decrees, it either falls one or two gradations until half an hour before sunrise, when all the lamps are extinguished. The calculations are based on the assumption of 640 watts to the horse-power, instead of 736, which is the theoretical efficiency of a German horse-power. If a number of diagrams are taken on this method for different periods of the year, the constant work can be ascertained. This knowledge is most valuable when calculating the most economical area for the mains, which is then easily accomplished by means of Forbes’ tables, which are based on Sir William Thomson’s well-known rule. The lines, 2 and 2 A show the constant work at the same two periods of the year from which the diagrams are taken. The constant work at the end of December will be found to amount to 20 per cent. of the total work, and that at the middle of June to 15 per cent. By summing up the average work for all the days in the year we obtain the cost per annum, and adding to this the expense of management, interest, &c., and knowing the local conditions, we can fix what proportion of the day’s work is admissible as loss. With the Edison system at Berlin, 5 per cent. is taken as average loss; thus, at the end of December, it amounts, with the maximum number of lights, to 18·8 per cent., and with the minimum to 1·1 per cent.; in the middle of July the maximum is 15·8 per cent., and the minimum 0·5 per cent. The dynamos must, of course, be of sufficient power to be able to overcome this loss, which only shows itself periodically; therefore the generating plant may be constructed to give, nominally, 20 to 30 per cent. less than the maximum work, and be capable of being pushed to the full amount for a short time only. THE POSITION OF CENTRAL-STATION LIGHTING.