throughout the district by means of five sub-stations; and from this board there branches a double system of mains, which run everywhere side by side, one-half the mains being connected to the first machine and one-half to the second, so forming an excellent arrangement for the prevention of total extinction of the light. The mains running to the sub-stations are on the divided system, which is introduced for the purpose of saving copper, as in a solid cable the loss of pressure is greatest when the full number of lamps is on, and decreases as the lamps are extinguished. With the divided main system it is intended to follow out Sir William Thomson’s formula, which equates the value of the loss of head, and the interest on the saving on the copper. If for a certain main this formula shows that a fall of 20 per cent. is the most economical condition for working, then, since by the divided main the pressure can be kept within a variation of 2 per cent. at the distant end, it follows that a considerable saving can be effected over an ordinary solid main. Special arrangements are adopted at Paddington to keep the pressure constant, a fall of potential being allowed for; thus at the engine-house the pressure is 150 volts, in the passenger station it is 120 volts, and at Westbourne Park it is 100 volts. The arc lamps are fed by the same mains, and are arranged two in series. A small installation at Kensington Court, erected two years ago, for the purpose of supplying the houses in the immediate neighbourhood, has rapidly developed, and underground mains have been led in many directions from the station, and a constant service of electricity is provided for by means of secondary batteries. As this is the first practical exposition of the secondary batteries’ system of distribution, it is proposed to describe the installation under that head. Central-stations are also at work in Liverpool, Leamington, Taunton, Exeter, and there are also five large installations nearly completed in London, besides the Kensington Court station, all of which will probably be in full swing before the end of the year. ELECTRIC LIGHTING from CENTRAL-STATIONS is now practically carried out on five different methods. I. BY SECONDARY GENERATORS OR TRANSFORMERS, WITH ALTERNATING CURRENT. II. THE EDISON OR PARALLEL SYSTEM, WITH CONTINUOUS CURRENT. III. THE SERIES OR BERNSTEIN SYSTEM. IV. MULTIPLE SERIES. V. DISTRIBUTION WITH SECONDARY BATTERIES OR ACCUMULATORS. CLASS I. SUPPLY BY SECONDARY GENERATORS OR TRANSFORMERS.