Great as are the difficulties of keeping up a continuous flame, they have been almost overcome by an arrangement introduced by Mr Renton, which preserves the cylinder of lime from cracking.  Gas has lately been introduced in the lighthouse at Hartlepool.  Hopes were entertained that electric lights might be introduced, but the great difficulty is to maintain an equable force, as the battery gradually declines in power. There are also other difficulties to be mastered.  The most successful experiments have been carried on in the South Foreland lighthouse, by an arrangement of powerful magnets.  The current thus produced passing through the carbon pillars, produces a splendid light, entirely eclipsing all other modes of illumination.  Years ago a limelight was so arranged as to be used on board ship for illuminating objects at a great distance.  By its means, an intended attack of torpedo vessels could be detected.  It was employed also in the Abyssinian expedition, for illuminating the advance camp when there was a possibility of it being attacked by Theodore's troops.  Now, however, electric lights are used on board all the first-class men-of-war, incandescent lamps being fitted for internal use, and arc lights for signalling and searching purposes.