one week off. Located in the engine-house, they also drove an air-pump to feed the pressure tank of a foghorn. In December 1891 the lighthouse tower was painted with a broad red band, to make it more conspicuous during the day. Foghorn Holmes also designed a reed fog signal for the new lighthouse, sounded from a separate foghorn house (east of the tower) through a pair of vertical trumpets (angled north-east and south-east, facing out to sea). It sounded one blast every 45 seconds. Compressed air for the foghorn was piped underground from the engine room, on the other side of the lighthouse, where the air compressor was driven by a common drive shaft from the engines powering the magnetos. A more powerful siren fog signal replaced the reeds in February 1879; it sounded one blast every minute. It required an additional and more powerful engine, which was installed in the engine room to drive the compressor for the new signal. Charts and engineering (1875) 20th century Conversion to oil In 1914 the pioneering electric light at Souter was replaced with the latest type of incandescent oil lamps (i.e. paraffin vapour burners). At the same time Chance Brothers provided a new, much larger lens system: a bi-form first-order catadioptric revolving optic (a bi-form lens is double-height, containing two lamps, one above the other). At the same time a new lantern was provided, to accommodate the taller optic. Red shades were placed around the lamps, within the lenses, so that the light now flashed red, once every 5 seconds. The 1914 optic remains in situ in the tower; it weighs 4.5 tonnes and floats on 1.5 tonnes of mercury. A separate lamp was used for the fixed red/white sector light, which continued to shine from its window lower down in the tower. Having removed the two Holmes magneto-electric machines from the engine room, the Corporation of Trinity House presented one to the Institution of Electrical Engineers in 1915; it is now on display at the Science Museum, London. The fog signal was improved in 1919 when a more powerful siren was installed, sounded through a pair of Rayleigh trumpets. (These replaced the twin Holmes trumpets formerly employed; the foghorn house was remodelled at the seaward corners to accommodate them). The siren gave a 4-second blast every minute. Hornsby oil engines were installed to drive the compressors for the new siren. Reversion