Two years ago I had an opportunity of making from the deck of the steamer “Pharos” some observations on the performance of the fog-siren at Sanda, off the Mull of Cantire, The instrument is worked by air at about 1¼ atmospheres pressure; and, though driven by a powerful air-engine, sounds for 7 seconds only per minute. One obvious defect of such an arrangement I saw to be the waste of energy in producing a current of air through the trumpet of the siren along with the oscillations.