Letters to Editors

Abstract

PERHAPS your correspondent, Charles Dennehy, M.R.C.S.I., R.M.S. Shannon, may find an interpretation of the nocturnal musical phenomenon (mentioned in NATURE, No. 28) experienced by iron ships when at anchor in seven or eight fathoms, with a bottom consisting of a heavy, dark sand and mud containing much vegetable matter, in the following natural system of gas-escape. In examining certain pools of water in the East, notorious for their poisonous qualities at certain seasons of the year, I was aware of intermittent risings of vast quantities of bubbles. The waters rested on vegetable deposits; if these were stirred up, large globules rose with considerable force, and I came to the conclusion that these air risings were due to the escape of gases from the decomposing vegetable matter. If any metallic body had met these bubbles as they rose, some sound would have been produced, the nature of it depending on various causes. The reason of the sound being heard on board ship between twelve and two, and not between two and four, is owing to a very simple, but beautiful rule of law: as the gases are at all times collecting, we might suppose that they would be at all times escaping, but as the surface of the bottom is of an elastic nature, the water pressure imprisons the gas as if it were within a valve; but when the force of the gas overpowers the water pressure, there is a bubbling escape till the collected gases are expended, and thus I account for the sounds continuing “about two hours, with but one or two very short intervals.” It is by no means improbable that the musical performance occurs more than once in the twenty-four hours, though the ordinary noises of shipboard prevent its being audible. I believe there is no other way of accounting for this incident; but the test I would propose is to stir up the bottom on a calm day with considerable force; if large quantities of air-bubbles arise, the sailors may rest satisfied that the concert is not given by ghost, mermaid, or siren, but simply by a continued contact of myriads of gas globules against the ship's bottom. The stirring up will not necessarily cause the sound, as the bubbles may be diverted by undercurrents.