At best, however, the bell was found to be an inefficient fog signal. In the first place its range was limited and uncertain. At times a bell could be heard for several miles and again its sound would carry seaward but a few hundred yards from the source, depending on the direction and velocity of the wind.
John Floherty, Sentries of the Sea, J.P.Lippincott Co., New York, London, p.38.
In order to make full use of the submarine bells, a ship must be equipped with receivers placed on each side of the hull close to the bow and connected by telephone with the bridge. The listener using double earphones can determine the direction of the submarine bell by the loudness with which the signals come in on one side or the other. Signals can often be heard on ships that have no receiving apparatus by placing an ear against the side of the hull well below the waterline. This device is sometimes used by fishermen.
At this moment, midnight, I can hear several vessels - there are four of them - feeling their way through the fog that shrouds the narrowing waters of Long Island Sound. At intervals of a minute, each of them lets go a long whistle blast. A deep-throated roar tells me that a large oceangoing freighter is in the group. Then comes a grunting blast from the air whistle on a Diesel-engined vessel of a smaller size, probably a trawler hastening to market.
John Floherty, Sentries of the Sea, J.P. Lippincott Co., New York, London, 1947, p.39.
TIME: post W.W.II
PLACE: Long Island, N.Y.