Still until the advent of on board electronic navigation equipment (radio direction finders, depth finders, radar and LORAN) sound signals were the mariners best friend during periods of reduced visibility…and certainly of more assistance that groping along with only the information supplied by a lead line. Navigating by sound is less than ideal and often, a tricky way to maneuver during reduced visibility. A signal “rated” for four miles might be heard at only two miles or, given the right atmospherics, 8 miles. Not only is it hard to tell the distance one hears a signal, it is impossible to predict that a signal can be heard at a certain range. And distances cannot be determined with any accuracy. A mariner thinks he hears a fog signal from one direction, when in fact it originated some 45 degrees to the right…or 30 degrees to the left. The sound from a fog signal might be heard at one mile, not at two miles and again at three. Cold and warm layers of air cause sound to deflect, skip, bounce, echo and sometimes stop cold. The propagation of sound is not a constant, especially during periods of fog. Near shore he listened for the crash of surf, used his lead line and often dropped the “hook” until visibility improved. When reduced visibility set in during the days of sail the mariner at sea, unable to see his stars, had only dead reckoning upon which to rely. Although not an exact science it was better than nothing during periods of reduced visibility prior to fog signals. The mariner or pilot familiar with an area could tell, more or less, where he was in a channel by the type of evidence stuck to the tallow. The tallow picked up bits of shell, small rocks, sand and other items from the bottom.
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In the bottom of the lead was a depression filled with tallow. When a ship approached a restricted channel or harbor entrance the leadsman constantly cast a lead line, which gave the navigator, pilot or captain a running commentary on the depth of the water. The early mariner also had his lead line to assist him to navigate into the ports of the world. This was certainly not adequate when running close to shore, but sufficient for crossing the ocean. Celestial navigation gave him an idea within a mile or two of his vessel’s location. The navigator also relied on stars when at sea. But light, no matter how powerful, cannot penetrate fog and other conditions of reduced visibility. Although the light signal from the lighthouse tower left much to be desired until the 18 th century, there was some sort of light to guide the seafarer.
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Since the mighty Pharos of Alexandria, Egypt (circa 280 B.C.) mariners have had, after a fashion, a light to guide them into port or clear of dangerous reefs. But, in fact, fog (correctly termed, sound) signals are relative newcomers to the field of navigational aids, and the most popular of them, the diaphone and diaphragms, are of the 20th century. It seems as though that sound must have always been part of the bayscape. Beeeohhhhh, the mournful sound of the fog signal hooting and echoing across a bay shrouded in gray.