MIT Institute Archives & Special Collections

Sonar chart, 1964

From Harold Edgerton's search for Spanish Armada wreck of 1588
Tobermory Bay, Scotland

Sonar chart Sonar chart Sonar chart

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| Position diagram
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Harold "Doc" Edgerton (1903-1990), who worked at MIT for 63 years as teacher, researcher, and head of the Stroboscopic Light Lab, was fascinated by all things unseen and undiscovered, including events too rapid for detection by the naked eye and mysteries of the marine depths, such as shipwrecks. Equipment and techniques developed or perfected by Edgerton have often been used to study sunken ships, e.g., Henry VIII's Mary Rose and the Union's Civil War ironclad Monitor. Sonar-scanning systems devised by him are capable of unveiling what lies beneath the ocean floor by drawing the pattern of mud-penetrating sound signals as they bounce back from objects buried in the seabed. The sonar chart and accompanying position diagram shown here are part of the field records of Edgerton's 1964 trip to Tobermory Bay, Isle of Mull, in western Scotland to search for the wreck of a Spanish galleon.

Edgerton's trip was at the request of Ian, Duke of Argyll, who owned rights to the wreck and its contents by virtue of a 1641 royal decree in favor of his ancestor. The galleon had sought refuge and revictualing at remote Tobermory in 1588 after Drake's English fleet routed the Spanish Armada and thwarted its attempted invasion of Britain. Relations between the Spaniards and Scots, cordial at first, turned ugly late in the year. A Scottish hostage on board is said to have ignited the galleon's powder store, causing a giant explosion that sent the vessel and its contents to the bottom of the bay. Thirty million pieces of eight were reputed to have gone down with the ship. The passing of generations and accumulation of silt gradually obscured the wreck's location.

The Edgerton team spent the week of August 3-8, 1964, traversing the bay by boat with an eight inch diameter 12 KC sonar transducer suspended by cable at a depth of forty to fifty feet, emitting signals at a pulse rate of 0.1 milliseconds. Return signals were picked up and recorded on board by an EG&G Recorder Type 245 prototype. Results were inconclusive because of previously dredged holes and the presence of a thick build-up of shells, which interfered with the transit of the sonar signals.

Several pages from Edgerton's report to the Duke of Argyll.

The papers of Harold Edgerton (MC 25) contain lab notebooks, field notebooks, charts, diagrams, data, reports, and letters tracing the step-by-step development of Edgerton equipment and the day-by-day course of his field research at various locations around the world. The papers are available for use in the Institute Archives and Special Collections, 14N-118.

Object of the Month: June 2000; November 2007

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