S O S – Ships in distress shall use the following signal repeated at brief intervals:

The 1906 International Wireless Telegraph Convention established the signal as the call for a ship in distress and was incrementally adopted after the agreement was approved in 1908. This 1935 book on the early history of the S O S signal used by ships in distress is held in the collection.  SOS To The Rescue by Karl Baarslag, Oxford University Press.

Hear the S O S signal at  Wikipedia.—Chris Cottrill