Brewer's: St. Elmo
called by the French St. Elme. The electric light seen playing about the masts of ships in stormy weather. (See Castor And Pollux.)
And sudden breaking on their raptured sight, Appeared the splendour of St. Elmo's light.
Hoole's Furioso, book ix.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894