Brewer's: Elmo's Fire
(St.). Comazants, or electric lights occasionally seen on the masts of ships before and after a storm; so called by the Spaniards because St. Elmo is with them the patron saint of sailors. (See Castor And Pollux.)
Sudden, breaking on their raptured sight, Appeared the splendour of St. Elmo's light.
Hoole: Orlando Furioso, book ix.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894