E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The French St. Swithin, June 19th. (See SWITHIN.)
In 1725, Bulliot, a French banker, made a bet that, as it rained on St. Gervaiss Day, it would rain more or less for forty days afterwards. The bet was taken by so many people that the entire property of Bulliot was pledged. The bet was lost, and the banker was utterly ruined.