In Perthshire there are several wells and springs dedicated to St. Fillan, which are still places of pilgrimage. These wells are held to be efficacious in cases of madness. Even recently lunatics have been bound to the holy stone at night, under the expectation that St. Fillan would release them before dawn, and send them home in their right minds.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Madness from Fact Monster: