It was Dr James Gully's water cure at Malvern. "I feel certain that the Water Cure is no quackery," Darwin wrote.

That winter he had been sick every week. His hands trembled and he thought he was dying. So he went to Malvern with his wife, his children and all the servants. At a quarter to seven he was scrubbed with a rough towel in cold water: "very like a lobster". A wet compress all day. His feet in cold water with mustard. Heated by a spirit lamp until he streamed with sweat, then rubbed with a cold dripping sheet. No sugar, no butter, no tea, and the snuff: "6 pinches of snuff for all this week, which is my chief comfort". By May even that was gone: "the cruel wretch has made me leave off snuff, that chief solace of life".

Six weeks became sixteen. He felt better, walked seven miles in a day and called himself "a mere walking & eating machine". He even stopped thinking about barnacles. At home in Down he built his own cold douche and kept at it, "frost or no frost".

Was it the cold water, or the rest, the walks and no work? Darwin felt certain anyway.

Cold water is a shock to the body. Go gently, and never alone.

What would you credit: the cold water or the rest?

Sources: Darwin's letters of 1849 to Susan Darwin, J. D. Hooker, W. D. Fox and J. S. Henslow, transcribed by the Darwin Correspondence Project (darwinproject.ac.uk).
Sounds: Freesound (CC0); Yle Arkisto, CC BY 4.0.