In Cornwall, Madron Well near Penzance had till recently—probably still has—a large thorn-tree growing against the wall of the baptistry which encloses the well. Young children suffering from skin-complaints are dipped in the well and carried round it three times, after which rags from their clothing are laid beside the streamlet and hung on the tree. This should be done about the beginning of May—the first Sunday if possible.
From Notes and Queries, May 23, 1942.