The sea as a letter box: missives cast upon the waters. Posting letters at St Kilda, 1910. During those months in which vessels do not call at the island of St Kilda, in the Outer Hebrides, letters are dispatched by being placed in a waterproof, buoyant case and cast upon the waters. Usually this mail packet is picked up on the coast of Norway to be forwarded to the Foreign Office. Four packages out of six reach their destination.