A Caesar shift with substitutes for the vowels: the cipher of Thomas Edmondes's letter from the camp before Laon, 9 July 1594 (Hatfield House, CP 27/30)

The letter that Thomas Edmondes, Elizabeth I's agent in France, sent from the royal camp before Laon on 9 July 1594 (Hatfield House, Cecil Papers CP 27/30) is partly in cipher. The Historical Manuscripts Commission printed its fifteen cipher passages in 1892 as strings of letters, without a decipherment, and no reading of them has been published since. This note gives the key and a reading. The cipher is a Caesar shift of seven places on the 24-letter Elizabethan alphabet, with additional substitutes, mostly for the vowels, taken from the slots of rare letters; in several pairs the scribe distinguishes two values of one letter by the weight of the ink. The 1892 transcription merged those pairs and contains copying errors, which is why the printed strings resisted solution. About 97% of the cipher text is now read. It records the Duke of Montpensier's case for English support of his command in Brittany: the province's convenience for correspondence with England, Huguenot Poitou on its flank, port towns "to giue us footing", and a promise from the governor of Rennes to declare for the Protestant religion. This preprint fixes the result; a full edition will follow once a possible contemporary duplicate in British Library Stowe MS 166 has been examined.