control would forward the observations to a central meteorological station which would rebroadcast them as Cyphered Ship Synoption, the synoptic using the international meteorological code. The encyphering of the synoptic was by means of a trigram substitution from one of a series of six tables changing 6 daily, each series of tables being used twice, the second appearance being a month after the first. This trigram substitution was broken by Archer's party in the Meteorological Section and the resulting synoptic decodes (known to us as Archery) gave a crib to the W W's.
12. A Diagram may make this clearer
13. The importance of these possible cribs, and of all short signal cribs, lay in the fact that in sending them the 4 wheel enigma was used as a 3 wheel machine, as described in Chap. III Para. 20. The message settings in the various indicator books for the short signals were all 3 letter and it is probably for this reason that these signals remained 3 wheel.
14. To enable us to use these cribs from Met. three things were necessary (1) The Met. messages and the Enigma message had to be correctly paired ("tied up") with each other. (2) The Met. section had to break the trigram substitution (3) We had to capture or break cryptographically the code and indicator list contained in the