55A DRAGON Figs 58(xxvi-xxx) (Details apply to Dragon 2 only)
(a) Purpose and method
The purpose is to set a common crib P, of up to 10 letters, in a given de-
chi D, i.e. to find a stretch of D (if there is one) where the underlying plain
text is P, so that P + D = PSI' , which when the extensions are removed, yields
Dragon adds P to a stretch of D in all positions in turn: in each position
it contracts P + D, i.e. omits repeated letters, and then compares each impulse of
the result, independently, with the corresponding PSI wheel : it all five can be
fitted the machine stops and displays the settings of D and all PSI's at the last
letter of the crib.
(b) Use of motor or limitation
Although the majority of repeated letters in PSI' are due to extension some
are not, and the method will obviously be more powerful if P + D is contracted
only at total motor dots.
Dragon 1 cannot do this; Dragons 2,3 can. In practice if the motor settings
are known it is unnecessary to use Dragon: the facility can however be used with
great benefit to forbid contraction at limitation dots.
(c) Setting up D, PSI, X2, Mu .
(i) The de-chi D is on a tape fed into a tape-reader. Dragon remembers it ten
letters at a time.
(ii) The crib P is plugged on a 10 x 5 array of jacks. The length of the crib can
be reduced by the top row switches: starting at the left, each switch thrown up
cuts out one letter. The only letter which can be cut out from the middle of the
crib is the fifth, by throwing its switch down.
There are actually two 10 x 5 arrays, selected by a switch, so that one can be set
up while the other is in use.
(iii) PSI . PSI is plugged up as usual: above each jack is a lamp to show the
setting at the end at a successful crib.
(iv) Mu, X2 Mu61, Mu37, X2 have each two rows of jacks, the lower for the
pattern, the upper for the setting at the start of the de-chi.
(d) The display
When a crib sets, this shews the settings, at the end of the crib, of Mu61,
Mu37, X2, D. D is measured in lines of 31.
The display also shows each position (1,2,....9).where P + D has been
(e) The de-chi display
Above the crib jacks is a display showing, in dots and