512 
PEOEESSOE JEVONS ON THE MECHANICAL 
the fourth position, so that any combination rod once condemned as contradictory so re- 
mains until the close of the problem, and its letters are no more seen upon the abece- 
darium. 
42. Any other proposition, for instance, B is C, can now be impressed on the keys, 
and the effects are exactly similar, except that the A b combinations are out of reach of 
the levers. The B subject key throws the b’s into the second, the C predicate key throws 
the Bc’s into the third, and the full stop throws the latter into the fourth, where they join 
the Ad’s already in that place of exclusion, while the remainder all return to the first 
position. 
The combinations now visible in the abecedarium will be as follows : — 
A 
A 
a 
a 
a 
a 
a 
a 
B 
B 
B 
B 
b 
b 
b 
b 
C 
C 
C 
C 
C 
C 
c 
c 
D 
d 
D 
d 
D 
d 
D 
d 
They correspond exactly to those previously obtained from the same premises (see § 24), 
except that each combination of A, B, C, a, b , c is repeated with D and d. If we now 
want a description of the term A, we press the subject key A, and all disappear except 
ABCD, ABCd, 
which contain the information that A is always associated with B and with C, but that 
it may appear with D or without D, the conditions of the problem having given us no 
information on this point. The series of consistent combinations is restored at any time 
by the full-stop key, the contradictory ones remaining excluded. 
43. Any other subject key or succession of subject keys being pressed gives us the 
description of the corresponding terms. Thus the key c gives us two combinations, 
a b c D, abed , informing us that the absence of C -is always accompanied by the 
absence of A and B. Of b we get the description 
a a a a 
b b b b 
C C c c 
D d D d 
whence we learn that the absence of B always causes the absence of A, but that C and 
D are indifferently absent or present. 
44. We can at any time add a new condition to the problem by pressing the full 
stop to bring the combinations as yet possible into the first position, and then impres- 
sing the new condition on the keys as before. Let this condition be 
Cis D. 
The effect will obviously be to remove such C d combinations as yet remain into the 
fourth position, leaving only five : 
