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The slips are furnished with little pins, so that when 
placed upon the ledges of the board, those marked by any 
given letter may be readily picked out by means of a 
straight-edged ruler, and removed to another ledge. 
The use of the abacus will be best shown by an example. 
Take the syllogism in Barbara ; — 
Man is mortal. 
Socrates is man. 
Therefore Socrates is mortal. 
Let 
A = Socrates. 
B = Man. 
C ~ Mortal. 
The corresponding small italic letters then indicate the 
negatives. 
a = not-Socrates, 
b — not-Man, 
c = not-Mortal, 
and the premises may be stated as 
A is B, 
B is G. 
Now take the second set of slips containing all the possible 
combinations of A, B, C, a 9 b, c , and ascertain which of 
the combinations are possible under the conditions of the 
premises. 
Select all the slips marked A, and as all these ought to be 
B’s, select again those which are not B, or b, and reject them. 
Unite the remainder, and selecting the B’s, reject those which 
are not C or c. There will now remain only four slips or com- 
binations : 
A 
a 
a 
a 
B 
B 
b 
b 
C 
C 
C 
c 
If we require the description of Socrates, or A, we take 
the only combination containing A, and observe that it is 
