The Swan-Pan of the Chinese (fig. 2) closely resembles the Roman abacus in its construction and use. Computations are made with it by means of balls of bone or ivory runp ing on slender bamboo rods, similar to the simpler board, fitted up with beads strung on wires, which is employed in teaching the rudiments of arithmetic in English schools.
The name of " abacus " is also given, in logic, to an instrument, often called the " logical machine," analogous to the mathematical abacus. It is constructed to show all the possible combinations of a set of logical terms with their negatives, and, further, the way in which these combinations are affected by the addition of attributes or other limiting words, i.e. to simplify mechanically the solution of logical problems. These instruments are all more or less elaborate developments of the " logical slate," on which were written in vertical columns all the combinations of symbols or letters which could be made logically out of a definite number of terms. These were compared with any given premises, and those which were incompatible were crossed off. In the abacus the combinations are inscribed each on a single slip of wood or similar substance, which is moved by a key; incompatible combinations can thus be mechanically removed at will, in accordance with any given series of premises. The principal examples of such machines are those of W. S. Jevons (Element. Lessons in Logic, c. xxiii.), John Venn (see his Symbolic Logic, 2nd ed., 18 94, p. 135), and Allan Marquand (see American Academy of Arts and Sciences, 188 5, pp. 303-7, and Johns Hopkins University Studies in Logic, 1883).
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