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AB'ACUS, noun [Latin anything flat, as a cupboard, a bench, a slate, a table or board for games; Gr. Usually deduced from the Oriental, abak, dust, because the ancients used tables covered with dust for making figures and diagrams.]

1. Among the Romans, a cupboard or buffet.

2. An instrument to facilitate operations in arithmetic; on this are drawn lines; a counter on the lowest line, is one; on the next, ten; on the third, a hundred, _e. On the spaces, counters denote half the number of the line above. Other schemes are called by the same name. The name is also given to a table of numbers, cast up as an abacus of addition; and by analogy, to the art of numbering, as in Knighton's Chronicon.

3. In architecture, a table constituting the upper member or crowning of a column and its capital. It is usually square, but sometimes its sides are arched inwards. The name is also given to a concave molding on the capital of the Tuscan pedestal; and to the plinth above the boultin in the Tuscan and Doric orders.

AB'ACUS PYTHAGORICUS, The multiplication table, invented by Pythagoras.

ABACUS HARMONICUS, The structure and disposition of the keys of a musical instrument.

ABACUS MAJOR, A trough used in mines, to wash ore in.