Volume 1: 1771 Edition of the Encyclopedia Britannica!
Abacus, a table strewed over with dust or sand, upon which the ancient mathematicians drew their figures. It is also signified a cupboard, or buffet.
Abacus, in architecture, signifies the superior part or member of the capital of a column, and serves as a kind of crowning to both. It was originally intended to represent a square tile covering a basket. The form of the abacus is not the same in all orders: in the Tuscan, Doric, and Ionic, it is generally square; but in the Corinthian and Composite, its four sides are arched inwards, and embellished in the middle with some ornament, as a rose or other flower. Scammozzi uses abacus for a concave moulding on the capital of the Tuscan pedcital; and Palladio calls the plinth above the echinus, or boultin, in the Tuscan and Doric orders, by the same name. See Plate I, figure 1, and ARCHITECTURE.
Abacus is also the name of an ancient instrument for facilitating operations in arithmetic. It is variously contrived. That chiefly used in Europe is made by drawing any number of parallel lines at the distance of two diameters of one of the counters used in the calculation. A counter placed on the lowest line, signifies 1; on the second, 10; on the third, 100; on the fourth, 1000; etc. In the intermediate spaces, the same counters are estimated at one half of the value of the line immediately superior, viz. between the first and the second, 5; between the second and the third, 50;, etc. See Plate I, figure 2, A B, where the same number, 1768 for example, is represented under both by different dispositions of the counters.
Abacus harmonicus, among musicians, the arrangement of the keys of a musical instrument.
Abacus logisticus, a right-angled triangle, whose sides forming the right angle contain the numbers from 1 to 60, and its area the facta of every two of the the numbers perpendicularly opposite. This is also called a canon of sexagesimals.
Abacus pythagoricus, the multiplication-table, or any table of numbers that facilitates operations in arithmetic.