Wikipedia 10K Redux by Reagle from Starling archive. Bugs abound!!!

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ABACUS (Gr. abax, a slab Fr. abaque, tailloir), in 
architecture, the upper member of the capital of a column.  
Its chief function is to provide a larger supporting surface 
for the architrave or arch it has to carry.  In the Greek Doric 
order the abacus is a plain square slab.  In the Roman and 
Renaissance Doric orders it is crowned by a moulding.  In the 
Archaic-Greek Ionic order, owing to the greater width of the 
capital, the abacus is rectangular in plan, and consists of a 
carved ovolo moulding.  In later examples the abacus is square, 
except where there are angle volutes, when it is slightly 
curved over the same.  In the Roman and Renaissance Ionic 
capital, the abacus is square with a fillet On the top of an 
ogee moulding, but curved over angle volutes.  In the Greek 
Corinthian order the abacus is moulded, its sides are concave 
and its angles canted (except in one or two exceptional Greek 
capitals, where it is brought to a sharp angle); and the same 
shape is adopted in the Roman and Renaissance Corinthian and 
Composite capitals, in some cases with the ovolo moulding 
carved.  In Romanesque architecture the abacus is square with 
the lower edge splayed off and moulded or carved, and the 
same was retained in France during the medieval period; but 
in England,in Early English work, a circular deeply moulded 
abacus was introduced, which in the 14th and 15th centuries 
was transformed into an octagonal one.  The diminutive of 
Abacus, ABACISCUS, is applied in architecture to the chequers 
or squares of a tessellated pavement . ``Abacus'' is also the 
name of an instrument employed by the ancients for arithmetical 
calculations; pebbles, hits of bone or coins being used as 
counters.  Fig. 1 shows a Roman abacus taken from an ancient 
monument.  It contains seven long and seven shorter rods 
or bars, the former having four perforated beads running 
on them and the latter one.  The bar marked 1 indicates 
units, X tens, and so on up to millions.  The beads on the 
shorter bars denote fives,--five units, five tens, &c. The 
rod O and corresponding short rod are for marking ounces; 
and the short quarter rods for fractions of an ounce. 

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 running 
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. 

FIG. 2.--Chinese Swan-Pan.  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., 1894, p. 135), and Allan 
Marquand (see American Academy of Arts and Sciences, 1885, pp. 
303-7, and Johns Hopkins University Studies in Logic, 1883). 

Source:  An unnamed encyclopedia from a project that puts out-of-copyright texts into the public domain.
This is from a *very* old source, and reflects the thinking of the turn of the last century.