No. LXXXIV.
Reading edition
An instrument whereby persons, ignorant in arithmetic, may perfectly observe numeration and subtraction of all sums and fractions.
NOTE.
Sir Samuel Morland has published a detailed account of two instruments of this kind in a tract entitled, The Description and Use of two Arithmetic Instruments, &c.—London, 1673. The Roman Abacus and Chinese Swan-pan are also instruments of a like description.
The Abacus was variously contrived; that chiefly used in European countries was made by drawing any number of parallel lines at pleasure, at a distance from each other, equal to twice the diameter of a calculus or counter. This placed on the lowest line, signified 1; on the second, 10; on the third, 100; on the fourth, 1000; on the fifth, 10,000; and so on. In the spaces between the lines, the same counters signified half of what they represented on the next superior line; viz. in the space between the first and second lines, 5; between the second and third, 50; between the third and fourth, 500; and so on. The abacus was also divided cross-wise into areolæ, and by this means subtractions were performed. The calculating instrument of Mr. Babbage is however much superior to any other contrivance yet suggested.