2) closely resembles the Roman **abacus** in its construction and use.

He wrote also treatises on the astrolabe (a copy of this is in the British Museum), on the **abacus** (three copies exist in the Vatican library, the library of Leiden University and the Bibliotheque Nationale in Paris), translations of the Kharismian Tables and an Arabic Introduction to Astronomy.

The name of " **abacus** " is also given, in logic, to an instrument, often called the " logical machine," analogous to the mathematical **abacus**.

To assist his lectures on astronomy he constructed elaborate globes of the terrestrial and celestial spheres, on which the course of the planets was marked; for facilitating arithmetical and perhaps geometrical processes he constructed an **abacus** with twenty-seven divisions and a thousand counters of horn.

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.

In his large study, the walls of which were hung to the ceiling with Persian rugs, bearskins, and weapons, sat Dolokhov in a traveling cloak and high boots, at an open desk on which lay an **abacus** and some bundles of paper money.