Philip Gray, age 12, of Linton, Indiana, for his question:

What is an abacus?

The abacus is the second adding machine to be invented. The first ever used was the fingers and toes. For early man, like everyone else, learned to count with the help of the digits on his hands and feet, Later he learned to count with pebbles and shells. It was natural that these little counters should lead to the invention of a counting frame. This was the abacus.

The abacus seems to have been invented by traders in scattered parts of the world. The splendid idea occured to the Egyptians, the Hindus and the Chinese. The invention is a smallish wooden frame, slatted across with a number of parallel rods. The counters are strung onto the rods like beads. Some people used balls of wood for the counters, others used shells and others used pebbles.

The counters are moved along the rods to solve problems of adding and subtracting, On one rod, each counter is equal to a single unit. Those on the next rod may be equal to five or ten units. An expert can work an abacus almost as fast as an up to date adding machine can give the answers.