Volvelle

A volvelle is a calculation device consisting of concentric moveable circles. The earliest types were typically made of parchment or paper, with one or more moveable circles attached to a backboard, often the inside cover of a book. The range of information on them was large. Some were like early slide rules, used to calculate the results of mathematical equations. Others worked out the phases of the moon or times of high and low tide. The volvelles that solved astronomical problems could have as many as six rotatable dials.

It was of course always a most expensive business to make these volvelles, which had to be cut to shape — often there were many little discs to a single volvelle — and fixed in place with considerable care. In spite of this care, it is quite common to find even in otherwise well-preserved examples that pieces of the volvelle have become detached and lost.

Science, 6 Oct. 1967.

Strictly, the term is applied only to such historical calculation devices. In more recent times, related ones, usually now called wheel charts or wheel calculators, have been used to display or calculate the details of everything from the date when a baby was due, through aircraft recognition and dieting data, to geography facts for school children. These were often given away as advertisements at trade shows or supplied with products. Americans of a certain age might recall the BAC (Blood Alcohol Content) calculators that were distributed in school so you could work out the safe limits for drinking. Even in the age of the computer, they’re by no means obsolete, being handy pocket-sized compendia of useful data for some specialist purposes.

The word is from the medieval Latin verb volvere, to turn. It is almost but not quite obsolete, appearing now only in works on historic scientific instruments or works of fiction.

There was a compass set in an ivory housing, a brass Persian astrolabe that must have been twenty inches across, a wooden volvelle, a series of plates that moved to follow the tides and phases of the moon, and an exquisite gold astronomical compendium, a marvellous small, beautiful box that held a compass, a sundial and even a wind vane.

The Pirate Devlin, by Mark Keating, 2010.