Museo Galileo
Virtual Museum
Manual division
Video   Text


Among the most important parts of mathematical, astronomical, and surveying instruments are graduated scales, which allow the accurate measurement of angles and distances. Until the mid-eighteenth century, the scales were divided by hand. A compass was used to mark on a circumference the divisions for 90, 60, and 30 degrees; these angles were then bisected to mark 15, 45, and 75 degrees. Proportional scales were used to obtain the intermediate subdivisions. The procedure required great skill. In the eighteenth century, only a handful of instrument-makers—most of them English, such as George Graham, John Bird, and Jeremiah Sisson—were capable of dividing with precision the scales of large astronomical instruments.