Galileo's military compass. Page from a 17th-century work by Italian mathematician, engineer, physicist and astronomer Galileo Galilei (1564-1642), describing the use of his design of military compass. Also known as a proportional compass or a sector, this instrument is used for trigonometry, surveying, navigation and aiming artillery such as cannon. The diagram shows trigonometry being used to indirectly measure the height of a tower on a cliff. The text is in Latin, and the page is from the work 'Le operazioni del compasso geometrico, et militare di Galileo Galilei' (Padua, 1606). For the instrument itself, see image C026/8917.