This is the title page of the 1640 printing of Galileo's Operation of the Geometrical and Military Compass, originally published privately in 1606. Galileo had invented a version of this geometrical compass a few years earlier and evidently gave copies of this manual to those who bought the compass. The compass had many uses, from performing square root calculations to determining ranges of cannons to solving surveying problems. Click here for more information on the compass.
On this page, Galileo demonstrates how to find the height of a distant object by using the compass twice to sight the object at different distances.