Geometric and Military Compass
Galileo invented the geometric and military compass, his first commercial scientific instrument, in 1597. The device, which resembled two rulers that moved over a third, curved piece, acted as an early calculator. Merchants could use it to work out monetary exchange rates. Shipwrights employed the device when testing hull designs in scale model. And it even found use on the battlefield, where soldiers could use the compass to determine the charge for a cannon.