Inventions that are credited to the ancient Greeks include the gear, screw, rotary mills, screw press, bronze casting techniques, water clock, water organ, torsion catapult, the use of steam to operate some experimental machines and toys, and a chart to find prime numbers. Many of these inventions occurred late in the Greek period, often inspired by the need to improve weapons and tactics in war. However, peaceful uses are shown by their early development of the watermill, a device which pointed to further exploitation on a large scale under the Romans.