This electromagnet releases a gravity lever onto the escapement above the master pendulum. The anchor became the standard escapement used in pendulum clocks. The clock tower was driven by a rotating waterwheel and escapement mechanism. A problem with the lever escapement is friction. Those three were weight driven, spring driven, and clocks with lever escapement. The fourth wheel also drives the "escape wheel" of the lever escapement. French watchmakers adopted the thinner cylinder escapement, invented in 1695. George Graham invented the deadbeat escapement for clocks in 1720. Therefore, the self-starting lever escapement became dominant in watches. Therefore, the advantages of this escapement over the lever are of an uncertain value. This is referred to as a coulisse-lever escapement. Mechanical metronomes don't need a battery, but run from a spring-wound clockwork escapement. The clock featured a double three-legged gravity escapement to the designs of Edmund Beckett. The verge was the only escapement used in clocks and watches for 350 years. This made the timekeeping strongly dependent on the force applied by the escapement.