is regulated by a rack and snail mechanism. The distance a rack is allowed to fall is determined by a snail-shaped cam, thereby regulating the number of times the bell is allowed to sound. There was a misconception during the 20th century that the rack and snail mechanism was invented by British clergyman Edward Barlow in 1675–6. In fact, the inventor is unknown. The snail-shaped cam is a part of the timekeeping train that revolves every twelve hours; often the snail is attached to the same pipe on which the hour hand is mounted. The diameter of the cam is largest at the one o'clock position, permitting the rack to move only a short distance, after which the striking train is stopped; it is smallest at the 12 o'clock position, which allows the rack to move the farthest. Striking stops when the last tooth of the rack has been taken up by the gathering pallet. Because the number of strikes on the hour is determined by the position of the snail which rotates in tandem with the hour hand, rack striking seldom becomes desynchronized. Rack striking also made possible the repeating clock, which can be made to repeat the last hour struck by pressing a button. Rack striking became the standard mechanism used in striking clocks down to the present. Parts of mechanism All hour striking mechanisms have these parts. The letters below refer to the diagram. Power source – This is usually identical to the device that powers the clock's timekeeping mechanism: in weight driven clocks it is a second weight on a cord (P), in spring driven clocks it is another mainspring. Although older one-day (30-hour) clocks often used a single weight or mainspring to drive both the timekeeping and striking trains, better clocks used a separate power source, because the striking mechanism consumes a lot of power and often has to be wound more frequently, and also to isolate the delicate timekeeping train from the large movements that occur in the striking train. Winding a striking clock requires winding both the timing and striking parts separately. Striking train – This is a gear train (G,H) that scales down the force of the power source and transmits it to the hammer mechanism which rings the gong. In antique clocks, to reduce the manufacturing cost, it was often exactly the same as the timing train that ran the clock's