Location: Myst Island
Apparently constructed at the same time as the giant gears, this clock was never actually used to keep time, just to help protect the Mechanical Age Linking Book. The clocktower rests offshore, and can be reached by a path of gears which rise from the water when the clocktower is set to the proper time.
Within the tower, a complicated "Combination lock" made of 3 horizontal gears is used to open the giant gears on the island's North side. It works using a counterweight system required to have the weight on the floor, and can be reset with a lever on a pulley opposite the weight-It should be noted that counterweight puzzles appears to be a "tradition" for the Myst series, as almost every game has at least one of them involved. To operate the lock, two levers with their wheels shaped like gears rotate two out of three-the middle gear once, wheels on top. The left one turns the upper and middle one, and the right turns the lower and middle. Once the correct code is set, a model of the giant gears opens in the center.
On the roof appears to be an ornament of some sort, seeming to resemble the famous Greek "Argo", being a boat shaped like the crescent moon with two passengers, and a long arrow on top.
Inside the clocktower for RealMyst, a person named "Doug" inscribed his name on the ceiling.