level on both sides is equal. The original gate machinery consisted of a huge drive wheel, powered by an electric motor, to which was attached a connecting rod, which in turn attached to the middle of the gate. The gates are hollow and buoyant, much like the hull of a ship, and are so well balanced that two motors are enough to move each gate leaf. If one motor fails, the other can still operate the gate at reduced speed. All but one chamber contains a pair of auxiliary gates, which can be used to divide the chamber in two. This design allows for the transit of smaller vessels less than long, such as canal tugs, without using the full quantity of water. The auxiliary gates were originally incorporated because the overwhelming majority of all ships of the early 1900s were less than long and therefore did not need the full length of the lock chamber. Mules From the outset, it was considered an important safety feature that ships be guided through the lock chambers by electric locomotives, known as mulas (mules, named after the animals traditionally used to cross the isthmus of Panama), running on the lock walls. These mules are used for side-to-side and braking control in the locks, which are narrow relative to modern-day ships. Forward motion into and through the locks is actually provided by the ship's engines and not the mules. A ship approaching the locks first pulls up to the guide wall, which is an extension of the centre wall of the locks, where it is taken under control by the mules on the wall before proceeding into the lock. As it moves forward, additional lines are taken to mules on the other wall. With large ships, there are two mules on each side at the bow, and two each side at the stern—eight in total, allowing for precise control of the ship. Mules are not used on the new expansion locks. The mules themselves run on paired broad gauge railway tracks. The track closest to the canal is used for towing and has rack tracks for geared operation. The track inland is used to return the mules to their starting point and does not have racks except on the steep inclines between lock chambers. Traction is by electric power, supplied through a third rail laid below surface level on the land side. Each