Prepaid postcard of 4c showing a ship being towed by a tug locomotive at the Panama Canal Lock.
The Panama Canal Locks, which lift ships up 25.9 m (85 ft) to the main elevation of the Panama Canal, were one of the greatest engineering works ever to be undertaken at the time. The total length of the lock structures, including the approach walls, is over 3 Kilometers (nearly two miles).
The locks, which have a total of six steps, limit the maximum size of ships which can transit the canal, known as Panamax. Each of these steps has two lock chambers, doubling the amount of traffic that can be handled; together they raise ships from sea level to a height of 25.9 m (85 ft)
Text Source: Wikipedia