There are two ways of overcoming this difficulty. At Liverpool great landing stages are built, floating in the water parallel to the shore. Running from these to the shore are great hinged gangplanks which permit the landing stage to rise and fall with the tides while these gangplanks, which are really more like bridges, hold them parallel to the shore and serve as bridges as well. A ship, made fast to one of these landing stages, rises and falls as the stage does, and the two maintain their relative positions to each other regardless of the stage of the tide. In Liverpool these stages are largely used for passenger ships. The other method, which is also in use at Liverpool as well as at many other ports, is to build a sea wall across the entrance to the docks, and in this sea wall to build a “lock,” or a water gate. When the tide is in, the water gate is opened and the harbour or the dock is flooded to the level of high tide. As the tide recedes this lock is closed and the water level behind it remains the same. Ships pass in and out, either at high tide, when the lock or gate can be left open for a time, or, if at other stages of the tide, by means of the lock, which, being made up of two gates at the opposite ends of a long, narrow, canal-like passageway, makes it possible for the ship to pass into the lock, where the water level can be made to coincide with the level of the dock or of the water outside. Then, by opening the inner or the outer gate, as the case may be, the ship can enter the dock or the unprotected waters outside. Equipment of both these types is to be found at a number of European ports, while still other ports, not troubled with a great range of tide, do not find it necessary to instal them. [Illustration: A NEW YORK HARBOUR LIGHTER _Lighters take various forms and perform various tasks. European lighters are more likely to have pointed ends. American lighters very often have square ends. Occasionally they have engines of their own, but generally they depend on tugs for power._ ]