There is another type of elevator that completely dwarfs anything used in office buildings. On some canals where it is necessary to make a sudden change of level of considerable extent, instead of using a flight of locks, a hydraulic elevator is employed to lift or lower not only the vessel but the water it is floated in. There are two huge tanks which ply between the upper and lower levels of the canal. These are so connected that as one rises the other descends. The tanks are big enough to take in the largest vessels that are likely to use the canal. They are fitted with water-tight gates at each end, and after a vessel has entered one of them, say at the lower level, the gate behind it is closed. Then hydraulic mechanism is operated to lift the tank, and when the top is reached the other gate is opened and the vessel sails out into the upper level. It seems like a tremendous undertaking to lift a heavy ship, but the two tanks balance each other and the only work that has to be done is to overcome the friction and inertia. No matter how heavy a vessel may be, it will not disturb the balance between the two tanks. Nor is it necessary to have a ship in each tank, for the same depth of water is maintained in the two tanks, and when a ship enters the weight of the tank is not increased, for the ship displaces its own weight of water. HYDRAULIC VARIABLE SPEED GEAR Water is frequently used to take the place of toothed gearing for the transmission of power, particularly where it is desirable to vary speed. One of the disadvantages of toothed gearing is that a change of speed can only be made by shifting gears. The speed cannot be varied gradually but is changed by abrupt steps. The guns of a battleship must be kept trained on the target, while the ship rolls in the waves under them, so as to be ready for a broadside at any instant. A telescope is secured to the barrel of the gun and a man known as a “pointer” tries to keep the cross-hairs of a telescope on the target by constantly elevating or depressing the gun. The speed of the elevating machinery must be constantly changing; now running at high speed, the next instant barely moving, and the next moment reversing.