Extending almost the length of the centre of a modern dry dock is a row of large wooden blocks, called keel blocks. These can be moved and are made fast when they are put in place. Often this row of blocks is paralleled on each side by a row of somewhat similar blocks called bilge blocks which run along tracks laid at right angles to the line of keel blocks. The bilge blocks can be moved individually along these tracks by means of ropes and pulleys. These ropes are extended up the sides of the dock so that, even when the dock is filled with water, each individual bilge block, and there are scores of them in each row, can be moved back and forth by men beside the dock. When it is necessary for a ship to be docked her docking plans are given to the man in charge of the dock. He then arranges the keel blocks so that the line along their tops is the same as the line along the keel of the ship. Certain marks are then made at the top of the dock’s side walls to show just how far the ship is to be hauled into the dock. When these arrangements are completed the dock is flooded, the gates are opened, or the caisson is floated out and the ship is very carefully and very slowly hauled into the dock. She never goes in under power, for the clearance between her sides and the sides of the dock is often very small, and the greatest of care must be taken to keep her from coming in contact with the masonry. When she has been hauled up to the point marked on the dock side she is carefully made fast with cables, and the entrance to the dock is closed. The ship must be riding on an even keel, for if she is listing—that is, leaning to one side or the other—she may damage herself when the water is pumped out and she comes to rest on the keel blocks. As the water level is reduced her keel slowly settles on the keel blocks which support the whole weight of the ship, but in order to prevent the ship from toppling over sideways the bilge blocks are pulled carefully under her. As they are slightly higher than the keel blocks they touch her bottom at some distance from the keel, and as there is a row of them on each side they keep her securely upright. Care must be taken that none of these bilge blocks come in contact with the ship where any of her numerous underwater valves project, for if that happened the valves would be damaged. The docking plan referred to, however, shows where such protuberances are and such accidents need not occur.