To form concrete walls under tide level, the most practical way will probably be to work inside a timber cofferdam, if the depth is not too great. The wall would be dealt with in short lengths. If it is wished to dispense with a cofferdam, the concrete can be deposited by means of cranes and special skips, which have doors at the bottom arranged to open when the bottuni of the sea is reached. The concrete is thus deposited quietly in position, and if the currents are not strong, the cement is not washed out before it has time to set. Another way, used in the construction of piers of breakwaters, is to sew the freshly made concrete up in long bags, like sausages, and then drop them into position. The bags protect the cement from being washed away. This method could only be used if the wall were very thick, and could not be depended on to make a water-tight wall.