INSET WALLS system of corvée labour was doubtless adopted by the conquerors. Primarily in a sea-wall dead weight is matched against momentum. Walls, if properly looked after, do not fail by direct breaching blows. If percolation takes place under the seat of a wall it is usually noticeable, and can be arrested in time. The surest manner of effecting this is to dig a trench on the landward side and fill it with well-rammed clay puddle. In the majority of instances in which breaching has taken place in sea-walls, such breaching has been caused by insufficient height. If a wall is supported in such a position that heavy spray passes over its crest, this may quite easily set up scour at the back, and a comparatively small run of water scoring its way down the back of a wall, especially if the wall be newly constructed, will sometimes so weaken its section that it gives way. The effect of a stream of water passing over a wall in this manner is to cause a rill down its inward slope, and this sets up weir action at the toe of the wall. By slow degrees the material of which the wall is composed oozes away into a flat slope, and the final result is a breach. It is frequently the case where a line of river walling is threatened with erosion from some local cause that it is a cheaper expedient to abandon the attempt to maintain the length of main wall in question, and to construct behind it what is termed an inset wall, i.e. a horseshoe care has to be exercised in the location of embankment. Great such an embankment,