 The caissons—always submerged, as at Combe Hay—were without bottoms and were arranged so that their sides dropped into deep underwater channels formed by "minor walls" inside the main walls.The vertical movement of the two caissons was effected by a balance pipe ("the channel of communication") passing under the lock floor between the two caisson chambers (but with a slight upturn to reach above the water levels), so that causing an increase of water level in one caisson ("the water of compression") displaced air through the pipe, thus forcing a corresponding decrease in the water level in the other.This increased the buoyancy of the latter caisson, which accordingly rose as the first sank.