The ordinary mine-sweep, the function of which was to catch the mooring rope of the mine and drag the mine clear of the channel, was, of course, ineffective against the mine on the bottom, and there was no guarantee that mines might not be released from the bottom and rise to a depth at which they were dangerous, _after the channel had been swept and reported clear_.
To deal with this danger a chain-sweep to work on the bottom was introduced, but its use presented many difficulties, especially over a rocky bottom.
When a regular swept and buoyed channel was in use the enemy had little difficulty in deciding on the positions in which to lay mines by reason of the presence of the buoys.

This fact constituted the principal disadvantage in the use of a buoyed channel, but in certain places where the traffic was heavy the procedure was inevitable, and it greatly simplified the work of the patrol craft and minesweepers; the only precautions possible lay in the use of alternative marked channels, and in the laying of defensive deep minefields outside the channel in which enemy submarines might compass their own destruction.

As rapidly as our supply of mines admitted, this latter device was adopted in positions where the minefields could not constitute a danger to our own submarines.

False buoyed channels with mined areas round them could also be laid in which to catch the submarine.

Another device was that of altering the position of light vessels and buoys with the object of putting a submarine on to a shoal.
The situation with which our patrol and minesweeping craft had to deal having now been stated, it remains to speak of the magnificent manner in which they accomplished their task.
I regret very deeply that, in spite of a strong desire to undertake the task, I have neither the information nor the literary ability to do justice to the many deeds of individual gallantry, self-sacrifice and resource performed by the splendid officers and men who manned the small craft.