Previously searchlights in destroyers had been used for this purpose.

The flares were not much used, however, from kite balloons owing to lack of opportunity, but trials which were carried out with flares from patrol craft, such as trawlers and drifters, demonstrated that they would be of value from these vessels, and when the Folkestone-Grisnez minefield was laid in November and December, 1917, it was apparent that the flares would be of use in forcing submarines to dive at night into the minefield to escape detection on the surface and attack by gunfire.
Manufacture on a large scale was therefore commenced, and during 1918 the flares were in constant use across the Straits of Dover.
ELECTRICAL SUBMARINE DETECTOR The existence of this very valuable device was due to the work of certain distinguished scientists, and experiments were carried out during 1917.

It was brought to perfection in the late autumn, and orders were given to fit it in certain localities.

Some difficulty was experienced in obtaining the necessary material, but the work was well in hand by the end of the year, and quickly proved its value.
SUBMARINE AGAINST SUBMARINE Prior to the year 1917 the only areas in which our own submarines operated against enemy vessels of the same type was in the North Sea, or occasionally in the vicinity of the Hebrides.

Grand Fleet submarines were used in the northern areas during 1916, and Harwich submarines operated farther south, but the number of underwater craft available was insufficient for any extended method of attack.