One of the most valuable inventions perfected during the last war was the paravane, which made it possible for ships of deep draught to pass through minefields in comparative safety. The apparatus consists of a torpedo-shaped body fitted with various devices to ensure that it keeps a straight course and maintains a certain depth. Paravanes are used in pairs, one being towed on either side of the ship. The tow-line is fitted with an apparatus that cuts through the mooring line of any anchored mine it encounters. The mine then floats to the surface and is exploded by rifle or gun fire. A pair of paravanes renders a ship practically immune from injury, even when passing through a field thickly sown with mines. The paravane was not perfected until the closing months of 1918, but it was then so effective that after September, 1918, only two ships, both light cruisers, were struck by mines, one of them in the Baltic after the Armistice. It is believed that the Germans never knew the secret of the device that had rendered their minefields ineffective until long after the war.
The paravane is used also by minesweepers. Most of the minesweeping in the last war was done by fishermen, among whom were men of sixty years of age or more. During the crisis of September, 1938, a volunteer minesweeping service was formed, and old trawlers were taken over to give the men training in this form of work.