In 1883 The London Metropolitan Police advertised for an idea to replace the policeman’s rattle, a cumbersome means of communication for the bobby on his “beat”.
Joseph Hudson invented a novel whistle for the purpose. It could be held in the mouth leaving the hands free a clear advantage over the rattle.
The Metropolitan Whistle is still made by the same company. Chrome-plated solid brass. Supplied with History Card.
(See the 1884 "Metropolitan" poster in "Artworks / Posters" department.)