Since most men were reluctant to join the navy due to low pay and the requirement of lifetime service, groups of 'press-gangs' would go around pubs on the coast of Britain, kidnapping men and forcing them into lifetime service. Anyone found in possession of the "King's shilling" was thought to be a loyal subject, and thus a candidate for the royal navy.
The press-gangs would travel to various pubs, and drop shillings into the beer mugs of hapless victims. When the victim got to the bottom of their beer, they would find the king's shilling, and the press-gang waiting for them outside to drag them off. Fancier pubs invented the glass bottomed beer mug so that their patrons could see if there was a king's shilling in the bottom, and refuse the drink.
Press-ganging was one of the factors leading to the war of 1812. Press gangs would land on the coast of America and would "accidentally" press-gang American civilians into navy service. Over 6000 American men were kidnapped in this manner in the early 1800s.