Nautical terms ,Press Gangs,the glass bottom theory

A solid pewter metal beer mug with a clear glass bottom.

Question: Why do Beer Mugs have Glass Bottoms? - There are two answers to this odd feature. The first is quite mundane and rather simple: It allows the bartender to see that you are approaching ‘empty’ as you take a swig, and therefore can offer you a refill (Some wags state that it also prevents 'drinking' from an empty mug and taking the space a productive customer would sit in). The second is more interesting, and has to do with the infamous ‘press gangs’ of King George III. It was the law that if a man ‘Accepted the King’s Shilling’ as a first day’s wage, then he was legally obligated to serve the king as a soldier or sailor. Unscrupulous press gangs would buy a potential victim a tankard of beer or ale, and then drop a shilling in the mug when he wasn’t looking. Swallowing, choking, or just finding the shilling was enough for the press gang. Thus, mugs began to have glass bottoms to give the customer a sporting chance to see that his drink was not tampered with.