STORY OF THE MOON UNDER WATER
The name came into being following the activities of Recruiting Sergeants in the 18th century who spent much of their time visiting Taverns in order to "Press" people into joining the British Army. Many of their methods were extremely dubious and they would go to any length in order to get men to sign up since their pay depended on it. Once a man had accepted the "Queen's Shilling" by fair means or foul, he was deemed to have become a recruit.
On entering a Tavern, a Recruiting Sergeant would very often sit next to some drunkard oaf and secretly drop the Queen's Shilling into the man's pewter tankard. When the drunkard lifted the tankard and the ale touched his lips that was it! He was regarded as having accepted the Queen's Shilling and was in the British Army. He was then carted off for service in some far-off campaign and almost certain death.
As means of countering this practice, the glass bottomed pewter tankard was introduced so that if a customer was in the least bit suspicious as to whether a coin had been dropped into his tankard, he could hold it up and look underneath before taking a drink. The sight of the Queen's Shilling under the ale with froth above was likened to that of "The Moon Under Water" hence the name.