I did some looking when I recently visited Boston and saw that, and no one seems to have an answer. Probably someone's spare change fell out of their pocket when they rudely took a seat on the stone years ago, prompting the next person to inexplicably throw near-worthless change at it, and so on.
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'He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me.'
-TJ
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