To the memory of John Boynes, late stone mason of His Majesty's Dock Yard, who was unfortunately drowned between the Island and Point returning from seeing Bonaparte in the Sound 13th July 1815, aged 35 years. (It seems as if the date may have been amended).
The Island is presumably Drake's Island and the Point is Devil's Point. This was the location of where the Bellerophon was anchored whilst awaiting orders with what to do with Napoleon.
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