The Tricky Passage

Vietnamese Trading Ships on route from Hoi An to Canton, would take on fresh water andsupplies at the circular rivers of Ha Long Bay, as discovered by Sailor Hung, because thesetreacherous waters were manageable for only those sailors that knew the way.To help identify the different rock formations he invented a ‘rock identifier’ similar to a pairof spectacles with dark interchangeable lenses that had the rock formations engraved on thelens so when looking through the lenses you could tell which rock you were looking at