From chapter 3
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From what I can tell of the quote, it means a full glass of alcohol,
"Though true cylinders without --within, the villanous green goggling
glasses deceitfully tapered downwards to a cheating bottom. Parallel
meridians rudely pecked into the glass, surround these footpads' goblets.
Fill to this mark, and your charge is but a penny; to this a penny more;
and so on to the full glass --the Cape Horn measure, which you may gulp down
for a shilling."
I have heard that after a trip round Cape Horn a triple shot was offered to the crew,
"One for the boy,
Two for the man,
And three for the full Cape Horn measure"
although that could have just ben an excuse to drink a triple!!