Finally, a story from the Cthulhu mythos! :-D

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The picture used is by David Garcia Forés, created specifically for this short story, and used here under the Creative Commons Attribution-ShareAlike 3.0 Unported license (https://creativecommons.org/licenses/by-sa/3.0/deed.en)

To follow along: https://www.hplovecraft.com/writings/texts/fiction/d.aspx

"Packet" is a type of sailing ship of the 18th and 19th centuries. "Supercargo" is a passenger on board a ship. So "packet of which I was supercargo" just means he was a passenger on a sailing ship. Or, given the late date of the story (WW1), it could have been a steamship.