Can someone give me an explanation of what the following quote from The Strange Case of Dr. Jekyll and Mr. Hydemeans? "The fog still slept on the wing...

Literally,
the quote is describing what London looks like outside the window of Mr. Utterson's home, where
he sits by the hearth with his friend Mr. Guest. The passage is saying that the city is covered
("drowned") in fog, which would not be uncommon for London. All that can be seen is
the glow of the many (at this time) gas lamps around London that look to Utterson like pustules
or pus-filled boils under human skin. Beneath the fog, though he can't see anything, he can hear
the sounds of London, which reassure him the life of the great city is going on. Traffic through
the arteries (main...