"Through Lud's famed gates, along the well-known Fleet,
Rolls the black troop and overshades the street;
Till showers of sermons, characters, essays,
In circling fences whiten all the ways:
So clouds replenished from some bog below,
Mount in dark volumes and descend in snow."
The "well-known Fleet" is the prison just mentioned, the side of which appears to have been visible at that time in Ludgate Hill, and where it was a joke (too often founded in truth) to suppose authors incarcerated.
"Few sons of Phœbus in the courts we meet;
But fifty sons of Phœbus in the Fleet,"