"Ne'er saw I, never felt, a calm so deep!
The river glideth at his own sweet will:
Dear God! the very houses seem asleep;
And all that mighty heart is lying still!"
The simile is "The river glideth at his own sweet will". It compares the flowing of the Thames River to a person acting with their own free will. This comparison emphasizes the peaceful and serene atmosphere of the scene, as if even the river is at rest.