In his first splendour (line 10)
The poem is about making the old seem new again. Even the sun is remade every morning. Each day is a new and transient world.
And all that mighty heart is lying still! (line 14)
This moment will not last long. The speaker catches London at a time after the sun has risen but before most people have awoken for work or play.
You've been inactive for a while, logging you out in a few seconds...