Question

"Dear God!" - Whom does the poet address in the poem "Upon Westminster Bridge? "

Answer 1

Dear God! the very houses seem asleep;
And all that mighty heart is lying still!

You would think the speaker couldn't possibly get more excited about this view after declaring it the most beautiful thing on earth, but no: he gets more excited.
He cries out to God as if he has just recognized something astonishing he had not noticed before.
He personifies the houses as asleep, when it's actually the people inside the houses who are sleeping at this early hour.
The city looks like one big, peaceful, sleeping body. Shh...don't wake it.
The "heart" of this body is "lying still" for the moment before the city awakens for a new day. The heart probably doesn't refer to anything specific, but rather the city's energy or vitality.
The last two lines mark a shift in tone with their two exclamation marks. The tone goes from amazed to Really Amazed!