What does “that mighty heart’’ refer to?

Reading Comprehension: Read the quoted parts carefully and answer the questions in English．Write your answers in the corresponding space．“The river glideth at his own sweet will：
Dear God! the very houses seem asleep；
And all that mighty heart is lying still!”
(from William Wordsworth’s “Composed upon Westminster Bridge”)
What does “that mighty heart’’ refer to?
【正确答案】：

IT REFERS TO LONDON.

【题目解析】：引文中的“that mighty heart’’指的是伦敦。