Change of tense from past to present creates a feeling of permanence or timelessness to a narrative, so (looking again in Chapter 3) author Dickens enters the narrative to add perspective adding comment on the human condition beyond the story itself. The story is proceeding with Jarvis Lorry on his way toward assisting Doctor Manette, imprisoned in the Bastille, and the comment here elevates the story to a general state of the human in its isolation and essential separation from others, despite how painful or undesirable this state is. Such a comment on human nature has to be made in the present tense, to indicate its timelessness. All of this takes place in the first paragraph of this chapter before the narrative itself resumes:
A wonderful fact to reflect upon, that every human creature is constituted to be that profound secret and mystery to every other. A solemn consideration, when I enter a great city by night, that every one of those darkly clustered houses encloses its own secret; that every beating heart in the hundreds of thousands of breasts there is, in some of its imaginings, a secret to the heart nearest it!
So, present tense is appropriate here not only to indicate the timelessness of the human condition, but also that condition as great equalizer amongst all people, no matter how poor or desperate a person may be. Possibly these kinds of statements account for one reason Dickens is a beloved author--the eternal democrat. Once he turns back to the story, which is set in a specific past reference frame, he turns back to past tense.
Early to mid-19th century authors were usually present as authors within their works, so it's not surprising to find Dickens refer to himself--as author--for example at the beginning of Chapter 3--"A solemn consideration, when I enter a great city by night, that every one of those darkly clustered houses encloses its own secret . . ." However, this is not the only way Dickens uses first person. At the end of the story, in Part 3, Chapter 13, titled "Fifty two," when Lucie, her father, and Charles Darnay are escaping the threat of the Bastille, due to Sydney Carton's heroism, Dickens uses first person as though he (and all readers) are alongside those anxiously escaping from mob violence, and this technique is unusual. It's unusual because he at this point does not identify himself as author here, apart from the story and as its student, but as one of the characters--as though to emphasize a timeless human sensibility of pity and tragedy over victims of human madness and cruelty. The result is very powerful writing at this point:
Houses in twos and threes pass by us, solitary farms, ruinous buildings, dye-works,tanneries, and the like, open country, avenues of leafless trees. The hard uneven pavement is under us,the soft deep mud is on either side. Sometimes we strike into the skirting mud, to avoid the stones that clatter us and shake us; sometimes we stick in ruts and sloughts there. The agony of our impatience is then so great, that in our wild alarm and hurry we are for getting and and running--hiding--doing anything but stopping. . . .
Dickens' twelfth novel "A Tale of Two Cities" appeared in weekly installments between April 1859 and November 1859. The novel uses the French Revolution as a backdrop to foreground Sydney Carton's unfulfilled romantic attachment and love to Lucie Manette. Dickens researched all the details of the French Revolution meticulously before starting to write "A Tale of Two Cities." His main source for the historical information was Thomas Carlyle's "History of the French Revolution."
Being an historical novel it was inevitable that Dickens intertwined the historical past with narrative present of the main plot of the novel. The novel is divided into three sections - Recalled to Life, The Golden Thread and The Track of a Storm.
Dickens begins his novel in the first chapter with a narrator giving his readers the necessary historical information in the past tense:
It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair, we had everything before us, we had nothing before us,
The plot of the novel begins in Ch.2 with a dramatic incident in the vivid present which serves to immediately capture the attention of the readers - Jerry Cruncher's heroic and successful attempt in giving the note to Mr.Jarvis Lorry at the nick of the moment. The chapter ends with Jerry Cruncher wondering aloud what Mr.Jarvis Lorry's reply to the note - RECALLED TO LIFE - meant. The dramatic incident which is played out then and there right in front of the eyes of the readers concludes with
"he [Jerry Cruncher] turned to walk down the hill.]
This sentence is in the third person past .
But the very next chapter begins with the narrator directly addressing his readers in the first person present as he wonders what secrets lie hidden in the bosom of each citizen of a metropolis:
A solemn consideration, when I enter a great city by night, that every one of those darkly clustered houses encloses its own secret; that every room in every one of them encloses its own secret; that every beating heart in the hundreds of thousands of breasts there, is, in some of its imaginings, a secret to the heart nearest it!
This rapid change of tense from past to present and objective third person narration to the first person present foreshadows the most important incidents of the novel - Dr. Manette's grim past and Charles Darnay of the Evremonde family becoming coincidentally his son in law in the sequence of incidents in the plot being enacted in present time.