In A Tale of Two Cities, what are some of the dramatic purposes of the scene in which the wine cask breaks, and the wine spills everywhere?Book...

Book the First's Chapter 5, "The Wine Shop," is what is
known as "set piece" because it stands on its own appart from the previous chapters. As
such, it portrays the dire poverty of the peasants with "cadaverous faces" who
frantically mop up every drop of wine from a broken cask and squeeze it into their
infants' mouths.  This chapter also presents a metaphoric tableau of the bloodshed of
the upcoming French Revolution with the symbolic breaking of the cask of wine.  The
"frolicsome embraces, drinking of healths, shaking of hands, and even joining of hands
and dancing" of the people will compare in a macabre way to the forthcoming delight
that The Vengeance and Madame Defarge take in the executions and bloodshed at the
guillotine. And, of course, the wine stains on the streets of Saint Antoine foreshadow
the blood that will flow in the streets of Paris. 

In
nearly an entire paragraph, Hunger is personified in this chapter with rhetorical
parallelism:

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It was prevalent everywhere.  Hunger was pushed
out of the tall houses, in the wretched clothing that hung upon poles and lines; Hunger
was patched into them with straw and rag and wood and paper; Hunter was repeated in
every fragment of the small modicum of firewood that the man sawed
off;....

Dickens uses the
metaphor of birds, "fine of song and feather," for the aristocracy which "took no
warning" of the events that presage their disaster:  "For, the time was to come, when
the gaunt scarecrows of that region should have watched the lamplighter...." Thus,
again, there is foreshadowing of the bloodshed to
come.

And, since the wine cask has spilled outside the shop
of Monsieur and Madame Defarge, Chapter 5 introduces this sinister couple to the
narrative.   Madame Defarge, wrapped in fur against the cold, sits with great composure,
knitting.  In an aura of subterfuge whenever people enter she coughs and lifts her
"darkly defined eyebrows over her toothpick by the breadth of a line."  This signals to
her husband to look around the shop.  There enters three Jacques, men who take the
anonymous name as revolutionaries. Then, Miss Manette and Mr. Jarvis Lorry emerge from
the shop and M. Defarge bends on one knee to kiss the child of his old master, taking
her hand to his lips.  Yet, there is a sinister foreshadowing in this act, too, as
Defarge shows no gentleness in his actions.

As Defarge
escorts Mr. Lorry and Miss Manette up the stairs where Dr. Manette is, there are
significantly three men peering through chinks into the room, the "three of one name." 
M. Defarge strikes three times upon the door, and he draw the key across the lock three
times. They enter where a "white-haired man sat on a low bench, ...making
shoes."

Using symbolism, parallelism,
foreshadowing, and metaphor, Charles Dickens employs these dramatic techniques in
Chapter 5 in order to presage important future events in the plot, introduce new
characters and link them with others as well as significant occurrences. Standing on its
own as a set piece, Chapter 5 also is an example of wonderful prose that utilizes
rhetorical devices.