Why does Dickens compare the piston of the steam engine in the factory to the head of an elephant? The word elephant suggests a circus to me.

Dickens uses another sinister image to describe the pistons of the steam engines; they constantly go up and down "like the head of an elephant in a state of melancholy madness" (Book 1, chapter 5). This unusual image suggests awesome amounts of energy and strength trapped in meaningless repetitive activity-which rather sums up the life of the factory.