Final answer:
Mark Twain's understatement injects humor and irony into the passage from Life on the Mississippi, providing a comical view on the relieved reaction to a character's failure and critiquing the perceived favoritism of fortune towards the undeserving.
Step-by-step explanation:
Mark Twain's use of understatement in the excerpt from Life on the Mississippi serves his purpose by employing a subtle dose of humor and irony to describe a situation that would ordinarily be considered tragic. The line "When his boat blew up at last, it diffused a tranquil contentment among us" uses understatement to express feelings of relief in an indirect manner. This comical twist in the passage helps to highlight the conflicted feelings that the community has towards the character, whose failure is met with a sense of unjust satisfaction.
Furthermore, the concluding remark on Providence's partiality to an "undeserving reptile" is a biting understatement that critiques how fortune seems to favor the less deserving. This adds depth to Twain's writing, for it encapsulates his satirical edge and his scrutiny of social attitudes while keeping the tone light and engaging.