Here's why:
* The boy's situation: The boy is in a state of "obscurity and misery," which suggests hardship and lack of opportunity.
* Becoming an apprentice engineer: This is often seen as a path to a good life and a promising career.
* The contradiction: The irony lies in the contrast between the boy's initial state of misery and the potential for improvement offered by his new apprenticeship. The words "obscurity and misery" create an expectation of further decline, while "apprentice engineer" suggests a path to success.
The irony highlights the stark contrast between the boy's situation and the potential offered by his new path, making the reader question whether the apprenticeship will truly lead to a better life.
It's important to note that this device is often used in Twain's works to critique social injustices and highlight the absurdities of life.