A metaphor for the subjective soul, trying to find out what's really happening in the world. We the readers are in the same position -- this quote, which is presented as witnessed fact (something Pedro "had said peremptorily") has actually passed from the speaker, to the railhead engineer, to the Sulaco operator, to the chief engineer, to Decoud, to his sister, to a first-person narrator who has already skipped over some of Decoud's letter, and finally to us. And that's if we leave Conrad out of it!

Incidentally, Minister of the Interior was the office of the Duc de Morny, after whom Pedro models himself.