It was bad enough not to be backed up in his own schemes, but to feel that his chum knew something that he did not, was very hard on Sir Digby.
Now he recalled it, Arthur had all along been somewhat reserved about the business.

He had made sport of other fellows' theories, but he had never disclosed his own.

Yet it was evident he had his own ideas on the subject.

Was it come to this, that after all these terms of confidence and alliance, a petty secret was to come between them and cloud the hitherto peaceful horizon of their fellowship?
Digby, perhaps, did not exactly put the idea into these poetical words, but the matter troubled him quite as much.
Now, it is my intention, at this place, generously to disclose to the reader what was hidden from Sir Digby Oakshott, Baronet, and from everyone else at Grandcourt--namely, that Arthur Herapath was fully persuaded in his own mind that he knew the name of the arch offender in the recent outrage, and was resolved through thick and thin to shield him from detection.

He was perfectly aware that in so doing he made himself an accessory after the fact, but that was a risk he was prepared to run.