Which, by the way, was only true in a certain sense; for while I had given the young man to understand that, for his own benefit and advantage, I intended to make him perform the duty of master, and hold him responsible for the navigation, I had taken care to maintain a strict check upon his calculations and assure myself that he was making no mistakes.  Of which fact he was of course quite aware.  Wherefore his reply to my retort was simply to change the subject with some celerity.