Complex sentence: Thomson and Thompson are thus sent as part of the expedition in order to look out for him, but he never appears, the detectives concluding at the end of the book that he was discouraged to take action due to their presence.
Split into simpler sentences: Thomson and Thompson are thus sent as part of the expedition in order to look out for him, but he never appears. The detectives conclude at the end of the book that he was discouraged because of their presence.