“Good luck, Tom!” called his aged father, as he was ready to take off the next morning. “Bring back that chest!” “I’ll do my best!” was the answer. CHAPTER XXII A STRANGE MESSAGE THOUGH he had no more than very slight clews on which to pursue the robbers, Tom Swift was not without a definite plan on which he proposed to operate. As he had mentioned to Ned, he had obtained information which indicated that the big blue aeroplane, after the robbers had put the chest on board and had left Koku tied to the tree, had departed toward the west. Of course there was no guarantee that it would keep on this course, and absolutely no way of telling how long it would hold it. “But we can stop from time to time,” said Tom to Ned, “and make inquiries about the plane. A big blue aeroplane isn’t easily hidden from sight.” “It sounds like good dope,” agreed Ned. Koku was no stranger to aeroplane rides, and he felt perfectly at home in the _Blackbird_. Indeed, as those of you remember who have read the earlier books of this series, Koku was brought from his home in a strange land by an airship. He rather liked to ride in them. So Tom, Ned and Koku flew off on their strange quest. Up into the air soared the _Blackbird_. She was a powerful machine, and, as has been said, was roomy. Really, she was built to carry four, but on account of the size of Koku a partition between two cockpits had been taken out, making a large space where he could dispose of his enormous legs and big body. Before starting on the search for his chest of secrets, Tom had caused inquiries to be made about the missing Barsky. But the man seemed to have disappeared completely after leaving Tom in the cistern. “Though of course he might have been, and probably was, one of the gang that took the chest and bound Koku,” suggested Ned. For several hours the trio of adventurers soared along, not going so high but what they could from time to time make observations of the earth below them through field glasses. For of course it was possible that the blue aeroplane might be on the ground. She also might be soaring along in the air, and Tom and Ned did not neglect to scan the heavens for signs. Once they saw a plane coasting along, and gave chase. But when within good viewing distance they made out that it was one of the government mail-carriers, and they turned back on their original course. It was near noon when Ned heard Tom give a sudden exclamation. “See anything?” asked the young manager. “Nothing that gives me any pleasure,” replied Tom grimly. “I see a leak in one of the oil pipes and that means we’ve got to go down and mend it. Lucky I discovered it in time!” An aeroplane engine, or for that matter an auto motor, that does not receive copious and continuous lubrication is going to overheat, bind and stop in a surprisingly short time.