All in all, the summer slipped away only too quickly, and it came time for Paul and Bob to go back to school again with Sky-Bird II not more than half finished.  It is true that the long fuselage of the craft was done, with its graceful curves and splendid, roomy, enclosed cabin, accommodating five persons; but all concerned were a little disappointed that more progress had not been made.  Mr. Giddings had been quite a frequent visitor at the fair-grounds all through the summer, lending a voice of encouragement throughout the operations.  He looked really concerned, however, when Paul and Bob had to return to Clark Polytechnic Institute for the new term of study.