It was a good thing that the flyers went over their engines.  John found a loose coupling in one, and a stretched fan belt in the other. Had they gone on in this condition trouble would have been sure to visit them.  It was small wonder, however, that something should not be out of good working order, for these faithful pieces of mechanism had been given the hardest kind of usage day in and day out, each in its turn, and sometimes working together, in this long flight around the earth.  Their final test had been the storm.  More than once the boys had marveled at the remarkable efficiency of their motive power.  What a tribute to the mechanical genius of modern man had these engines paid!  They were almost human in intelligence, more than human in their untiring zeal.