In the second volume, _The Boy Aviators on Secret Service_, the reader was taken to the mysterious Everglades region of Florida where the young inventors once more demonstrated their ability to grapple with emergencies. They proved that they were patriotic sons of Uncle Sam by discovering and putting out of commission a factory that was making dangerous explosives without the consent of the Washington Government.
It was a long jump from Florida to the depths of the Dark Continent, but the occasion arose necessitating their taking this trip to Africa. If you want to learn how theirs was virtually the first aëroplane to soar above the trackless heart of Africa, how they found the hidden hoard of priceless ivory secreted by slavers in the wonderful Moon Mountains, what strange things came about through their being hunted by the vindictive Arab slave trader, with many other interesting adventures, you can do so by procuring _The Boy Aviators in Africa_.
Through the coaxing of their warm chum, Billy Barnes, the boys were next induced to enter in a competitive race across the continent, and it can be easily understood that the pages of this book, _The Boy Aviators in Record Flight_, fairly teem with exciting incidents and thrilling adventures. Crossing the great Western cow country, they met with many difficulties from sand storms to treacherous cowboys and renegade Indians that threatened to end their game voyage. But the same indomitable spirit that had carried Frank and Harry through so many trials allowed them to meet with the glorious success they so richly deserved.
From one series of adventures like this it was easy for the wide-awake young air pilots to engage in others. A story of an old Spanish galleon caught in the grip of that mysterious Sargasso Sea, where the circling tides have held vessels amidst the floating grass for centuries, fascinated them, and they set out to explore the dismal region that has been the graveyard for countless ships. Of course, the lure lay in the fact that a vast treasure was said to be aboard this old galleon; and the hunt for it, together with the opposition caused by a rival expedition, makes great reading for boys who have red blood in their veins. It is all set down in _The Boy Aviators' Treasure Quest_, which has been voted one of the best of the entire series.