The twenty-one balloons

Professor William Waterman Sherman just wants to be alone. So he decides to take a year off and spend it crossing the Pacific Ocean in a hot-air balloon the likes of which no one has ever seen. But when he is found after just three weeks floating in the Atlantic among the wreckage of twenty hot-air balloons, naturally, the world is eager to know what happened. How did he end up with so many balloons . . . and in the wrong ocean?