Call me silly, but I think that the level of the story drops steadily from book one through this one. Now, after the Riverboat is built, dirigibles come into the picture (now why didn;t I think of that!?). Okay, Sam Clemens and his cohorts are hot on the heels of King John the dastardly. Are they going to get him? The enterprising dirigible pilot Jill Gulbirra (of the Clemens lot) finally reaches the mysterious tower at the center of the polar sea. I won;t spoil it for you by giving it away. Let me just tell you that it is ..ahem.. less than revealing. But the absolutely worst sin of this book are the endless parentheses dedicated to Peter Frigate's dreams and reminiscences which say absolutely nothing. At the start of the book, Farmer points out the autobiographical genesis of Frigate's character but he still is (at least so far) a very minor character that simply does not justify the soporific dream sequences.