I've just read the original 1930 version of this ninth Hardy Boys novel back-to-back with the 1965 revised version. The original, which was written by the great Leslie McFarlane under the Stratemeyer Syndicate's house pseudonym Franklin W. Dixon, has 24 chapters and 210 pages, and the newer one (written by Tom Mulvey) is considerably shorter, with 20 chapters, 175 pages, and several illustrations. The plot of the original is far more entertaining, with a considerably higher content of adventure and intrigue and character-building events; in the newer one the Boys fly off to Montana and a Caribbean Island and encounter a random tornado while taking a helicopter ride, but it's much less engaging, much of the mystery relying on silly coincidences. There are quite a few more characters in the original, and we learn that Frank's girlfriend Callie Shaw is the class valedictorian, Tony Prito plays the accordion, Joe's girlfriend Iola Morton plays the violin, her brother Chet "doesn't care for girls" (didn't that raise an eyebrow or two in 1930?), Joe wears a size six shoe and weighs 125 lbs., Mrs. Hardy actually has a name (Laura; in the newer version she's just Mrs. Hardy), and so on. The Boys graduate from high school in this book, and there's much more continuity with other books in the series than in the newer stories. I was surprised that they also carry revolvers while in pursuit of the criminals. One odd thing that caught my attention was that Frank lost his sweater and pocketknife in the barn; I wonder when they got a barn? Also, airplanes and air mail were much more of a novelty then than they were in the 1960's, so some understanding of history is needed. Anyway, I'd say the original is appropriate for the average twelve-year-old and rate it at four, and the revision is better suited for the nine-year-old level and give it a two.