This was a very difficult book to obtain. There was a copy in the British Library, and another one in a Library in Dartmouth, Devon. For several years I tried at least weekly to find a copy via Abebooks or eBay, with no success. The copy belonging to the Ballantyne family had disappeared, not to put too fine a point on it. Eventually a kind family in Canada offered to scan the pages of their copy, and send the images to me, and this is the result. Ballantyne did indeed try out some diving equipment, so as to obtain a first-hand feel for diving. It is related that something went wrong, too much air was sent down, and he surfaced rapidly upside down. A similar episode is related in the book. Ballantyne's style often gives rise to two or even three stories continuing simultaneously, and here we have the adventures of one Rooney Machowl, an Irishman who decides to move from his ship's carpenter trade to that of diving. In fact divers should always have another trade, or they wouldn't be much use under the water. In addition there is the aspiration of Edgar Berrington to win the hand of a fair young lady, there are the events happening to the young lady's father, and then again the events happening to the young lady's companion. So it is all fairly convoluted. But you'll certainly learn a lot about diving, as the art stood in 1876. It is rather strange that Ballantyne, having written this book, which ran to several printings, did not much mention diving in any other of his books.