What,"" asks the author at one point, ""is the purpose of a porpoise?"" The answer is given (and proven throughout the text):""... to delight."" This may seem like an off-beat topic for juvenile delectation, but the amount of study currently centering on dolphins is bringing up undersea secrets that are being applied to submarine construction, animal psychology, fishing procedures and possibly to the retrieval of space capsules. The playful dolphin is a catch and fetch creature who has responded to training. They are a powerful drawing attraction at Florida marinas and the widely publicized Lilley investigation into their communication underlines their intelligence. (Their brains are bigger than those of the chimpanzees). The author's obvious delight in her subject comes across in a text that is outstandingly well written for younger readers. Extensive research has resulted in anecdotes about dolphins from all over the world; history and literature have been combed for their dolphin stories; and marine experiments are described in a way that shows how trained observation leads to theory and tested theory leads to fact.