Are there enemy spies following Professor Bullfinch? Is he working on a secret government invention? Danny and his friends, Joe and Irene, decide they must warn the Professor of possible danger. They track him down to an old barn, where he's hidden his workshop. Once inside all they find is a weird and fascinating machine. Danny tries out one of two levers—but he isn't prepared for what happens next! He and his friends are trapped inside the machine and shrinking fast...

Danny Dunn and his friends search for a legendary serpent in Central Africa.

Cover has small rip, last page ripped but intact

As a boy detective Danny Dunn has lots of competition but the young scientist performs creditably here. Amateur sleuths are unlikely to lose any sleep over the crime or its What could be a more obvious setup than an uncrackable safe? And who could be a more likely suspect than the store manager, Mr. Anguish, who asked Professor Bullfinch to design it? Yet the gadgetry Danny improvises would be the envy of any junior Sherlock; having learned the principle of Professor Bullfinch's watchdog safe (the owner's individual smell is its uncrackable combination) Danny builds his own radiometer from an old vacuum cleaner and is soon hot on the trail of the missing Mr. Anguish. With help from his pal Irene, occasional poetic encouragement from his non-scientist buddy Joe, and with fatherly, black Detective Ellison to keep their investigations in line, Danny keeps up a brisk pace. And young investigators will want to track down the pedigree of the Professor's chemical bloodhound. (Kirkus Review)

Danny Dunn goes fishing to try out a new invention and winds up saving a dam.