Dixon, Franklin W

Frank and Joe suspect that there is something sinister going on at the site of an Alaskan waterfall when they and their friends are harassed and warned to stay away.

Frank and Joe, the Hardy boys, solve a mystery involving a magician, American secret agents, an international spy ring, and a Scottish castle.

Frank and Joe investigate a walled estate guarded by savage dogs, where a businessman is hiding in fear of his life.

Relates seven cases in which the Hardy brothers use police technology to track down criminals. Details of these techniques and procedures are given in the last five chapters.