Disloyalty in trusted servants is one of the most disheartening things that can happen to a public performer. But it must not be thought that I say this out of personal experience: for in the many years that I have been before the public my secret methods have been steadily shielded by the strict integrity of my assistants, most of whom have been with me for years. Only one man ever betrayed my confidence, and that only in a minor matter. But then, so far as I know, I am the only performer who ever pledged his assistant to secrecy, honor and allegiance under a notarial oath.

Miracle Mongers and Their Methods (1920)

https://books.google.com/books?id=l0u-YrUcJeMC