The Historama was a magic TV-screen where Shazam the Wizard could see past, present and future. 
The Historama is a book that sits beside Shazam on his throne which he can call upon to see a great many things. It functions like a crystal ball, allowing the user to see anything on any plane in which it is commanded to produce. That is to say that one can call upon the book and ask it to show them any plane of existence desired and the book will show them. To activate the Historama one must say, "Historama show me...".
It's powers and limitations include:
- Remote Sensing
- Extra-dimensional Detection
- True Sight
- Dimensional Viewing
- Temporal Viewing: The ability to look into the past, and functions just like Dimensional Viewing.
These powers allow the Historama to "cleave through the barriers" to show the occupants of the Rock of Eternity what transpires in other dimensions and even other times.
When Shazam died and left the Rock of Eternity in the hands of his champion, Captain Marvel, he also left with him the Historama. When the Tenth Age of Magic came into reality and replaced the Ninth, Captain Marvel then known as just "Marvel" decided to forgo the traditional book and in it's place stood many television and computer screens that served the same purpose.
Links and References