[Jonty30]

It is only theoretical to see if it’s possible. It’s another thing to see if it was of practical use, in the event of an invading army.

[saltlick]

"In The Lord of the Rings, a series of seven beacons is used as a signaling device between Gondor and Rohan. [1] In the film adaptation of The Return of the King, Gandalf has Pippin light the beacon closest to Minas Tirith. The series is then lit, thereby notifying Rohan's King Théoden that Gondor calls for help in the battle against Sauron."

http://en.wikipedia.org/wiki/Beacon