Erie Canal. The story that brought this to my attention was on the This Day in History website. The picture at the right was used on the Erie Canal website but was used there by the permission of the Memorial Art Gallery of the University of Rochester.
Besides the building of the canal as a major engineering feat and an incredible accomplishment for the workers who dug it mostly using hand tools, the thing that caught my eye was how they were able to signal New York City that Governor Clinton was leaving the opening ceremonies [near Buffalo, NY] on a trip that would span the length of the canal route to New York City. They were able to send the signal in only 81 minutes by using cannon that were spaced along the entire route. As the first cannon was fired, it was heard at the next cannon placement so it fired and this continued all the way to New York City. It was the fastest communication of an event in the world at that point. A practical telegraph system wouldn't be available for 19 more years. This was a very limited signal, though. It could have meant anything (including that New York City was under attack!) but the meaning of the signal was decided before hand. The arrival of the signal meant, "Remember what we discussed before? That Governor Clinton was going to travel here on the new canal? Well, he's started."
It reminds me of the system described in The Return of the King (part of the Lord of the Rings trilogy) when the kingdom of Gondor sent a message to its ally Rohan by lighting huge bonfires along the route. Of course, that book wasn't published until the 1950's. But I wonder if somewhere in history, people were able to send messages over long distances using flags or fires in a similar way?