I just got back from Colonial Williamsburg and I have the answer. It is a gate closer. Unfortunately I did not have my camera with me but they are on every picket gate down there. How it works is a post is set in the ground inside the fenced area. Then a piece of chain is connected to the post on one end and to the gate on the other. The ball hangs and holds the chain in a v shape at rest. When the door opens the chain straightens and when you let go of the gate the weight pulls the chain back to a v and puts pull pressure on the gate so it won't blow open. I find this a simple but an effective and smart idea. If you google "gate closer" or "Williamsburg picket fence" you will probably find it.