What was used as a warning system in the old north church

I believe the answer is: One lantern would be lit if the British were coming by land and two of them lit if they were to come by sea.

The purpose of using lantern is so the people are able to prepare a surprise attack when there are outsiders who meant to attack them.

If the warning is made using sounds such as bells, the attacker would knew and the chance for a surprise attack would be gone.

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