The plan might well have died then, had not Whiston hit on the idea of combining sound and light. If the proposed signal guns were loaded with cannon shells that shot more than a mile high into the air, and exploded there, sailors could time the delay between seeing the fireball and hearing its big bang—much the way the weather wise gauge the distance of electrical storms by counting the seconds elapsed between a flash of lightning and a clap of thunder. - Longitude

A short demo of how this would work, demonstrating basic geometry.