High school physics. We had a guy set off a large aerial mortar firework on one side of the river and had five people measure from the flash to the sound with stop watches. We threw out the two most extreme times and averaged the middle three. Then distance per second x time in seconds down to the hundredths. We came up within 30 feet of what the US Coast and Geodetic survey map had as the distance on a topographical map. Not far off for a group of high school kids and a 1.4 mile wide river. At the 4th of July a little town along the river has an impressive fireworks display for such a small town. We went down to the river to watch one night. The problem is that we were 7 miles south. The sound took over a half minute to get to us.