In 1776, John Adams proposed that fireworks be used to celebrate Independence Day. After 221 years, most people still don't understand how they work, said John A. Conkling, executive director of the American Pyrotechnics Association and one of the leading fireworks analysts in the United States.

Even though the spectacular displays now are triggered by computer, the basic composition of fireworks has remained the same: Potassium nitrate, sulfur, charcoal, aluminum and an assortment of chemicals produce the spectrum of colors that light up the sky.

The challenge of choreographing a great fireworks show, Conkling said, is in the patterns, colors and heights selected. Shell size determines how high the fireworks will travel and how long they will last. The shells are staggered throughout the show so that they explode above or below the smoke left by earlier bursts. The computer sends firing cues every 1/10 of a second to maintain a consistent pattern of bursts in the sky.

THE SHELLS The 21-minute display features 2,495 shells, 30% of which will be fired during the final three minutes of the show.

FACTS ABOUT THE MALL SHOW * WHEN: 9:10 p.m. Friday * WHERE: By the Reflecting Pool; an 840-foot buffer zone separates spectators from the firing site. * BUDGET: $92,000, sponsored by Target Stores Inc. * COST OF EACH SHELL: From $25 to $180 for a six -- inch shell to up to $450 for a 12 -- inch shell. * WEATHER: Wind speed must be less than 20 miles an hour; in the event of strong winds or inclement weather, the show will be postponed until Saturday. Height of explosion

Shell diameter Quantity in show 1,000 -- 1,300 ft.

12 inches

100

10 inches

152

800 -- 1,000 ft.

8 inches

547

400 -- 600 ft.

6 inches

1,324

300 -- 400 ft.

4 inches

72

3 inches

300 Washington Monument, 555 ft.

Fireworks explode every 1.5 to 2.5 seconds during the main part of the show and every 1/2 second to one second during the final three minutes, the grand finale. A firework travels for 3.1 to 7.5 seconds before bursting in the sky. The amount of black powder (made from potassium nitrate, charcoal and sulfur) and the length of the time fuse determines how high the shell travels and when it will explode. After bursting, a firework may remain visible for two to five seconds. The six- to 12-inch shells are fired from four trailers filled with sand, which supports the 35-pound steel mortars that hold the shells. The three- and four-inch shells, which are strictly noisemakers and explode close to the ground, are fired during the grand finale from wooden crates and racks. ANATOMY OF A FIREWORK Each shell contains an assortment of "stars" -- each about the size and shape of a jawbreaker -- laid out in a precise order to produce a specific pattern. If the stars are out of position -- even 1/16 of an inch -- the pattern of the firework will be affected. Multi-break shells consist of more than one chamber, separated by cardboard disks and ignited by time fuses. Each chamber has its own bursting charge that lights and throws out the stars. The container must burst open with tremendous force; the longer the shell can resist the explosion, the bigger the display will be. Electrical charges ignite a fuse that fires a lift charge, which blows the shell out of its steel pipe anchored in the sand. The blast also lights a time fuse at the shell's base.

SOURCES: American Pyrotechnics Association, Atlas Advanced Pyrotechnics Inc., National Council on Fireworks Safety and Nova Online, http://www.pbs.org/nova/kaboom

CAPTION: Fireworks pictured above, from left: Brocade Crown to Crackling Stars, Butterfly, Brocade Crown (green), Scatter, Blue Peony with Brocade Center.

CAPTION: FOURTH OF JULY EVENTS ON THE MALL (This graphic was not available)