Some history first.  The Declaration of Independence took the first part of the year of 1776 to write and was signed on July 4.  The United States Constitution was written in the summer of 1787 in Philadelphia.  The United States began official business on March 4, 1789, and the Bill of Rights  was adopted and certified on December 17, 1791.

The United States at that time thought it had finished war with Great Britain, but friction developed with regard to trading with both the British and French, which brought the United States into war with the British again, beginning on June 18, 1812 and ending February 17, 1815.  During that period, the battle at Fort McHenry in the Chesapeake Bay area of Baltimore was fought, and Francis Scott Key, a 35-year-old attorney, witnessed the battle from a boat in the harbor.  He wrote the poem "The Star-Spangled Banner" on September 14, 1814.  Later that year, the poem was put to the music — "To Anacreon in Heaven," a song written by John Stafford Smith, and it was adopted by the United States in 1931 as our national anthem.  Within that song are the words "and the rocket's red glare, the bombs bursting in air."

We think of rockets as a modern-day device.  To understand what rockets were in those days, an explanation of gunpowder is necessary.  The Chinese developed gunpowder in the 12th or 13th century.  Gunpowder then consisted of charcoal, potassium nitrate, and sulfur.  It has been improved with various modifications and chemicals over the years.  Gunpowder does not explode; it burns rapidly and produces a lot of gas from the burning.  Following that thought, if there is an immediate outlet for the gas, it can be used as a propellant.  The Chinese developed rockets by using hollowed out bamboo poles and long sticks affixed to them to act as guides.  A portion of gunpowder wrapped in paper acted as a fuse.

The British adopted modified versions of these rockets, to use in the late 1700s and 1800s, and they were named Congreve Rockets, after William Congreve, a British artillery officer who improved their range and accuracy.  They are much like the bottle rockets and model rocket engines of today.  For the most part, they were ineffective, used primarily to start fires.

The "bombs bursting in air" are a different story.  These bombs were fired from cannons or mortars at the enemy.  Originally, they were hollowed out cannonballs, usually not more than 24 pounds, that had been filled with small metal parts incorporated with an explosive and a fuse.  These devices, used to kill and wound as many as possible, were invented by a British Major, Henry Shrapnel.

Originally, there was a paper fuse that was lit before firing the cannon ball, but that design was not reliable and effective, as the Francis Scott Key poem illustrates with "bombs bursting in air."  In later years, during the Civil War, the cannonballs were equipped with the Bormann fuse, a pewter cylinder packed with gunpowder, and the top of the fuse was designated with a number from 1 to 5 to denote the time in seconds after ignition when the ball would explode.  The gunner would punch out one of the numbers with a punch and hammer to open the fuse to the required amount of fuse gunpowder.  The fire resulting from the firing of the cannon would ignite the pewter housed fuse.  These were used only for a short time before impact fuses were invented and breech loading cannons and artillery came on the scene.    

The shape of the projectiles changed near the end of the Civil War from the round balls to the modern bullet shape with some extra designs.  During this time, rifling was added to the barrels of long guns and canon and artillery to spin the bullet projectiles to make them fly accurately. 

The National Anthem has four verses, and here is the last verse:

O thus be it ever when freemen shall stand
Between their lov'd home and the war's desolation!
Blest with vict'ry and peace may the heav'n rescued land
Praise the power that hath made and preserv'd us a nation!

Then conquer we must, when our cause it is just,
And this be our motto: "In God is our trust,"
And the star-spangled banner in triumph shall wave
O'er the land of the free and the home of the brave.

Happy Fourth of July.

Thanks to Britannica on the web as a helpful source for this article.

Image: Georgia National Guard via Flickr, CC BY 2.0.