Object lesson No 37

3rd November 2000 at 00:00
The physical law of action and reaction that makes rockets soar and Bonfire Night revellers go "ooh" and "aah" was first properly explained by Isaac Newton in the 17th century. But a Greek named Archytas was using the same principle to wow crowds 2,000 years earlier by sending a steam-powered wooden pigeon along a wire.

Soon afterwards the Chinese discovered the explosive properties of saltpetre and mixed it with charcoal and sulphur to make black powder - the fuel of all fireworks. By 1232 they were terrifying Mongols at the battle of Kai Keng by launching barrages of "fire arrows" against them. Soon they were launching rockets just for fun - and to ward off evil spirits.

A few years later an English monk called Roger Bacon became the first westerner to record experiments with black powder, declaring: "If you light it, you will get thunder and lightning."

Cannons were preferred for warfare, with rockets reserved for spectacular sideshows; Oriental and Italian manufacturers led the way. Based on hollow balls and multi-tiered cylinders respectively, and put together with cardboard, string and glue, these two types of construction are still commonly used. But they must be handled with care; firework makers have to wear cotton clothes to avoid creating stati electricity.

In the 16th century, the German firework maker, Johan Schmidlap, reached new heights in rocketry with his multi-staged step rocket, which presaged the technology that would power missiles and land a man on the moon.

Early rockets carried goose quills stuffed with steel powder to spout golden sparks and entertained English monarchs; Queen Elizabeth I was so amused she appointed a royal firemaster.

Today, DIY displays launched from milk bottles in the back garden pale in comparison with modern spectaculars where what look like rockets are technically mortars, fired from tubes set in sand.

Chemical salts of sodium, strontium, magnesium and copper produce the yellow, red, white and blue visuals and nature gives many of the familiar effects their names: the "willow", which trails streams of sparks, the outward-spreading "palm tree" and the "chrysanthemum", which explodes into spherical blooms. Potassium perchlorate gives out the high-pitched screeches, while powdered titanium makes sure things go with a bang.

And what really gets people fired up is not the complicated chemistry but the simple pleasure of seeing the rocket's red glare illuminate the night sky.

You don't have to be a rocket scientist to work that one out.

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