Today, we ring the Liberty Bell. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Some technologies are so evocative! Bells, for instance: The ghostly Shropshire Lad hears the bells that once called his love to church, called her to wed him -- then called her to his funeral. Finally he screams, "Oh noisy bells, be dumb! I hear you." Edgar Allan Poe celebrates the "tintinnabulation that so musically wells from the bells bells bells" -- from sleigh bells, alarm bells, wedding bells. Bells signal the turnings and changes of our lives -- from the first hour of school to hailing a new government.

The Liberty Bell is a microcosm of political and technological history. It was also a kind of laboratory for both politics and technology. The technology of bell-making pushes foundrymen to their limits. Bells took their familiar modern form in the 13th century -- with a flattish top, concave sides, and a somewhat thickened lip; but they've always been hard to make. For one thing, the shape is hard to cast. Bronze of 77 percent copper and 23 percent tin seems to be the best metal to use. That mix strikes a delicate compromise between tone quality and brittleness. The bronze Liberty Bell is 3 feet high, and it weighs a ton.

It was actually made long before the American Revolution. It was ordered from an English bell foundry in 1752 to celebrate Pennsylvania's 50th anniversary. Yet its inscription from Leviticus foresaw its role: it says,

Proclaim liberty throughout all the land unto all the inhabitants thereof.

The English bell cracked even before it was put in its tower. So a Philadelphia foundry undertook to recast it. They tried make it less brittle by using extra copper, but that bell had a dull sound. So they put the tin back in and cast the third and final bell. It first rang in 1757 at a meeting in which the legislature sent Ben Franklin to England with a list of grievances. From then on it rang the great events of the Revolution -- it was muffled to toll a death knell for English taxation -- it called rebel meetings -- it celebrated victories.

But it didn't ring on the 4th of July. The 4th was merely an intended date. The actual proclamation was delayed until July 8th. Then the bell did indeed ring out. And it went on signaling the great events of our land until 1835. Finally, as it tolled the death of Chief Justice John Marshall, it cracked.

But it was now 78 years old. It was a creature of trial and error. It started out English and saw us through to an established America. It followed an odyssey of political change. In the end it became the perfect national symbol.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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