Engines of Our Ingenuity


No. 1609:
FRANKLIN'S HEAT EXPERIMENT

by John H. Lienhard

Click here for audio of Episode 1609.

Today, Benjamin Franklin thinks about theoretical and applied science. 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.

Ben Franklin's breadth and intelligence were dazzling by any measure. His life embodies the entire shape of the American Revolution, all the way from intellectual adventurism in the early eighteenth century to an established new nation.

Franklin devoted his life to understanding and harnessing the forces and fluxes around him -- electricity, music, politics -- it hardly mattered, for he was omnivorous. Historian I.B. Cohen tells how, when Franklin was twenty-three, he worked with a scientific club that eventually became the American Philosophical Society.

Franklin and a friend named Breintnal contrived an experiment. One sunny winter day, they laid colored cloth patches and a pane of glass out on the snow and noted how deeply each eventually sank into the melting snow below it. The white cloth hardly sank at all. The darker each patch was, the deeper it sank. The black cloth and the glass pane sank deepest.

Breintnal published those results eight years later. Then, 32 years afterward, Franklin wrote about them, adding yet another twist. He also focused a burning glass on both white and black paper. White paper absorbed less heat, and it took longer to catch fire than dark paper.

Those results wouldn't be entirely clear until much later. Anything black absorbs light, while anything white reflects it. The catch is that some surfaces respond differently to light and heat. Take skin, for example: Equatorial peoples have dark skin whose pigment protects them from sunburn. You might think dark skin would absorb heat and make life miserable in hot countries. But the sun's rays carry more infrared heat than light. Dark skin reflects heat just as white skin does. If it didn't, Nigerians would all have to live in Stockholm.

Ben Franklin's pane of glass made a similar point. Light goes right through glass. But glass absorbs the heat radiated by the sun -- stops it in its tracks. In that sense, glass is opaque. It is black to infrared (or heat) radiation.

Franklin finished by saying: "What signifies Philosophy that it does not apply to some Use?" Then he went on to say what his tests suggested about dressing for cold and warm climates. Many years later, Franklin made his famous remark, "What good is a new-born baby?" That was an older and wiser Franklin offering a subtle answer to the younger Franklin's question, "What signifies Philosophy that it does not apply to some Use."

The questions raised by Franklin and Breintnal were not entirely new. They'd undoubtedly read older speculations by Boyle, Boerhaave and Newton on the action of heat and light. I admire Franklin for the way he always brought his wide-ranging curiosity back to the hard earth, but rooted in the hard earth is not where these young students of nature (and of government) started out.

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

(Theme music)


Cohen, I. B., Benjamin Franklin's Science. Cambridge, MA: Harvard University Press, 1990. Chapter 9.

This is a greatly revised version of Episode 141.



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The Engines of Our Ingenuity is Copyright © 1988-2000 by John H. Lienhard.