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No. 678:
Watt and Black

Today, a parable about the origin of ideas. 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. 

The steam engine was 70 years old in 1769. Then James Watt filed the patent that changed it utterly. Before Watt, you squirted cold water into a cylinder to condense the steam. That made a vacuum. It sucked the piston down in a working stroke. But it also cooled the cylinder. You wasted more steam reheating the cylinder than you used to refill it.

In one ingenious leap, Watt got around the problem. He exhausted the steam into a separate condenser. The condenser was always cool and it always held a vacuum. The cylinder stayed hot and the condenser stayed cold. That doubled steam engine efficiencies.

Watt was an instrument maker at the University of Glasgow. The great chemist Joseph Black taught there. He was a friend to Watt. They would talk together. Black had just found out about latent heat. He showed that water absorbs a huge lot of energy, without changing temperature, when you boil it. We like to believe that science leads technology. So we've presumed that Black gave Watt the key idea he needed to invent the separate condenser. 

One of Black's students fed that notion after Black died. He wrote up Black's lecture notes and published them as a textbook. He dedicated the book to Watt. He called him, "Black's most illustrious pupil." He said Watt owed his invention to "the instructions ... you received from Dr. Black."

All that supported our belief that science gives birth to technology. But in the last few years we've learned it's not really what happened at all.

Watt worked six years giving birth to his new engine. He spent part of that time in his lab, experimenting with steam. He learned -- on his own -- that the heat from just one pound of steam will bring six pounds of water to its boiling point.

Then he told Black what he'd done. So Black answered that his result was because of latent heat. Later, Watt wrote,

Thus I stumbled upon one of the material facts by which [Black's] beautiful theory is supported.

This is not a story about a fight for priority. There was no argument between Watt and Black. New ideas do suddenly well up among many people. Watt came upon latent heat one way -- Black another. Watt used the concept to change our world before he'd seen it formalized. And that's really no surprise. After all, it's in the nature of invention that it must, one way or another, precede understanding.

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

(Theme music)

Fleming, D., Latent Heat and the Invention of the Watt Engine. Philosophers and Machines. (Otto Mayr, ed.) New York: Science History Publications, 1976, pp. 121-123.


From The Steam Engine Familiarly Explained, 1836

From The Steam Engine Familiarly Explained, 1836


The University of Glasgow
Photo by John Lienhard

The University of Glasgow