Yes, in principle the water at the bottom of a waterfall is slightly warmer than the water at the top. As it falls, the water gains energy of motion; when it crashes into the pool below, that energy turns into heat. James Prescott Joule, who proved that work and heat are two forms of the same thing, worked out how much: by his estimate a fall of 817 feet would warm water by one degree Fahrenheit, so the Niagara river’s fall of 160 feet should raise its temperature by about one fifth of a degree. Warmer, then, but barely.
A brewer with a thermometer
Joule was born in Salford on 24 December 1818, the son of a wealthy brewer, and as an adult he managed the family brewery. Science began as a serious hobby. In the 1840s it became something more.
His best-known experiment was simple to describe and very hard to do well. A falling weight, pulled down by gravity, turned a paddle wheel inside an insulated barrel of water. The paddles churned the water, and the water got warmer. Mechanical work had turned into heat, and always in the same ratio: so much falling weight for so much warming. In 1850 he published a refined value of 772.692 foot-pounds of work for every British thermal unit of heat.
The waterfall as a laboratory
If a paddle wheel could warm water, so could gravity on its own. In an 1845 letter to the Philosophical Magazine Joule suggested that readers living “amid the romantic scenery of Wales or Scotland” could check his result by measuring “the temperature of the water at the top and at the bottom of a cascade”. If his views were correct, he wrote, a fall of 817 feet would generate one degree of heat, and the Niagara would be warmed by about one fifth of a degree.
The honeymoon, as the story goes
In 1847 Joule married Amelia Grimes, and the couple went on honeymoon. William Thomson, the young Glasgow professor later known as Lord Kelvin, had heard Joule speak that year and was intrigued but sceptical. The story goes that Thomson met the newly-weds by chance at Chamonix, and that a few days later the two men set out to measure the temperature difference between the top and bottom of the Cascade de Sallanches. The attempt, so the account runs, proved impractical. The difference they were after was a fraction of a degree.
Why it mattered
The waterfall was a vivid way of saying what the paddle wheel had already shown. Energy does not vanish. It changes form, from height to motion to heat, and it can be counted at every step. That idea grew into the law of conservation of energy and the first law of thermodynamics.
Joule died on 11 October 1889. The Short follows his question from a barrel of water to a mountain cascade.
The bottom of the waterfall is warmer, by a sliver of a degree. And that’s how we found out.
