English physicist James Joules' famous 1847 Niagara Falls experiment, in measuring the mechanical equivalent of heat.
In experiments, Niagara Falls is a frequented site of experimentation, in the early history of thermodynamics, on whether or not there is a difference in temperature between the top water and bottom water. The impetus is such that the measured data points of the change in temperature and the change in height of a unit mass of water would yield for a measurement of the mechanical equivalent of heat.

Joule
English physicist James Joule suggested that water at the bottom of Niagara Falls, which are 50 meters high, should be warmer than that at the top; he estimated that the rise in temperature should be about 0.10 K. [4] In the early 1840s, Joule suggested to Manchester chemistry professor Lyon Playfair that they take a trip to Niagara Falls to “ascertain the difference of the temperature of the water at the top and bottom of the fall.” [1] Joule latter commented on this: [7]

“I have disputed with Dyer on these matters for a good 20 years … I once invited him to go with me to Niagara and offered to pay our joint expenses if a higher temperature was not found at the bottom than at the top of the falls.”

Joule’s finally conducted the experiment while visiting a waterfall (Niagara Falls?) with his wife on their honeymoon in 1847, during which he produced a thermometer with which to measure the change in the temperature of the water as it fell. [2]

Mayer
In circa 1865, American physicist Alfred Mayer, during a visit to Trenton and Niagara Falls, found that on misty days the water was heated by the arrest of motion after the descent, as one would expect in accordance with the first law of thermodynamics. If the atmosphere were dry, however, the evaporation more than offset the heating effect. [3]

Maxwell
In 1868, Scottish physicist James Maxwell commented in a letter to Irish-Scottish physicist William Thomson: [6]

“I hope Tait will start the determination of Joules equivalent with mercury coming down a wide tube from a cistern and flowing through a difficult passage into a lower cistern. One foot of mercury is as good as 400 of water so an experiment in a room will be much better than Niagara Falls.”

Student problems
Chemical thermodynamics students are frequently assigned the task of calculating the temperature change using the data of: height equals 50 meters, heat capacity of one mol of water, 0.018 Kg, equals 80 J/K, acceleration equals 9.8 meters per second squared. [4] Another version of the question is: “How much warmer is the water at the bottom of Niagara Falls than at the top, knowing that the height is 160 feet?” [5]

See also
Thermodynamic anecdotes

References
1. Shachtman, Tom. (1999). Absolute Zero and the Quest for Absolute Cold (pg. 91). Mariner Books.
2. Joule – Anecdotes.com.
3. Mayer, Alfred G. and Woodward, Robert S. (1916). “Biographical Memoir of Alfred Marshall Mayer (1836-1897)” (32-pgs). Washington: National Academy of Sciences, Jan.
4. Smith, E. Brian. (2004). Basic Chemical Thermodynamics (pg. 20). Imperial College Press.
5. Draper, Charles H. (1906). Heat and the Principles of Thermodynamics (pg. 244). Blackie and Son.
6. Maxwell, James. (1868). “Letter to William Thomson”, Feb 20; in The Scientific Letters and Papers of James Clerk Maxwell: 1862-1873, Volume II (pg. 346-47). Cambridge University Press.
7. Cardwell, Donald S. L. (1991). James Joule: A Biography (Niagara Falls, pgs. 76, 210). Manchester University Press.