## Using cannon balls to estimate the age of the Earth

Stories from Physics for 11-14 14-16

Georges-Louis Leclerc, Comte de Buffon, a naturalist and mathematician, provided an early estimate of the age of the Earth by observing cooling cannon balls. In a 1775 work, the Frenchman describes making a number of iron spheres of various diameters and heating them till they were white hot. He recorded the time taken to reach two points: a temperature when they could be held comfortably and room temperature. Buffon argued that the time taken to reach the two temperatures was proportional to the balls’ diameter and estimated that it would have taken the Earth “ninety-six thousand and six hundred seventy years and one hundred and thirty-two days” to reach room temperature. Buffon’s estimate preceded Kelvin’s famous erroneous estimate of the age of the Earth (see end of Kelvin’s achievements on page 28) via a similar method.

### References

**Using cannon balls to estimate the age of the Earth**

J.-P. Poirier, About the ages of the Earth, *Comptes Rendus Geoscience*, vol. 349, no. 5, pp. 223-225, 2017