Introduction

The important questions that relate to the Earth's thermal regime and energy budget were raised a long time ago and some are still waiting for a complete answer. These past debates have more than historical interest. Our present understanding of the Earth's dynamics is based on the answers that were given to these questions.

People who live in volcanic areas always had the intuition that temperature increases with depth in the Earth. That it must be so everywhere became clear to scientists and engineers with the development of coal mining and the construction of deep tunnels in the nineteenth century.

Among the many advances in physics during the nineteenth century, development of the theory of heat conduction and of thermodynamics had immediate implications for the understanding of the internal structure and evolution of the Earth. The scarcity of data did not hamper physicists in speculating about the temperature regime inside the Earth.

Kelvin and the age of the Earth

When Fourier first published Théorie Analytique de la Chaleur, the temperature gradient of the Earth was estimated to be ≈20 Kkm-1, a value not very different from our present estimates. Fourier analyzed the temperature inside the Earth and concluded that the Earth had retained most of the heat from its formation. This conclusion was the basis for the calculation by Lord Kelvin of the age of the Earth (Thompson, 1862). Kelvin's study triggered a very serious debate between physicists and geologists and has received much attention from the historians.