Letter, quoted by Vladimir Dobrushkin, "Biography of Joseph Fourier" & Elena Presitini, The Evolution of Applied Harmonic Analysis (2004) p. 42. - Joseph Fourier

We shall describe... the principal results of the prolonged action of the solar rays upon the terrestrial globe. ...[T]he state of the mass has varied continually in proportion to the heat received. This variable... internal temperature... has approached... nearer to a final state... subject to no change. Then each point of the solid sphere has acquired, and preserves... a fixed temperature, which depends only on the situation of the point... The final state of the mass, the heat of which has penetrated all... parts, can... be compared to... a vessel which receives by openings at the top, liquid from some constant source, and permits exactly an equal quantity to escape by orifices.
Thus the solar heat has accumulated in the interior of the globe and is... continually renewed.

The mobility of the air, which is rapidly displaced... and... rises when heated... diminish the intensity of the effects... [of] a transparent and solid atmosphere, but do not entirely change their character.

If we consider further the manifold relations of this mathematical theory to civil uses and the technical arts, we shall recognize completely the extent of its applications. It is evident that it includes an entire series of distinct phenomena, and that the study of it cannot be omitted without losing a notable part of the science of nature. The principles of the theory are derived, as are those of rational mechanics, from a very small number of primary facts, the causes of which are not considered by geometers, but which they admit as the results of common observations confirmed by all experiment.