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Rh to various depths according to their duration and amplitude. Suppose, for the sake of example, a block of stone transported from the equator to our latitudes; its cooling will have commenced at the surface, and then become propagated into the interior; and if the cooling has extended throughout the whole mass, because the time of its transportation has been very short, that body thus transported to our climate will present the phænomenon of an increase of temperature with the distance from the surface. The earth is in the case of this block of stone;—it is a body coming from a region the temperature of which was higher than that of the place in which it now is; or we may regard it as a thermometer moveable in space, but which has not had time, on account of its magnitude and according to its degree of conducting power, to take throughout its mass the temperatures of the different regions through which it has passed. At present the degree of temperature of the globe is increasing below the surface; the contrary has in former times been, and will hereafter be, the case: besides, at epochs separated by many series of ages this temperature must have been, and will in future be, much higher or lower than what it is at present; a circumstance, which renders it impossible that the earth should always be habitable by man, and has perhaps contributed to the successive revolutions the traces of which have been discovered in its exterior crust. It is necessary to observe that the alternations of temperature of space are positive causes which have an increasing influence upon the heat of the globe at least near its surface; while the original heat of the earth (chaleur d'origine de la terre), however slow it may be in dissipating, is but a transitoiy circumstance, the existence of which it would not be possible at the present epoch to demonstrate, and to which we should not be forced to have recourse as a hypothesis except in the case of the permanent and necessary causes being insufficient to explain the different phænomena."

The following are the titles of the different chapters of the work, together with a short abstract of the contents of each.

I. Preliminary Notions.—After having given the definition of temperature and many other definitions, it is explained how we have been led to the principle of a continual radiation and absorption of heat by the molecules of all bodies. The interchange of heat between material particles of an insensible magnitude, but yet comprising immense numbers of molecules, cannot disturb the equality of their temperatures when it actually exists. From this condition we conclude, that for each particle the ratio of the emitting to the absorbing power is independent of the substance and of density, and that it can only depend on temperature. In the case of the inequality of temperatures, we give the general expression of their variations during every instant, equal and contrary for two material particles, radiating one toward the other. We