M. POISSON ON THE MATHEMATICAL THEORY OF HEAT. 125 
tending 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 ex- 
tended throughout the whole mass, because the time of its transporta- 
tion has been very short, that body thus transported to our climate 
will present the phenomenon 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 itasa 
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 dorigine de la 
terre), however slow it may be in dissipating, is but a transitory circum- 
stance, the existence of which it would not he 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 neces- 
sary causes being insufficient to explain the different phenomena.” 
The following are the titles of the different chapters of the work, to- 
gether with a short abstract of the contents of each. 
Carter 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 im- 
mense numbers of molecules, cannot disturb the equality of their tem- 
_ peratures when it actually exists. From this condition we conclude, that 
_ for each particle the ratio of the emitting to the absorbing power is inde- 
_ pendent 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 
