Temperature of the Terrestrial Globe. 13 
The mobility of the air, which is rapidly displaced in every direc- 
tion, and which rises when heated, and the radiation of non-lumin- 
ous heat into the air, diminish the intensity of the effects which 
would take place in a transparent and solid atmosphere, but do not 
entirely change their character. The decrease of heat in the higher 
_ regions of the air does not cease, and the temperature can be aug- 
mented by the interposition of the atmosphere, because heat in the 
state of light finds less resistance in penetrating the air, than in re- 
passing into the air when converted into non-luminous heat. We 
shall now consider that peculiar heat which our globe had at the 
time of the formation of the planets, and which continues to be dis- 
sipated at the surface under the influence of the low temperature of 
the planetary space. 
‘The opinion of an internal fire as a perpetual cause of many re- 
markable phenomena, has been renewed in every age of philoso- 
hy. The end we have in view at this time, and which the latest 
progress of mathematical science enables us to reach, is to ascertain 
exactly by what laws a solid sphere heated by a long immersion in 
a medium, would discharge that primitive heat if it were transported 
into a space of a constant temperature inferior to that of the first me- 
dium. The design of the experiment is, to ascertain if the present 
temperature of the surface of the globe can yet undergo any sensi- 
le changes. The form of the terrestrial spheroid, the regular order 
of the lower strata, made manifest by experiments with the pendulum, 
their density increasing with the depth, and divers other considera- 
tions, concur in proving that a heat of very great intensity has at some 
previous period penetrated every part of the globe. This heat is dis- 
sipated by radiation into surrounding space, which has a temperature 
much below that of the congelation of water. Now the mathemat- 
ical expression for the law of cooling shows that the primitive heat 
contained in a spherical mass of as great dimensions as the earth, 
diminishes much more rapidly at the surface than in parts situated 
ata great depth. These latter preserve almost all their heat for 
an immense period of time. There can be no doubt respecting the 
truth of these results, because we have calculated these times for 
metallic substances, which have a greater conducting power than 
those of which the globe is composed. 
But it is evident that theory alone can only teach us the laws to 
which phenomena are subject. It remains now to inquire whether 
in penetrating the earth, we find any indications of a central heat. 
