OF THE EARTH AS A MASS, 15 
But in a memoir read to the Paris Academy of Sciences, on the 
4th of August, 1818, Laplace communicated some new views upon 
the subject. From the results of observations on the vibrations 
of the pendulum, he showed, that not only is the matter of the 
interior of the earth more dense than what is at the surface, but 
the density goes on regularly and uniformly increasing from the 
surface to the centre, and the densities are equal at equal distances 
from the centre in all directions. 
These truths having been ascertained and settled, it appears less 
probable than before, that the high specific gravity of the earth is 
owing simply to its composition and to the existence of metallic 
matter in its interior ., Upon this supposition the facts would re- 
quire, not merely that the interior mass should be metallic, but 
that the metals of which it is constituted should be disposed in 
concentric shells, the lighter resting upon the heavier, and that 
these shells should individually be of such thickness, as to produce 
a regular and uniform increase of density as the centre is ap- 
proached. 
The density of the gases is in proportion to the force by which 
they are compressed. Solids and liquids do not obey the same 
law, but there can be little doubt that they are all compressible 
in a greater or less degree. Laplace finds that if we suppose the 
chemical constitution of the earth to be substantially the same in 
all its parts, and that the specific gravity of the materials of which 
it is composed, is moderately increased when they are subjected 
to pressurej we shall be able to explain, without difficulty, all the 
phenomena which depend upon the density of the earth. 
8. Temperature of the Earth. — It is found by observations 
made in the mines of England, France, Germany and Mexico, 
and in Artesian wells, that the earth is hotter at considerable 
depths than at the surface. The temperature increases at a rate 
which is different for different points, but is supposed toaveragea 
degree of Fahrenheit's thermometer, for about forty -five feet of 
descent. If the heat continues to increase uniformly at this rate, 
it is evident that the temperature of boiling water (212°) will be 
found at a level not very remote from the surface, as also that at 
the depth of a few miles, the rocks themselves are in a state of 
fusion. The hot springs that gush from the sides of the moun- 
tains in most countries, and of which there are several within 
the limits of the United States, area proof that an internal source of 
heat is not confined to the central and western parts of Europe 
and to Mexico. This subject will receive a fuller discussion on a 
subsequent page. It may be enough to mention here, that it is the 
common belief of the ablest geologists, that the interior of the 
earth is a mass of liquid fire. The mean temperature of the sur- 
face of the globe, is estimated by Sir John Leslie, at 67 degrees. 
9. Composition of the Earth, — It is evident that the fifty-four 
simple bodies must all belong to the mineral kingdom, and consti- 
