of experiments on the pendulum. 
87 
of the simplicity of the progression which they exhibit, being 
in the first instance precisely equal to 1 and varying 
but slowly from this value. 
Now if we suppose, with Mr. Laplace, the mean density 
of the earth to be to that of the superficial parts as 1.55 to 1, 
it appears from this table, that the height of the modulus of 
elasticity must be about .59 4; that is, more than 12 million 
feet, while the modulus of the hardest and most elastic sub- 
stances, that have been examined, amounts only to about 
10 million. It follows therefore, that the general law, of a 
compression proportionate to the pressure, is amply sufficient 
to explain the greater density of the internal parts of the 
earth ; and the fact demonstrates, that this law, which is true 
for small pressures in all substances, and with regard to elas- 
tic fluids, in all circumstances, requires some little modifica- 
tion for solids and liquids, the resistance increasing somewhat 
faster than the density: for no mineral substance is suffi- 
ciently light and incompressible to afford a sphere of the 
magnitude of the earth, and of so small a specific gravity, 
without some such deviation from the general law. A sphere 
of water would be incomparably more dense, and one of air 
would exceed this in a still greater proportion : indeed, even 
the moon, if she is really perforated, as has sometimes been 
believed, and contains cavities of any considerable depth, 
would soon have absorbed into her substance the whole of 
her atmosphere, supposing that she ever had one. It may 
be objected, that the resistance of solids to actual compres- 
sion may possibly be considerably greater than appears in 
our experiments, since we are not absolutely certain that they 
