398 JOURNAL OF SCIENCE. 
the surface of the earth is explained by the agency of natural 
forces, still at work at the present day. With this the founda- 
tion of modern Geology was securely established, and though in 
some respects the great Scotch philosopher went too far, his 
system was, nevertheless, the only true one on which his succes- 
sors could build that branch of knowledge, now claiming a pro- 
minent rank amongst its sisters as an inductive science. And 
when William Smith, the modest English land surveyor, in 1790 
published his “ Tabular View of the British Strata,” in which the 
first attempt was made to connect certain fossils with certain 
strata, an attempt turning out a masterpiece of patient research — 
and skill, a further great step was made in advance, and instead 
of merely theorising on disconnected facts, the greater portion of 
geological students began to rely more upon the facts collected 
by them and others, than upon speculative views, however fasci- 
nating they might be. 
In entering upon a short review of the physics relating to the 
great system of which our earth is only a very inconsiderable 
speck, we find that although men of the highest scientific merit 
had tried to explain the origin and nature of the Cosmos, and the 
laws by which it is governed, not one speculation had been 
adopted at the time of the publication of the Cyclopedia of 
Chambers as possessing all the necessary precision for the entire 
satisfaction of inductive reasoning. 
It was only at the end of last century that Pierre Simon 
Laplace published his two great works, “ Exposition du Systeme 
du Monde” in 1796, and “La Mécanique céleste” in 1799. 
This cosmogony, usually called the “ Nebular Hypothesis,” has 
hitherto stood the test of inquiry nearly a whole century ; all the 
facts—and they are innumerable—tending invariably to testify at 
least to the great probability of its general correctness. In jus- 
tice I ought here to mention that Immanuel Kant published -in 
1755 his cosmical theories in his work “ Allgemeine Naturge- 
schichte und Theorie des Himmels,” in which the great Kcenigs- 
berg philosopher came to the same. conclusions, afterwards so 
convincingly demonstrated by the French mathematician. 
But when we leave the Cosmos and confine ourselves to our 
small planet, we find ourselves surrounded by such difficulties 
that we appear just as far now from a true conception of the 
constitution of the earth’s interior as our predecessors were at 
the beginning of this century. 
Numerous theories, based upon careful calculations, as to the 
thicknessof thecrust of the earth have been advanced. Some phys- 
icists give to our earth so thin a crust that it has been compared to 
the rind of an orange, the fruit enclosed in it representing the 
molten matter of the globe ; others affirm that the crust is of much 
greater thickness, whilst there are some who maintain that our 
planet has cooled so thoroughly that it now forms a mass of 
rock of various density from the surface to the very centre. 
Other theories (or better stated hypotheses) giving to our globe 
a crust of more or less thickness, with a hard metallic nucleus in — 
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