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POPULAR SCIENCE REVIEW. 
intensity have been reported in the Pacific Ocean off the coast 
of Mexico, as well as in the Atlantic, between Africa and South 
America. 
Notwithstanding that the records of such phenomena in more 
ancient times are extremely defective, a retrospect of such as 
have been observed indicates that they have not diminished in 
frequency in later periods, and the tabular statements of known 
European eruptions and earthquakes made by Professor Phillips 
and Mr. Mallet respectively, show that their number has gradu- 
ally increased per century, from the fourth up to the nineteenth, 
and that since that early period by far the greatest number in 
any one century has occurred in the last and present century. 
It is not to be wondered at, therefore, that the question. What 
does the central mass of our sphere consist of ? should be one 
possessing more than ordinary popular as well as scientific 
interest; and for this reason it is here proposed to submit a 
concise sketch of the opinions which from time to time have 
been brought forward by different writers on the subject, along 
with a notice of the arguments used in their support or advanced 
in opposition, thereby to enable some independent if not 
impartial judgment to be formed by our readers. 
The greatest depth hitherto attained by direct explorations 
into the substance of the earth’s surface has not yet reached 
5000 feet, and it is scarcely to be expected that any much 
greater depth can be arrived at, for which reason, notwithstand- 
ing the many and valuable data resulting from such explorations, 
it appears self-evident, in order to pursue this enquiry further, 
that we must mainly rely upon the less direct evidence furnished 
by calling in the assistance of the natural sciences. 
According to the different hypotheses advanced at various 
times with respect to the physical condition of the earth’s mass, 
our sphere has been respectively represented as a globe — 
1 . Composed of a relatively thin external crust or shell, filled 
with matter in a state of molten liquidity ; 
2. Nearly, if not quite, solid to the core ; 
3. Composed of a solid external shell separated from an 
equally solid nucleus by a zone of intermediate molten matter ; 
4. Consisting of an external solid shell, filled with enormously 
compressed gaseous matter. 
The first of these four hypotheses, or that which regards the 
Earth as being a sphere of molten matter, surrounded by a 
comparatively thin solid external shell or crust, is the one more 
generally accepted by geologists and such men of science as 
prefer basing their deductions entirely upon facts elicited from 
the direct examination of so much of the earth’s exterior as is 
accessible to man. Amongst the facts advanced in support of 
this theory, the following are probably the most important. 
