Clay pole on the Interior of the Earth. 3S3 
terior of the earth. All the mines and wells and bore-holes that 
have been sunk are but so many punctures in the skin revealing 
nothing of the great central mass below. No auger has ever 
fathomed more than a single one of the four thousand miles 
that separate us from the centre. And the data thus obtained 
from this single mile are too slender, the material is too scanty 
to warrant wholesale induction regarding the greater inacces- 
sible residue of the sj^here. Direct examination fails and we 
are therefore driven to employ indirect methods — to infer from 
feeble manifestations at the surface the nature and the intensity 
of the energies at work below — to extend cautiously but im- 
mensely the results of petty experiments in our laboratories 
until they match in magnitude the vast operations in the great 
subterranean laboratory of nature — .and finally to deduce from 
the known properties of matter its necessary or probable be- 
haviour in unknown and unattainable conditions. All this is 
working vmder immense difficulties and at enormous disadvant- 
ages; and progress toward the solution of this interesting ^^rob- 
lem is proportionately slow. 
And when in addition to all this we reflect that in order to 
deal with the subject a man should possess knowledge of a high 
order in mathematics and physics, and ability to use readily the 
most powerful engines at the command of these two sciences 
not less than those methods more strictly geological, we can 
understand why so few enter the field and why the results of 
their labors are so small and so uncertain. 
Yet there is progress. From year to year some new fact is 
made known on evidence that commands general assent. Some 
new inference is deduced that deserves examination. Some 
new possibility is suggested that promises to reward investiga- 
tion. Through these three stages all ovn- knowledge regarding 
the condition of the earth's central mass is destined by the na- 
ture of the case to pass. 
Not very long ago it was an accepted doctrine that the earth 
was composed of a hot and liquid internal sphere surrounded 
by a solid shell of comparatively small thickness. This doctrine 
may yet be recognized in some of our popular semigeological 
literature. But it has passed into oblivion as a tenet in geology, 
