Geoscience Reference
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out that in his opinion 2 million years was an adequate length of time
for the uniformitarian geological processes that had been described by
geologists such as Charles Lyell. Eight years later he argued that the
Sun 'has not illuminated the earth for 100,000,000 years, and almost
certain that he has not done so for 500,000,000 years', and that it was
probably closer to the truth that the Sun had been operational for
between 20 and 60million years. Towards the end of his life, following
persuasion by his associate Peter Guthrie Tait (1831-1901) he argued
that the Sun was no older than 20 million years. This by association
gave a limit to the age of the Earth.
Cooling of the Earth
Like Buffon, Thomson believed that the primordial Earth was molten
throughout, and that it solidified from its centre outwards as the
internal heat migrated through the rocks by conduction and heat
was lost from its surface. Once solid, but still hot, it lost heat by
conduction rather than convection, which sees heat carried through
a fluid volume by a flow of matter. In 1862 he carried out experiments
to determine the conductivity of various rock types, and took 7,000
degrees Fahrenheit as the temperature of fusion of rocks, an estimate
that had been determined a short time earlier. In this research he was
greatly influenced by the work of French physicist Jean Baptiste Joseph
Fourier (1768-1830), which he had studied during his continental
sojourn in 1845. Fourier had written that the source of the Earth's
heat was three-fold: from primitive internal heat, from heating by
the Sun, and from heat in the Universe. Thomson also drew on work
that had revealed the temperature gradient in the Earth and noted
that temperatures increased approximately one-fiftieth of a degree
Fahrenheit per foot that one went down. This had been investigated
by taking temperature readings from various depths in mines or from
boreholes. This work was not very accurate until the invention in
the 1830s of specialised thermometers designed for the task. Even by
the 1860s results were not very conclusive, and it was believed that the
temperature at the Earth's centre was approximately 3,800 degrees
