John F. W. Herschel in his
Discourse on Natural Philosophy, published in 1830. In
preceding works the gradual diminution of the earth's central
heat was almost the only cause assigned for the acknowledged
diminution of the superficial temperature of our planet.

[166] We are indebted to Baron Alex. von Humboldt for having
first collected together the scattered data on which he founded
an approximation to a true theory of the distribution of heat
over the globe. Many of these data were derived from the
author's own observations, and many from the works of M. Pierre
Prevost, of Genera, on the radiation of heat, and from other
writers.--See Humboldt on Isothermal Lines, Mamoires d'Arcueil,
tom. iii. translated in the Edin. Phil. Journ. vol. iii. July,
1820.

The map of Isothermal Lines, recently published by Humboldt and
Dove (1848), supplies a large body of well-established data for
such investigations, of which Mr. Hopkins has most ably availed
himself in an essay "On the Causes which may have produced
Changes in the earth's Superficial Temperature."--Q. Journ.
Geol. Soc. 1852, p. 56.

[167] Sir J. Richardson's Appendix to Sir G. Bach's Journal,
1843-1845, p. 478.

[168] Malte-Brun, Phys. Geol. book xvii.

[169] On Isothermal Lines, &c.

[170] Rennell on Currents, p. 96. London, 1832.

[171] Ibid. p. 153.

[172] Ibid. p. 25.

[173] Scoresby's Arctic Regions, vol. i. p. 208.--Dr. Latta's
Observations on the Glaciers of Spitzbergen, &c. Edin. New
Phil. Journ. vol. iii. p. 97.

[174] Rennell on Currents, p. 95.

[175] Humboldt on Isothermal Lines.

[176] Journ. of Travels in S. America, &c. p. 272.

[177] Darwin's travels in S. America, p. 271.

[178] Mr. Hopkins raises the question whether, in South
Georgia, the descent of glaciers to the margin of the sea might
not have been mistaken by Capt. Cook for the descent of the
snow-line to the sea level. Quart. Journ. Geol. Soc. p. 85,
1852. The great navigator is generally very accurate, and there
seem to be no observations of more recent date either to
confirm or invalidate his statements.

[179] After all these modern discoveries, the area still
unexplored, within the antarctic circle, is more than double