the area of Europe. The surface of the latter contains about
2,793,000 square geographical miles. The unexplored antarctic
region, as calculated for me by Mr. Gardner, in 1840, equalled
about 7,620,000 square miles.

[180] On icebergs in low latitudes, by Capt. Horsburgh, by whom
the sketch was made. Phil. Trans. 1830.

[181] Scoresby's Arctic Regions, vol. i. p. 234.

[182] This follows, observes Herschel, from a very simple
theorem, which may be thus stated:--"The amount of heat
received by the earth from the sun, while describing any part
of its orbit, is proportional to the angle described round the
sun's centre." So that if the orbit be divided into two
portions by a line drawn _in any direction_ through the sun's
centre, the heat received in describing the two unequal
segments of the eclipse so produced will be equal. Geol. Trans.
vol. iii. part. ii. p. 298; second series.

[183] On Isothermal Lines.

[184] A full consideration of the effect of changes in physical
geography on the distribution and extinction of species is
given in book iii.

[185] For calculations founded on astronomical data, see
Young's Nat. Phil., Lect. xlvii.; Mrs. Somerville's Connex. of
Phys. Sci., sect. 14, p. 110. Laplace, endeavoring to estimate
the probable depth of the sea from some of the phenomena of the
tides, says of the ocean generally, "que sa profondeur moyenne
est du meme ordre que la hauteur moyenne des continens et des
isles au-dessus de son niveau, hauteur qui ne surpasse pas
mille metres (3280 ft.)" Mec. Caleste, tom. xi. et Syst. du
Monde, p. 254. The expression "du meme ordre" admits in
mathematical language of considerable latitude of
signification, and does not mean that the depth of the water
below the level of the sea corresponds exactly to the height of
the land above it.

It appeared from the observations of Sir James Ross,
communicated to me in 1849, by himself, and his fellow voyager,
Dr. Joseph Hooker, that in latitude 15 degrees 3 minutes S.,
longitude 23 degrees 14 minutes W. (the island of Trinidad, the
nearest land, being 486 miles distant, and bearing S. 47 W.),
they sounded with a weight of 76 lbs., and 4600 fathoms of line,