[179] After all these modern discoveries, the area still unexplored, within the antarctic circle, is more than double 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 lat.i.tudes, 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...o...b..t, 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 surpa.s.se pas mille metres (3280 ft.)" Mec. Celeste, tom. xi. et Syst. du Monde, p. 254. The expression "du meme ordre" admits in mathematical language of considerable lat.i.tude 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 lat.i.tude 15 3' S., longitude 23 14' 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, which ran out to the very end, without finding bottom. Here therefore in mid-ocean the depth exceeded 27,600 feet. One of the shallowest soundings ever obtained in the open sea during the same survey, struck bottom with 2677 fathoms, or 16,062 feet, lat.i.tude 33 21' S., longitude 9 4' E. The surveyors arrived at the conclusion, that at a moderate distance from the sh.o.r.e, the depth of the great ocean always exceeds 4000 feet.

During the American survey in 1849, a much greater depth, or 5700 fathoms (34,200 feet), was sounded in the Atlantic by Lieut. Walsh, without reaching the bottom, in lat. 31 59' N., long. 58 43' W., or between the Bermudas and the Azores. But the deepest soundings yet published were taken Oct. 30th 1852, by Capt. Henry M. Denham, R. N., who reached bottom at 7706 fathoms (46,236 feet), lat. 36 49' S., long. 37 6' W., the nearest land being at the mouth of the River Plate. A weight of 9 lbs. was attached to the line, which was one-tenth of an inch in diameter; the day was calm, and the line took 9 hours 24 minutes to run out. When the bottom was struck the line was raised 50 fathoms, and then allowed to run out again. It struck at the same point as before, verifying the observations.

Nevertheless some experienced surveyors have remarked that the experiment would have been more satisfactory had the weight been greater. The highest summits of the Himalaya are about 28,000 feet; the Pacific, according to this sounding, is probably at some points twice as deep as the Himalaya are high.

[186] Mr. Hopkins, reasoning on data furnished by Dove's Isothermal maps, has arrived at the very interesting conclusion, that both on Snowdon and the lower mountains of the West of Ireland the snow-line would descend to within 1000 feet of the sea level, and glaciers reach the sea, if we could simply a.s.sume the three following geographical changes:--

1st, The diversion of the Gulf stream from its present northerly course; 2dly, the depression of the existing land of Northern and Western Europe, to the amount of no more than 500 feet; and 3dly, a cold current from the North sweeping over the submerged area. Quart. Journ. Geol. Soc. 1852, p. 85.

[187] Daniell's Meteorological Essays, p. 103.

[188] Observed by J. Crawfurd, Esq.

[189] In speaking of the circulation of air and water in this chapter, no allusion is made to the trade winds, or to irregularities in the direction of currents, caused by the rotary motion of the earth. These causes prevent the movements from being direct from north to south, or from south to north, but they do not affect the theory of a constant circulation.

[190] See Scoreby's Arctic Regions, vol. i. p. 378.

[191] Ibid. p. 320.

[192] This is shown by projecting a map on the horizon of London, that is to say, by supposing the eye of the observer to be placed above that city, and to see from thence one half of the globe. For it so happens that from that point, and no other, we should behold the greatest possible quant.i.ty of land; and if we are then transferred to the opposite or antipodal point, we should see the greatest possible quant.i.ty of water.

(See figs. 3 and 4.) A singular fact, first pointed out by Mr.

James Gardner, namely, that only one twenty-seventh part of the dry land has any land opposite to it, is intimately connected with this excess of land in one of the two hemispheres above alluded to. Thus, in fig. 3, the land shaded black in part of China answers to that portion of the extremity of South America and Tierra del Fuego which is opposite or antipodal to it, whilst the dark spots in the northern and central parts of South America represent Borneo, Sumatra, and other antipodal islands in the Eastern Archipelago. See Gardner, Geol. Soc.

Proceedings, 1833, vol. i. p. 488.

[193] Humboldt on Isothermal Lines

[194] Humboldt, Tableaux de la Nature, tom. i. p. 112.

[195] Ad. Brongniart, Consid. Generales sur la Nat. de la Veget. &c. Ann. des Sciences Nat., Nov. 1828.

[196] Sir J. Richardson, Proceedings of Geol. Soc. No. 7, p.

68, March, 1828.

[197] Ad. Brongniart, Consid. Generales sur la Nat. de la Veget. &c., Ann. des Sci. Nat., Nov. 1828.

[198] See a Memoir on the Alps, by Professor Sedgwick and Sir Rod. Murchison, Trans. of Geol. Soc. second ser. vol. iii.

accompanied by a map.

[199] See Proceedings of Geol. Soc. vol. ii. p. 334.

[200] It may be observed, that the facts and inferences exhibited in this map bear not merely on the theory of climate above proposed, but serve also to ill.u.s.trate the views explained in the third book respecting the migration of animals and plants and the gradual extinction of species.

[201] See Sir R. Murchison's Paper on the Alps, Quart. Journ.

Geol. Soc. vol. v. and my Anniversary Address for 1850, ibid.

vol. vi.

[202] Allgemeine Literatur Zeitung, No. cx.x.xix. July, 1833.

[203] In this estimate, the s.p.a.ce within the antarctic circle is not taken into account: if included, it would probably add to the excess of dry land; for the late discoveries of Capt.

Sir James Ross, who penetrated to lat. 78 10' S., confirm the conjecture of Captain Cook that the acc.u.mulation of antarctic ice implies the presence of a certain quant.i.ty of terra firma.

The number of square miles on the surface of the globe are 148,522,000, the part occupied by the sea being 110,849,000, and that by land, 37,673,000; so that the land is very nearly to the sea as 1 part in 4. I am informed by Mr. Gardner that, according to a rough approximation, the land between the 30 N.

lat. and the pole occupies a s.p.a.ce _about equal to that of the sea_, and the land between the 30 S. lat. and the antarctic circle about one-sixteenth of that zone.

[204] See papers by Mr. Smith of Jordanhill, F. G. S., and the author, Proceedings Geol. Soc. No. 63, 1839, also that of Prof.

E. Forbes, before cited, p. 86, note.

[205] The theorem is thus stated:--"The eccentricity of the orbit varying, the total quant.i.ty of heat received by the earth from the sun in one revolution is inversely proportional to the minor axis of the orbit. The major axis is invariable, and therefore, of course, the absolute length of the year: hence it follows that the mean annual average of heat will also be in the same inverse ratio of the minor axis."--Geol. Trans. second series, vol. iii. p. 295.

[206] Ann. du Bur. des Long. 1834.

[207] Poisson, Theorie Mathemat. de la Chaleur, Comptes Rendus de l'Acad. des Sci., Jan. 30, 1837.

[208] Quart. Journ. Geol. Soc. 1852, p. 62.

[209] Proceedings Roy. Astronom. Soc. No. iii. Jan. 1840.

[210] See a Memoir on the Temperature of the Terrestrial Globe, and the Planetary s.p.a.ces, Ann. de Chimie et Phys. tom. xxvii.

p. 136. Oct. 1824.

[211] Sir H. Davy, Consolations in Travel: Dialogue III. "The Unknown."

[212] Quart. Journ. Geol. Soc. 1852.

[213] Buckland's Bridgewater Treatise, p. 409.

[214] Owen's Report on "British Fossil Reptiles, to Brit. Soc."

1841, p. 200.

[215] Quart. Journ. Geol. Soc. No. 6, p. 96.

[216] See Hitchc.o.c.k's Report on Geol. of Ma.s.sachusetts, and Lyell's Travels in North America, chap. 12.

[217] See Manual of Geol. by the Author, index _Microlestes_.

[218] This figure (No. 8) is from a drawing by Professor C.