irrigation some idea may be formed from an estimate that the Amazon drains an area of 2,500,000 square miles; that its mouth is 96 miles wide; and that it is navigable 2,200 miles from its mouth. Somerville's Physical Geography, vol. i. p. 423. Indeed, it is said in an essay on the Hydrography of South America (Journal of Geograph. Society, vol. ii. p. 250), that ‘with the exception of one short portage of three miles, water flows, and is for the most part navigable, between Buenos Ayres, in 35° south latitude, to the mouth of the Orinoco, in nearly 9° north.’ See also on this river-system, vol. v. p. 93, vol. x. p. 267. In regard to North America, Mr. Rogers (Geology of North America, p. 8, Brit. Assoc. for 1834) says, ‘the area drained by the Mississippi and all its tributaries is computed at 1,099,000 square miles.’ Compare Richardson's Arctic Expedition, vol. ii. p. 164.
175
The Oregon, or Columbia as it is sometimes called, forms a remarkable botanical line, which is the boundary of the Californian flora. See Reports on Botany by the Ray Society, p. 113.
176
For proof that the mean temperature of the western coast of North America is higher than that of the eastern coast, see Journal of Geograph. Society, vol. ix. p. 380, vol. xi. pp. 168, 216; Humboldt, la Nouvelle Espagne, vol. i. pp. 42, 336; Richardson's Arctic Expedition, vol. ii. pp. 214, 218, 219, 259, 260. This is well illustrated by the botanical fact, that on the west coast the Coniferæ grow as high as 68° or 70° north latitude; while on the east their northern limit is 60°. See an Essay on the Morphology of the Coniferæ, in Reports on Botany by the Ray Society, p. 8, which should be compared with Forry on the Climate of the United States and its Endemic Influences, New York, 1842, p. 89.
177
‘Writers on climate have remarked that the eastern coasts of continents in the northern hemisphere have a lower mean temperature than the western coasts.’ Richardson on North American Zoology, p. 129, Brit. Assoc. for 1836: see also Report for 1841, Sections, p. 28; Davis's China, vol. iii. pp. 140, 141; Journal of Geograph. Society, vol. xxii. p. 176.
178
The little that is known of the early state of the North-American tribes has been brought together by Dr. M'Culloh in his learned work, Researches concerning America, pp. 119–146. He says, p. 121, that they ‘lived together without laws and civil regulations.’ In that part of the world, the population has probably never been fixed; and we now know that the inhabitants of the north-east of Asia have at different times passed over to the north-west of America, as in the case of the Tschuktschi, who are found in both continents. Indeed, Dobell was so struck by the similarity between the North-American tribes and some he met with nearly as far west as Tomsk, that he believed their origin to be the same. See Dobell's Travels in Kamtschatka and Siberia, 1830, vol. ii. p. 112. And on this question of intercourse between the two continents, compare Crantz's History of Greenland, vol. i. pp. 259, 260, with Richardson's Arctic Expedition, vol. i. pp. 362, 363, and Prichard's Physical History of Mankind, vol. iv. pp. 458, 463, vol. v. pp. 371, 378.
179
From general physical considerations, we should suppose a relation between amount of rain and extent of coast; and in Europe, where alone we have extensive meteorological records, the connexion has been proved statistically. ‘If the quantity of rain that falls in different parts of Europe is measured, it is found to be less, other things being equal, as we recede from the sea-shore.’ Kaemtz's Meteorology, 1845, p. 139. Compare pp. 91, 94. Hence, no doubt, the greater rarity of rain as we advance north from Mexico. ‘Au nord du 20°, surtout depuis les 22° au 30° de latitude, les pluies, que ne durent que pendant les mois de juin, de juillet, d'août et de septembre, sont peu fréquentes dans l'intérieur du pays.’ Humboldt, la Nouvelle Espagne, vol. i. p. 46.
180
‘The difference between the climates of the east and west coasts of continents and islands, has also been observed in the southern hemisphere but here the west coasts are colder than the east, while in the northern hemisphere the east coasts are the colder.’ Meyen's Geography of Plants, 1846, p. 24.
181
Mr. Darwin, who has written one of the most valuable works ever published on South America, was struck by this superiority of the eastern coast; and he mentions that ‘fruits which ripen well and are very abundant, such as the grape and fig, in latitude 41° on the east coast, succeed very poorly in a lower latitude on the opposite side of the continent.’ Darwin's Journal of Researches, Lond. 1840, p. 268. Compare Meyen's Geog. of Plants, pp. 25, 188. So that the proposition of Daniell (Meteorological Essays, p. 104, sec. xiv.) is expressed too generally, and should be confined to continents north of the equator.
182
The trade-winds sometimes reach the thirtieth parallel. See Daniell's Meteorological Essays, p. 469. Dr. Traill (Physical Geography, Edin. 1838, p. 200), says, ‘they extend to about 30° on each side of the equator:’ but I believe they are rarely found so high; though Robertson is certainly wrong in supposing that they are peculiar to the tropics; History of America, book iv. in Robertson's Works, p. 781.
183
‘In the northern hemisphere the trade-wind blows from the north-east, and in the southern from the south-east.’ Meyen's Geog. of Plants, p. 42. Compare Walsh's Brazil, vol. i. p. 112, vol. ii. p. 494; and on the ‘tropical east-wind’ of the Gulf of Mexico, see Forry's Climate of the United States, p. 206. Dr. Forry says that it has given to the growth of the trees ‘an inclination from the sea.’
184
Respecting the causes of the trade-winds, see Somerville's Connexion of the Physical Sciences, pp. 136, 137; Leslie's Natural Philosophy, p. 518; Daniell's Meteorological Essays, pp. 44, 102, 476–481; Kaemtz's Meteorology, pp. 37–39; Prout's Bridgewater Treatise, pp. 254–256. The discovery of the true theory is often ascribed to Mr. Daniell; but Hadley was the real discoverer. Note in Prout, p. 257. The monsoons, which popular writers frequently confuse with the trade-winds, are said to be caused by the predominance of land, and by the difference between its temperature and that of the sea: see Kaemtz, pp. 42–45. On what may be called the conversion of the trades into monsoons, according to the laws very recently promulgated by M. Dove, see Report of British Association for 1847 (Transac. of Sections, p. 30) and Report for 1848, p. 94. The monsoons are noticed in Humboldt's Cosmos, vol. ii. p. 485; Asiatic Researches, vol. xviii. part i. p. 261; Thirlwall's History of Greece, vol. vii. pp. 13, 55; Journal of Geograph. Society, vol. ii. p. 90, vol. iv. pp. 8, 9, 148, 149, 169, vol. xi. p. 162, vol. xv. pp. 146–149, vol. xvi. p. 185, vol. xviii. pp. 67, 68, vol. xxiii. p. 112; Low's Sarawak, p. 30.
185
Lyell's Principles of Geology, pp. 201, 714, 715; see also Somerville's Physical Geography, vol. ii. p. 71. And on this confining power of the Cordillera of the Andes, see Azara, Voyages dans l'Amérique Méridionale, vol. i. p. 33. According to Dr. Tschudi, the eastern chain is properly the Andes, and the western the Cordillera; but this distinction is rarely made. Tschudi's Travels in Peru, p. 290.
186
On the rain of Brazil, see Daniell's Meteorological Essays, p. 335; Darwin's Journal, pp. 11, 33; Spix and Martius's Travels in Brazil, vol. ii. p. 113; Gardner's Travels in Brazil, pp. 53, 99, 114, 175, 233, 394.
187
Dr. Gardner, who looked at these things with the eye of a botanist, says that near Rio de Janeiro the heat and moisture are sufficient to compensate even the poorest
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