It is evident from the observations of Captains Beechey and Findley, and still more particularly from those of the French Captain Kerhallet, that the remarkable subdivision of the main equinoctial current, flowing from east to west into two branches, one directed to the N.W., the other to the S.S.W., commences at a considerable distance from the Capes of St. Roque and St. Augustin. This bifurcation has always, and with good reason, been ascribed to the protruding convexity of the South American continent at these two promontories. It would be an important step gained in verifying the theory of currents, could the precise distance be ascertained by chronometer. It is apparently like an "actio in distans," probably a phenomenon of what is known as "packing." As the frigate, on leaving Rio de Janeiro is to make for the Cape of Good Hope, the opportunity will present, should she steer sufficiently southerly, for many interesting observations with respect to the connecting current W.N.W. and E.S.E. which encounters that from Madagascar and Mozambique, close to the Cape, more especially with regard to the temperature of the sea.
If the frigate is intended to approach the small cluster of islands of Fernando de Noronha, E. of Pernambuco (Lat. 3° 50′ S.), I would recommend to that excellent geognostic, Dr. Hochstetter, the hornblendic phonolithe rock found there, far from a volcanic crater, but with trachytic dykes and basaltic amygdaloid. The flat little island of St. Paul (Peñedo de San Pedro), 1° N. Lat., singular to say, is not volcanic at all, containing, like the Malouin or Falkland Islands, slaty green-stone passing into serpentine.
Should the frigate alter her course and cross the Equator more to the eastward, without touching at Rio de Janeiro, she might possibly fall in with the Marine Volcanic region, (Lat. 0° 20′ S., Long. 22° W.), which has quite lately become famous again by the U. S. Expedition of the Brig Dolphin (1854), commanded by Lieutenant Lee. On 19th May, 1806, columns of black smoke were seen issuing from the sea by Krusenstern, and volcanic ashes were gathered, after a singular bubbling of the sea from 1748 to 1836, according to careful investigations by Daussy.
As the frigate is commissioned to visit Ceylon and the Nicobar Islands, she cannot sail direct from the Cape to Australia; and the hope must therefore be abandoned of her visiting the small basaltic islands, known as Prince Edward's (47° 2′ S., 38° E.), and Possession (46° 28′ S., 47° 30′ E.), belonging to the Crozet's Group, or the two islands, long confounded with each other, of Amsterdam (Lat. 37° 48′ S.) and St. Paul (Lat. 38° 38′ S.) The latter island, the more southerly of the two, (a very characteristic drawing of which was given by Willem de Vlaming so far back as 1696), is supposed to be volcanic, not only by its form, which will at once remind the geologist of Santorin, Barren Island, and Deception Island, (one of the New Shetland group), but also in consequence of the eruption of steam, and the flames occasionally observed there.
As for Amsterdam, which consists of a single densely-wooded mountain, the puzzle remains for solution as to how, during the expedition of D'Entrecasteaux in 1792, the whole island seemed, during two entire days, enveloped in smoke; whereas, on landing there, the naturalists of that expedition were satisfied that the mountain was not an active volcano, and that the columns of steam issued out of the ground near the shore! As yet, the phenomenon remains entirely unexplained.
If we examine any map of the Indian Ocean, we may trace the continuation of the Sunda group from Sumatra, N.W., through the Nicobar, and Great and Little Andaman Islands, and thence through the volcanoes of Barren Island, Narcondam and Cheduba, nearly parallel with the coasts of Malacca and Tenasserim, all on the eastern part of the Bay of Bengal. The minor volcanoes just enumerated will present valuable opportunities of geological enquiry.
Along the coasts of Orissa and Coromandel, the western portion of the Bay of Bengal is quite free of islands, Ceylon, like Madagascar presenting rather the type of a continent.
Off the W. coast of the peninsula of India, (that is opposite the Neilgherrie hills, and the coast of Canara and Malabar), there is a series of three archipelagoes, extending from 14° N. to 8° S., viz., the Laccadives, the Maldives, and the Chagos, which appears, as it were, continued through the banks of Sahia di Malha, and Cargados Carajos, to the volcanic group of the Mascarenhas and Madagascar. As the first-named archipelagoes, so far as is yet known, consist solely of coral, and are, consequently, true "atolls," or reef-lagoons, the bottom of the ocean should be examined over a large extent, adopting the ingenious hypothesis of Darwin, that it is to be considered as an area of subsidence, rather than an elevated region.
It would also be a matter of great importance to get observations respecting terrestrial magnetism, particularly so as to define the position of a given segment of the magnetic equator. Capt. Elliott, as the result of his comprehensive studies, (1846-49), ascertained that the magnetic equator passes through the north end of Borneo, and thence nearly due W. to the northern extremity of Ceylon. In this region the curve of minimum intensity is nearly parallel to the magnetic equator, which intersects the Continent of Africa near Cape Guardafui—according to Rochet d'Héricourt, in lat. 10° 7′ N., long. 38° 5′. E. Between this point and the Bight of Biafra nothing is known.
The South Asiatic islands comprise Formosa, the Philippines, the Sunda group, and the Moluccas. The great and little Sunda Islands and the Moluccas embrace 109 volcanoes, with fiery eruptions, and 10 what are called mud-volcanoes. This is not a mere estimate, but is the result of an enumeration by Junghuhn, who, within the last year (1856), has returned to Java, and thoroughly equipped by M. Pahud, Governor-General of the Indian Netherlands, will be of great assistance to the Imperial Expedition.