صور الصفحة
PDF
النشر الإلكتروني

enough to furnish me with the enclosed note on the few specimens brought by me from Tristan da Cunha.

Note by Mr. Layard.

The specimens of natural history brought from Tristan da Cunha by Captain Nolloth may be thus enumerated.

Two earthworms, Lumbricus.

Two specimens of Oniscus; one found some distance from the sea, the other on the beach.

A species of Julus, very small.

A small brown slug, apparently unprovided with any shield, Limax. A portion of a species of crayfish. Captain Nolloth states that the islanders informed him that living crabs or crayfish were never found on the island; once only has a specimen of the king-crab containing flesh been thrown on the beach.

Two caterpillars. Decomposition has much injured the colours of these, but enough remains to enable me to identify one of them as similar to a specimen I myself took on the island in 1842, and figured. It was undoubtedly the larva of a Noctua, probably an Agrotes. I found my specimen feeding on wild sorrel. Captain Nolloth states he found one species on the cabbages, the other, in vast quantities, on the ground.

Shells.-Captain Nolloth has brought three species, each differing from those found here, viz:

A Chiton about an inch and a half in length, with spicula along the sides.

A pretty little Siphonaria, shaped like a capulus.

And three Ranella closely resembling the common species of these shores. These are said never to be seen alive, but to be cast up with the remains of the crabs and crayfish. One specimen, however, has not been many days deserted by its proper inhabitant, and still contained a considerable portion of the epidermis.

Birds.-The birds procured are the common albatross (D. Exu lans) and the smaller species known by the name of the "mollymaw" (Diomedea melanophrys tern), two species of petrel (Procellaria), and two species of tern, (Sterua stolida).

Captain Nolloth also brought the eggs of these birds, and of the king penguin (Eudyptes chrysocome lath), which he likewise saw on the island.

But among the eggs is one of peculiar interest, being the egg of a Gallimele, called by the islanders the "island hen." Corporal Glass mentioned to me in 1842 that the only bird they had besides sea-fowl was a water hen. Can this egg belong to the apterous bird, which is known to have existed on the island some years since?

Geological Specimens.-These consist of granite, obsidian, pumice lava-a substance which fuses easily under the blow-pipe, and even catches fire, being composed of sulphur,-and several rounded water. worn red stones, of great hardness, similar to some I have received from the Crozettos. Captain Nolloth was informed there was no

granite on the island, and did not detect the single specimen here enumerated until it had been washed and cleansed from the coating of mud which covered it.

Plants.-Captain Nolloth has brought back a considerable number of species of Ferns. There has not been time to ascertain which of them have been described by Carmichael, who visited the island some years ago. They, however, all differ from those in this country, as do also the plants which have reached this alive, and have been sent to the Botanical Gardens.

The fragments of the sofa appear to be mahogany, but they are too small to be properly identified.

E. L. LAYARD,
Curator South African Museum.

THE INDIAN OCEAN CONSIDERED WITH REFERENCE TO THE WANTS OF SEAMEN.

The Indian Ocean is bounded on the North by the South coasts of Asia; on the East it is separated from the Pacific by the islands of the Indian Archipelago; on the West Africa separates it from the Atlantic Ocean, and its Southern boundary is generally considered to be a line drawn from the Cape of Good Hope to Bass Strait.

The peculiar form of the land which surrounds this extensive basin prevents the currents of air and water being so regular as they are in the Atlantic Ocean.

The S.E. Trade Wind.-In the Northern hemisphere of the Indian Ocean, and even in part of the Southern, trade winds are not met with. In this ocean they are only found between the parallels of 10° and 28° S. from the West coast of Australia to within a few degrees East of Madagascar.

Limit of the S.E. Trade Winds.-The Southern limit of the S.E. trade winds is influenced by temperature: it changes to the North or South by two or three degrees according as the sun has North or South declination.

It is the same also with regard to their equatorial limit, which, under the same conditions, approaches the equator more or less, and sometimes even reaches it.

If the sun is in the Northern hemisphere, and at his greatest distance from the equator, the trade winds have a tendency to blow more from South; that is, they vary from S.E. to S.S.E.: on the contrary, when the sun is in the Southern hemisphere, the winds take a more easterly direction, varying from E.S.E. to East, and sometimes to E.N.E.

Variable Winds. From the parallel of 10° S. lat. to the coast of India, periodical winds are met with in the Indian Ocean. The trade

winds of this sea are only regular at a considerable distance from the land, because the continents and large islands obstruct the regular course of these atmospheric currents.

Variable Winds between 28° and 60° South Latitude.-South of 28° S. lat. is the zone of the variable winds. In this region, between 28° and 60° S. lat. there is a constant strife between the inferior currents of air flowing from the pole towards the equator and those which are returning from the equator towards the pole. The latter currents, occasioned by the trade winds, have received, as already observed, the name of tropical currents. This opposition, in which the tropical wind generally prevails, produces the variable wind of this zone.

In the Southern hemisphere of the Indian Ocean, Southward of the trade wind, as in the same hemisphere of the Atlantic, the prevailing wind is N.W.; that is, this wind is met with during the greater part of the year.

Prevailing Winds between 60° and 70° South Latitude.-It may be stated generally, that in the zone comprised between the parallels of 60° and 70° S., the prevailing winds during summer are mostly from South, and in winter from West.

Periodical Winds or Monsoons of the Indian Ocean.-The trade winds which blow fresh in the Atlantic South of the Equator, and sometimes even South of the parallel of 6° N. lat., are, as we have previously stated, interrupted by the peculiar formation of the land forming the basin of the Indian Ocean on N.E., N.W., and East. The influence of these lands combined with the difference constantly existing between the temperature of the continents and that of the adjoining sea, occasions the phenomena known by the name of the Monsoons.

Monsoons exist in several quarters of the globe; but in no sea do they blow with such regularity, or are they so well established, as in the Indian Ocean, especially in that part North of the equator. There they extend from the coast of Africa beyond the China Sea, and even penetrate the Pacific Ocean; although in the Northern part of the China Sea and further East they are less regular than in the Indian Ocean.

The monsoons generally blow towards the continents during the summer, and in an opposite direction in the winter. Thus, the S.W. monsoon, which blows in the North part of the Indian Ocean from April to October, corresponds with the season when the sun, having attained a high North declination, has warmed the surface of the countries of India; while during the winter in this hemisphere, when the sun has a South declination, the N.E. monsoon prevails.

Duration of the Monsoons.-The monsoons blow in one direction during one half of the year, or rather from the middle of April to the middle of September; and from the middle of October to the middle of March in the opposite direction.

S.W. and N.E. Monsoons.-In the North Indian Ocean the S.W. monsoon begins in the middle of April and terminates in the middle of September. The N.E. monsoon succeeds, and lasts from the mid

dle of October to the middle of March. The S.W. monsoon brings rain and bad weather. The wind is stronger during this monsoon than during that from N.E., when the weather is generally fair and

serene.

Zone of S.W. and N.E. Monsoons.-The zone comprising the S.W. and N.E. monsoons is included between the equator and the tropic of Cancer. It extends from the East coast of Africa to the coasts of India, China, and the Philippine Islands, and their influence is often found in the Pacific Ocean as far as the Marianne Isles; that is, in 145° E. long. In the North they sometimes extend as far as the Japan Isles. The limits of the monsoons are not everywhere the same, and their change does not always take place at the same period. It must be observed that in the Gulf of Bengal the wind varies more in strength and direction than in the Indian Ocean, where it rarely fails at the expected times, which are generally at the changes of the

monsoons.

It is also observed, that in that part of the Indian Ocean where the monsoons are found, the wind has more Southing near the coast of Africa, and more Westing on the coast of India, during the S.W.

monsoon.

South of the equator the S.E. monsoon commences in the middle of April and terminates in the middle of September; the N.W. monsoon, varying to W.S.W., replaces it, and commencing in the middle of October terminates in the middle of March. This last monsoon is the time of squalls and bad weather.

In the Indian Ocean the S.E. and N.W. monsoons blow in a zone comprised between the equator and the parallels of 8° or 9° S. lat.; but on the coast of Australia, and in the West part of the Pacific Ocean, this zone extends as far as 12° and 14° S. latitude. It begins in the West near the coast of Africa, and terminates in the East on the meridian of the islands of Pomoutou and Nouka-Hiva, situated in the Pacific Ocean in about 138 W. longitude.

The N.W. monsoon rarely blows either strong or regularly, except in the months of December and January, at which time it occupies in the Indian Ocean a zone comprised between 10° or 12° S. lat., and 2° or 3° N. lat. This monsoon is also subject to many irregularities. In proportion as we approach Sumatra its Northern limit recedes more South from the Equator, and leaves a space between itself and this great circle, to be occupied by calms and variables.

The S.E. monsoon, which is the period of the fine season South of the equator, may be considered as an extension of the S.E. trade winds, which then blow as far as the equator, when the sun is near the tropic of Cancer.

The places where the S.E. and N. W. monsoons are found with the greatest force and regularity, are in the Java Sea, the Sea of Timor, and among the Moluccas, and especially near New Guinea. These monsoons are felt on the North coast of Australia, between Melville Island and Cape York, as well as in Torres Strait, where the N.W.

NO. 8.-VOL. XXV.

3 н

monsoon begins at the end of October. In these seas they are often called the West and East monsoons.

Change of Monsoons.-The change of monsoons takes place between the latter end of March and September, and the beginning of April and October. In some latitudes it takes place a week or two sooner than in others. The change is gradual, and is generally accompanied by squally and tempestuous weather. However, South of the equator, if the weather is sometimes bad at the change of the monsoon, the wind is never strong. When the monsoon has nearly terminated, the highest clouds are observed taking an opposite direction to that of the wind, and this happens some weeks before the change is sensible in the lower regions. The monsoons penetrate far into the interior of the continents bounding the Indian Ocean on the North, and their direction is then influenced by the form of the coasts and islands, the chains of mountains, or other causes.

Seasons in India.--The direction of the monsoons in India regulates the dry and rainy seasons. The rainy season of the West coasts corresponds with the S.W. monsoon, and that of the East coasts with the N.E. monsoon; in other words, the winds from the sea in general cause rain, while those from the land bring fine weather.

Monsoons on the Coast of Madagascar, East Coast.-It has been already observed that the S.E. trade wind does not extend as far as the coast of Madagascar. On the East coast of this island, and in the channel separating it from the coast of Africa, we find the monsoons regularly established.

N.E. Monsoon varying to N.N.E.-On the East coast of Madagascar the N.E. monsoon varying to N.N.E., blows from November to April, and the S.E. monsoon from April to November. This last period is the time of the fine or dry season. However, in the S.E. part of Madagascar, we generally find N.E. winds, which, veering round the coast, blow as far as Cape St. Mary, and West of this cape and off the S.W, of Madagascar, blow from the S.E. to South during the whole year; but do not reach far to the North along the West coast of this island. Land winds are not known at Madagascar ; on all the coasts of this island, the winds above-mentioned prevail throughout the year, and those to which we shall presently allude in the Mozambique Channel. These winds commence about eight or nine o'clock in the morning, freshen up towards noon, when they blow very strong, and continue so till three o'clock, then gradually abating towards sun-set.

Monsoons in the Mozambique Channel.-In the Mozambique Channel, and chiefly between the parallel of the bay of Sofala and the equator, the S. W. monsoon begins in April and terminates in November. S.W. and S.S. W. Monsoon.-This monsoon forms the period of the fine season. At the entrance of this channel we find the wind generally from South to S.S.W., veering Westward as we go Northward; and they unite at last with the W.S.W. and S.W. winds, blowing at the same time North of the equator. In the North part

« السابقةمتابعة »