Abbildungen der Seite
PDF
EPUB

October, 1853, only 2° 49' 20" westerly. The following are the results:—

September Observation. October Observation.

Westerly Variation. Westerly Variation.

Obs., 5 means, Azimuths 2° 58' 0" Variations, West.. 2° 43' 0"

Amplitude .. 2 48 4 Azimuths 2 48 3

Obs., 3 means, Azimuths.. ..2 53 0 Amplitude ....2 51 0

Amplitude ..2 45 ,6 Azimuths 2 45 0

„ „ 2 50 0 Amplitude .. ..2 54 0
„ 2 52 10
„ 2 47 10
„ „ 2 46 O

Means of '27 Observations.

Means of 27 observations with a most beauti-1

ful instrument, with Col. Everest's im-l„„ ,„,..,, „T . . . .

provements, in September and October, 2 49'20" Westerly variation.

1853, at Aden J

Means of 130 observations during the survey) 5 g Q Westerlv

of Aden in 1834 J

Variation diminished in 19 years.. .. 2 12 40

This difference of variation is also proved by taking the true and magnetic bearing to fixed points; the "means" of numerous magnetic bearings to different fixed points now differing from the magnetic bearing on the survey 23 12', while the true bearings all agree.

The variation having diminished 2° 12' 40" westerly at Aden, renders it almost certain that a still greater change (even, I imagine, \ of a point) has taken place in the N. part of the Red Sea since the survey; which \ of a point, during a long and dark night, with steam-rate at 10 knots, will place a steamer 6 or 7 nautical miles from her supposed position; and might cause, particularly if assisted with a slight current, incalculable mischief. My anxiety for the public good has prompted me to make these observations, and I do not hesitate to say I am confident they are correct.

It is also probable that the variation has changed during the last twenty or thirty years along the African, Arabian, Persian, Beloochistan, and Indian coasts; which, however, is not of such vital importance to the navigator as while navigating at a rapid steam-rate between the narrow limits of Red Sea dangers.

It may be observed that steamers can ascertain the variation by observation; but, from long and tried experience, I assert such observations cannot be trusted sufficiently for Red Sea navigation. Azimuth compasses cannot be pointed to a nicety, and all vessels have more or less local attraction, which is not always sufficiently attended to.

I would respectfully suggest that the change of variation at Aden be made public as soon as possible.

VII.—On the Physical Geography of the Red Sea.
By Dr. Buist, of Bombay, F.r.g.s.
Read April 10, 1854.

Boundaries.— The Red Sea is one of the most remarkable estuaries on the surface of the globe; separating the N.E. portion of Africa from Arabia for the space of above 1280 miles, itseems at no very remote period to have been connected with the Mediterranean. Opening into the Indian Ocean, through the Gulf of Aden, it commences at the Straits of Babelmandeb, lat. 12° 307 N., long. 43° 40' E., extending in a north-westerly direction till it terminates in the two Gulfs of Akabd and Suez, the upper extremity of the former being in lat. 29° 36' N., long. 35° 2' E., that of the latter in lat. 30° 2' N., long. 32° 38' E. from the Strait to Suez in a direct line is 1230 miles; the greater strait itself measures in breadth 13 miles, the lesser If miles; the two together, which constitute the entrance to the sea, 14f miles, or including the island of Perim, which separates them, 16^ miles. Its entire circuit measured round both gulfs is 4020 miles, its area 108,154 miles, and its cubic content probably about 800,000 miles.* Its greatest breadth under the parallel 17° N., that is one-third up the sea, is 192 miles, and it narrows pretty uniformly towards both extremities, being 72 miles across at Ras-Mahommed, where the peninsula of Mount Sinai splits its upper extremity into two, and nearly a similar breadth at Gibbel Zugar, under the 14th

Gulf of Suez.—The Gulf of Suez from its upper extremity to its entrance is 167 miles in length ; its greatest width is under 30; at its mouth it is about 17 miles from shore to shore; its area in all is about 2000 square miles. Its greatest depth is about 50 fathoms, its average about 22 ; excluding the shallows at Suez, it occupies from 8 to 10 miles at its upper extremity.

Gulf of Akaba.—The Gulf of Akabd, is about one-third the area of that of Suez, or 800 miles; it is 100 miles in length, 16 across at the widest, and 7 at the strait. It is more than double the depth of its sister gulf, being about 120 fathoms for twothirds of its length, reaching at one point the depth of 200 •fathoms without bottom. Its mean depth altogether is probably not less than 70.

Depth of the Red Sea.—Although two-thirds of the area of the Red Sea have never been sounded, and no sufficient data exist from which to form a judgment of its depth, there is no reason to

* The figures representing area, circumference, and content are to be received as approximations merely—especially the circumference; the area will probably be found not very far from truth.

[graphic]

believe it to be very great. There are soundings all around its shores from 10 io 20 miles out from Judda, in lat. 21° 30', to the approach of the volcanic region in lat. 173, and frequent cross soundings have been obtained from this southward to the strait. From 21° to 27° 30', or for nearly 400 miles in length, there is a space from 50 to 70 miles in breadth, or an area of about 20,000 miles, that seems never to have been sounded; and this part has never been examined since the original survey was made 20 years ago. Being free from islands, reefs, and shoals, it occasioned no alarm to the navigators, and the surveyors had no time to devote to mere questions of physical geography. The" greatest depth that seems ever to have been tried is 400 fathoms, lat. 25° 20', at which no bottom could be found; and there appears to be a gulf from 5 to 10 miles wide down the centre of the sea, varying from 150 to 250 fathoms, with abrupt and precipitous sides. The average depth of the central region of the sea to an extent of about 40 miles or so may probably be about 100 fathoms; the average depth of the whole sea probably falls short of 40. A reef or shallow runs across from Mocha, lat. 13° 30', to the African shore; it has been very carefully sounded all along; it affords an average depth of from 25 to 30 fathoms; its greatest depth being 40, near mid-channel. From this the sea deepens again to 125 fathoms* as it approaches the strait. The great strait, as already mentioned, is 16 miles across; its average depth is about 80 fathoms, its greatest 125; and for a breadth of nearly 6 miles in mid-channel the depth exceeds 100 fathoms. The narrow channel being that which vessels from Aden almost always prefer in ascending the Red Sea, varies from 12 to 17 fathoms in depth, but there is 30 feet of water up to both shores with a fine sandy bottom, so that navigators feel no apprehension in traversing it at any hour. The Gulf of Aden, which continues the communication from the Straits to the Arabian Sea, is a funnel-shaped estuary above 900 miles in length and nearly 200 across from the N.W. point of Africa to the Arabian shore. Its general characteristics are similar to those of the lower part of the Red Sea—it is remarkable for the violence of its currents, which will be noticed afterwards, and for the depth of its central channel and shallowness of its shores.

The Tides at Suez are about 5 feet at neap and 7 at spring.* The rise and fall at Ras-Mahommed is about 5 feet—high water at 6 o'clock. The direction of the wind makes a, difference of about 4 feet in the depth of the water near Suez: the banks which are left dry by the N. wind are well covered with a breeze

* The depths are taken from the chart, but the greater part of the soundings are marked as having " no bottom," so the actual depths are certainly beyond this,— how much we know not.

from the S. We know little of the tides over the rest of the sea. At Aden, 80 miles without the strait, where continuous observations have been taken by gauge for three years, the tides rise 4 feet at neaps, and nearly 8 at springs; high-water occurring at TSO; the sweep of the tides, as well as the hour of high-water, being remarkably irregular. The Red Sea probably derives its name from large portions of it, as well as of the Arabian Sea, being covered with patches, from a few yards to some miles square, of a blood-red colour, derived from a species of animalcule particularly abundant in the spring months, and which dye the upper waters of the most intensely blood-red hue that can be conceived. There is nothing else about the sea that is red, and a considerable expanse of this, encountered by an early navigator who had not met with any similar phenomenon elsewhere, would seem warrant enough for its name.*

Islands.—Though islands are numerous along both shores of the Red Sea, they are for the most part of inconsiderable size,

* Since the preceding remarks were in type I have fallen in with a paper by Ehrenberg on the bloody appearance of water, a translation of which appeared in 1831 in the tenth volume of Jamieson's Edinburgh Philosophical Journal. After enumerating instances quoted by the ancients of red snow, red rain, and rivers and seas covered with blood, he quotes a list of our experiences on these subjects in modern times. I take the following entire; he comes to the same conclusion as that wjich I have arrived at as to the origin of the name of the Red Sea:—

"In 1823 I was for a number of months at Tor, on the Red Sea, in the vicinity of'Mount Sinai. On the l(Jth December I there observed the striking phenomenon of the whole bay which forms the harbour of Tor of a bloody colour. The main sea beyond the coral reef that encloses the harbour was, as usual, colourless. The short waves of the calm sea, during sunshine, carried to the shore a bloody coloured slimy mass, which it deposited on the sands, so that the whole bay, fully half a league in length, at the ebb of the tide exhibited a blood red border of more than a foot broad. I took up some of the water itself with glasses and carried it to my tent at hand on the sea shore. It was immediately discovered that the colouring was caused by small flakes, scarcely distinguishable, often greenish, sometimes of a lively green, but for the most part of a dark red colour, although the water itself was not stained by them. This very interesting appearance attracted my attention as explanatory of the name of the Red Sea, a name hitherto so difficult of explanation. I, for many days and with perfect leisure, accurately examined the appearances and made microscopical observations on the colouring mass. The flakes consisted of small spiral or lougish irregular bunches of oscillatorise threads, which were enclosed in a gelatinous sheath, and the flakes neither resembled one another nor the threads in each flake. In the glasses placed beside me I observed that the flakes during the heat of the day and in sunshine floated together on the surface of the water. During the night and when the glasses were shaken they descended to the bottom. After some time they returned to the surface. The observation made by Dr. Englehardt on Lake Murten was very similar to this appearance, and the delineation of the single threads by De Candolle exhibits a very close relation to it. De Candolle informs me he has preserved no dried specimen of that substance, for which reason no comparison can be made. The gelatinous covering and the union of many threads into very small spiral groups give to the substance of the Red Sea a peculiar character, which entitles it to form

a particular genus of alga The appearance of the Red Sea was not

permanent, but periodical. I observed it several times: on the 25th and 30th December, 1843, and on the 5th January, 1844."

and they have been included in the measurement of the area: their structure will come to be spoken of along with that of the rocks on the,opposite shores.

Temperature.—The surface temperature of the Red Sea, agitated by the paddles of the steamers, varies from 60° to 85°. By a series of hourly observations made on it in April, 1840, when it ranged from 80° to 85°, it was in general from two to three degrees hotter than the air from midnight to dawn, and about as much colder from noon to sunset. Captain Newbold's observations for May, 1844, give from 82° to 84° as the midnight and noon temperatures, those of the air at the same hours being from 85° to 90°. The matter is one that has been but little attended to, though it is probable that the Red Sea scarcely differs in this from the outer ocean.

Appearance and Saltness of its Waters.—As the Red Sea is entirely surrounded by a hard, sandy, or rocky shore, without a rivulet, and scarcely a drop of rain falling into it from year to year, its waters are remarkable for their transparency and purity, even over a long expanse of shallows. Where the sea is deep and distant from the land, its colour is of the most intense blue, changing from greenish-blue to bluish-green, green, and light green, as the coral reefs approach the surface—the corals themselves being mostly white. It was long supposed to be considerably salter than the general ocean, a supposition now proved to be unsound. In 1837 Dr. Malcolmson found the water off Cossir of specific gravity 1035, indicating a degree of saltness greater than that at Suez, but not so great as that of many parts of the Atlantic. The water at Mocha and Camran hardly at all differs from that of the outer sea. In 1848 Mr. Morris, engineer, obtained for me specimens of the water from seven different stations, nearly equidistant from each other all the way down: they were examined by Dr. Giraud, Professor of Chemistry at Bombay, and the following were the results:—

[table]

Dr. Giraud gives the following note of the saltness of the sea from a variety of other localities. From this it will be seen that the Mediterranean at Marseilles is of the same saltness as the Red Sea at Suez, while the Atlantic in the latitude of the Canaries is Tsvtt more salt.

« ZurückWeiter »