A Voyage of Discovery and Research in the Southern and Antarctic Regions/Volume 1/Chapter 2
CHAPTER II.
CHAPTER II.
On each side of the tropic of Capricorn, which we 1839.crossed on the evening of the 19th, we experienced Dec. 19.sharp squalls regularly between 8 a.m. and noon, and sometimes they occurred about 3 a.m. The cumulostratus clouds, of which Sir John Herschel has given so admirable a description, were in all cases the origin of these sudden gusts of wind. The cumuli, like beautiful rolls of wool, were first seen to windward, at the height of 3° 46′, resting upon an ill-defined misty-looking cloud, about one degree high. They formed an irregular arch as they rose; and driven towards us by the wind, we could see the rain falling abundantly. When it had attained 35° of altitude, the squall struck the ship, and compelled us to lower the topsails, and take in all the sails but the foresail. They generally occurred three or four times in the forenoon. During their continuance the wind always veered more to the northward, and they were always preceded by a very perceptible increase of the short sea that obtains in this part of the ocean.
A curious phenomenon was witnessed by many Dec. 20.of the officers at 8h. 30m. this evening. It was a beautiful clear night, not a cloud to be seen in any part of the heavens, yet we had a light shower of rain of more than an hour's continuance. The temperature of the dew-point by Daniell's hygrometer being 72°, that of the air 74°.
In addition to our almost daily experiments on the temperature of the ocean to the depth of six hundred fathoms, we had made several fruitless attempts to obtain soundings as we passed through the tropics. These repeated failures were principally occasioned by the want of a proper kind of line, but they served to point out to us that which was most suitable. I accordingly directed one to be made on board, three thousand six hundred fathoms, or rather more than four miles in length, fitted with swivels to prevent it unlaying in its descent, and strong enough to support a weight of seventy-six pounds.
1840.
Jan. 3.On the 3d of January, in latitude 27° 26′ S., longitude 17° 29′ W., the weather and all other circumstances being propitious, we succeeded in obtaining soundings with two thousand four hundred and twenty-five fathoms of line, a depression of the bed of the ocean beneath its surface very little short of the elevation of Mount Blanc above it.
As I shall have occasion hereafter to enter more fully into the general question of the maximum depth of the sea, I shall take that opportunity of describing the method by which the soundings were obtained, and the accuracy with which the several depths were determined. We were at this time only forty-five miles to the north-west of a spot where a shoal is marked on the chart as having been discovered in 1701. The great depth of water would seem to throw some doubt on the reality of this supposed danger, but being exactly to leeward of the assigned position we were unable to ascertain the fact.
In latitude 21° S., and longitude 15° 30′ W., we Jan. 8.recrossed the line of least magnetic intensity. By taking advantage of every change of the wind, which we found to vary from S.S.E. to E.S.E. at different periods of the day, we advanced on an average between twenty-three and twenty-four miles each day, and eventually anchored in St. Helena Roads at 3° 30′ p.m. on the 31st January.Jan. 31.
The chief purpose of our visit being to establish a permanent magnetic observatory, and to land Lieutenant Lefroy of the Royal Artillery, and the party under his command, together with all the instruments intended for their use, I obtained permission from His Excellency Lieutenant-General Middlemore to make an examination of the lands Feb. 2.belonging to the crown; and having selected a spot near the house that had been erected for the residence of the Emperor Napoleon, but which had never been occupied by him, as a site for the observatory, it was immediately granted by the governor. All the arrangements for the lease of the premises, and the contracts for the necessary buildings, were completed in a few days by the diligent and ready co-operation of the Board of respective Officers, consisting chiefly of Lieut.-Col. Trelawny of the Royal Artillery, and Captain Alexander of the Royal Engineers, to whom I am more especially desirous to express thus publicly my thanks for the prompt and effectual means they adopted to forward the service with which I was charged.
Here as invariably on all volcanic islands, it was found quite impossible to obtain correct measures of the magnetic elements, by reason of the large amount of the disturbing influence of the rock itself. So powerful indeed was it in this particular instance, that even at the distance at which our ships were anchored it produced such anomalies in the results of our observations for the deviation of dip, variation, and intensity, as to mask the ordinary effects of the ship's iron. The comparisons of our magnetic instruments were just as unsatisfactory, for no two places could be found, however near to or distant from each other, where accordant results could be obtained even with the Feb. 9.same instrument. The importance, therefore, of St. Helena as a magnetic station will be manifested more by the detection of the momentary, irregular, and secular changes than by absolute determinations; and for meteorological purposes it cannot fail to be of essential advantage. Taking leave of our kind friend the governor, Col. Trelawney, Capt. Blackwell, and the officers of the 91st regiment, to whom we are all indebted for their attentions and many acts of friendship, we weighed on the morning of the ninth, after sending our letters and despatches on board the "Bombay" for conveyance to England, and proceeded on our voyage.
The trade wind prevented our fetching so far to the eastward as I wished, so that Ave crossed for a third time the line of least magnetic intensity in lat. 21° S. and long. 8° W. Our slow progress through this, magnetically speaking, very interesting region afforded us the opportunity of obtaining a vast number of observations, which having been transmitted to England from the Cape of Good Hope were placed in the hands of Lieut. Colonel Sabine of the Royal Artillery, and published, under his supervision, in the Philosophical Transactions[1] of the Royal Society, to which I must refer the scientific inquirer for the observations themselves, and also for a more detailed account of the results. It is enough to state here, in the words of Col. Sabine, "that the determination of the position of the line of least intensity is easier, and in some respects more sure, than that of an isodynamic line, because it is independent of the correctness of an assumed intensity at a base station. It is therefore to be expected that the position of this line will become in future years the subject of frequent examination, seeming to mark, from time to time, the progress of the secular change in its position. This may be done with the more interest and advantage, because there is reason to believe that its position is changing rapidly in the space referred to, particularly in the eastern meridians; and that the southern magnetic hemisphere, in so far as its boundary may be indicated by this line, is in that quarter of the globe gaining rapidly on the northern."
He further states, that whilst the general direcrection of the line of least intensity, drawn from observations[2] of Dunlop, Erman, and Sulivan, corresponding nearly to the epoch of 1825, is consistent with that deduced from our observations in 1840, yet its earlier position is every where three or four degrees south of that which would be inferred from the later determinations. Its average northerly movement therefore, during the last fifteen years, appears to have rather exceeded annually thirteen miles.
Feb. 15.A water-spout of small size, seen on the morning of the fifteenth, was followed by a heavy shower in the afternoon of an hour's continuance. The rain which fell at a temperature of 67°, in a few minutes reduced that of the air from 79° to 73.°
In lat. 26° 10′ S. and long. 12° 50′ W. we made another attempt at deep sounding, but failed, owing to the line being accidentally checked and breaking at twelve hundred and sixty fathoms.
Feb. 22.This evening, soon after dark, a number of cuttle fish sprang on board over the weather bulwark, fifteen or sixteen feet high; several passed entirely across the ship, and altogether not less than fifty were found upon the decks. We could not on this, as on a former occasion, ascribe their visit to the sea washing them into the vessel, the water at the time being quite smooth, and only a moderate breeze blowing.
Whilst running before a strong northerly breeze, Feb. 23we observed heavy clouds gathering to the S.W., and could scarcely reduce our sails before we experienced a most violent squall from that quarter, which was almost immediately succeeded by a calm of short duration. These sudden gusts of wind are not at all indicated by the barometer and are not unfrequently the cause of the loss of masts, when not prepared for in time. The northerly wind resumed its power until about noon, when it again shifted suddenly to the southward, so that we seemed to have got into another region of the variables. Before and during the heavy torrents of rain, which poured down for the next twenty hours with but little intermission, we were surprised that both the barometer and sympiesometer rose steadily. The temperature of the rain (as noticed also on the fifteenth) being 67°, again reduced that of the air from 74°5 to 69°.
In lat. 31° 20′ S. we crossed the meridian of Feb. 27.Greenwich on our eastward course.
Although the south-westerly breeze of the preceding Feb. 29.day hardly amounted to a moderate gale, we found that this morning we had run into a heavy swell from that direction. The result of several experiments gave only twenty-two feet for the entire height of the waves, or eleven feet above and below the general level of the ocean; the velocity of the undulations eighty-nine miles per hour, and the interval between each wave nineteen hundred and ten feet.
Steering for the shoal, called Kattendyk on our charts, in lat. 33° S. and long. 4° 52′ E, we kept the lead constantly going, but could not get soundings with from one hundred and twenty to one hundred and fifty fathoms, although we passed exactly over its presumed position; and the next day, in March 1.lat. 33° 10′ S. and long. 5° 50′ E., we could not reach the bottom with five hundred and eighty fathoms.
March 3.At 9h. 30m. a.m., when in lat. 33° 21′ S. and long. 9° E., being perfectly calm, we lowered the boats down and again succeeded in obtaining deep soundings: on this occasion, in two thousand six hundred and seventy-seven fathoms, and at a distance of about four hundred and fifty miles west of the Cape of Good Hope. The current was setting to the westward, at the rate of a mile per hour; and for several days previously we had experienced its influence fully to that amount.
March 7.This evening we observed a gradual diminution of the temperature of the air and the sea, as we approached the coast of Africa; and before midnight we entered a cold mist, which prevented our seeing to any considerable distance: the water appearing discoloured, we tried for, but did not obtain, soundings, with one hundred and thirty fathoms of line.
By 1 p.m., the next day, the temperature of the March 8sea had fallen from 70° to 56°5, that of the air being 65°, and the mist unpleasantly cold to our feelings. We were at this time in lat. 32° 21′ S. long. 17° 6′ E., therefore about forty-five miles from Paternoster Point, when we struck soundings in one hundred and twenty-seven fathoms, on a bed of fine dark sand. We had expected to have found an elevation in the temperature both of the air and sea on our approach to the African coast, by reason of the radiation of heat from its shores; but the cause of the depression became evident on the morning of the 9th, when, having sighted Cape March 9.Paternoster at daylight, we found we had to contend against a current increasing in strength and coldness of temperature as we neared the land. The existence of a body of cold water rushing from the eastward, round the Cape of Good Hope, has long been suspected; but, its extension so far to the northward has not, I believe, been before noticed. As we were several days beating up to the Cape we collected the following curious facts respecting it. Thus on the 7th, when distant one hundred and twenty miles from the coast, and before we perceived the effects of the current, the temperature of the air was 71°, that of the sea 70°, and the depth of water more than four hundred fathoms; which being placed in order will serve to explain the arrangement of the following table.
| Distance | Temperature. | Depth | ||||
| No. | Date. | off Shore. | Air. | Sea. | of Water. | Remarks. |
| Miles. | Fms. | |||||
| 1 | 7 | 120 | 71° | 70° | 400 | No soundings. |
| 2 | 8 | 90 | 65 | 63 | 130 | No soundings. |
| 3 | — | 45 | 65 | 56 | 127 | Temp, at that depth, 45°. |
| 4 | — | 10 | 59 | 54 | 65 | |
| 5 | 9 | 10 | 59 | 54 | 47 | |
| 6 | 10 | 60 | 64 | 61 | 200 | Temp, at that depth, 43°.5. |
| 7 | — | 20 | 61 | 55 | 130 | |
| 8 | 11 | 52 | 67 | 64 | 203 | |
| 9 | — | 32 | 60 | 54 | 142 | |
| 10 | 12 | 51 | 69 | 66.5 | 313 | |
| 11 | — | 36 | 67 | 67 | 202 | |
| 12 | — | 27 | 58 | 54.5 | 72 | |
| 13 | 13 | 7 | 63 | 55 | 58 | |
| 14 | — | 4 | 59 | 51.5 | 48 | |
| 15 | 14 | 27 | 62 | 57.5 | 115 | |
| 16 | 15 | 6 | 55 | 51 | 76 | W.N.W. from Cape. |
| 17 | 16 | 11 | 66 | 62 | 190 | W.S.W. from Cape. |
| 18 | 17 | 4 | 65 | 60 | 37 | In False Bay, S. E. from Cape. |
By a careful examination of the above experiments it will be manifest that the distance to which the cold water extends from the coast depends materially upon the depth of the soundings. It barely reaches forty miles from the shore, where the sea is more than three hundred fathoms deep, but spreads over double that distance in the shallower parts. At forty-five miles from the land, and at a depth of one hundred and twenty fathoms, the temperature was found to be 45°, that of the surface being 56°; and at sixty miles off the land, at two hundred fathoms, it was 43°.5, the surface being 61°.
All these circumstances combine to show that a northerly current of very limited extent, but of considerable force, exists from the Cape of Good Hope, along the western coast of Africa; which, in general terms, may be represented by a volume of water sixty miles wide and two hundred fathoms deep, averaging a velocity of about a mile an hour, and of the mean temperature of the ocean, running between the shores of Africa and the waters of the adjacent sea. The cloud of mist which hangs over this stream of cold water is occasioned, of course, by the condensation of the vapour of the superincumbent atmosphere, whose temperature is generally so many degrees higher than that of the sea. It is sufficiently well defined to afford useful notice to seamen of their near approach to the land.
We anchored on the evening of the 17th in Simon's March 17.Bay, and immediately commenced the necessary comparisons of our magnetic and other instruments, the sandstone formation of the country being more favourable for the purpose than that of any other place we had touched at since leaving England. Observations for the effect of the ship's iron on the various instruments employed were made in both ships, as also, by the kind and ready assistance of Captain Puget, on board of the "Melville," the first two-decker on which I had ever had an opportunity of experimenting, and in which it proved to be of very small amount.
We here found no difficulty in replenishing our stores and provisions, for by an especial order to that effect from the Commander-in-Chief, Rear-Admiral the Honourable George Elliot, C.B., and the active exertions in our favour of J. Deas Thompson, Esquire, the naval storekeeper, every want was promptly and liberally supplied, and every thing done for us that a cordial interest in our undertaking could suggest.
To His Excellency Lieutenant-General Sir George Napier I am also much indebted for offering me any portion of the crown lands that might be required for the site of the Magnetic Observatory, which I was directed to establish here, and leave under the superintendence of Lieutenant Eardley Wilmot, of the Royal Artillery, who, with his party of three assistants, and all the necessary instruments, were now disembarked from the Erebus. In making a selection of so much importance I gladly availed myself of the judgment and local knowledge of Mr. McLear, at present and for several years past the resident astronomer, and by his advice decided on a spot adjoining the Cape Observatory; thereby securing to Mr. Wilmot the co-operation of practised observers when long continued series of observations should render their aid desirable, and at all times relieving him from a considerable amount of labour, by rendering the astronomical observations of the observatory available for his purposes. I am truly obliged to Mr. McLear for his kindness on that occasion, and for the alacrity with which he offered and has given every assistance in his power to promote the erection of the necessary buildings, and forward the views and wishes of Mr. Wilmot in the discharge of his arduous and responsible duties.