It appears on inquiry, that the quantity of variation is no more known, any where within the continent of Africa, than within that of New Holland. And it happens moreover, that the lines of equal quantities of variation, do not run across Africa with that degree of regularity and parallelism, which takes place over great part of the Atlantic and Indian oceans (at least this is what appears clearly to my judgment): so that it became necessary to inquire, what quantity prevails in the surrounding seas; and what the general direction, as well as the particular nature, and tendency, of the curves, of the lines of equal quantities?
The variation lines on the globe have occupied a good deal of my attention at different periods of my life, and therefore the application of such new observations as the assiduity and kindness of my friends had procured for me on this occasion, was less difficult, than if the subject had been new to me. A dissertation on the subject, would be out of place here; and therefore I shall only give the result of my inquiries, in abstract; after premising, that the theoretical part belonging to the interior of Africa, is founded on a supposed continuation of those lines of equal quantities, whose tendency has been already ascertained, in the surrounding seas. I am perfectly aware, that some may regard the assumption as too great: but they will no doubt admit, at the same time, that it is difficult to conceive a more probable arrangement: and what is much more to the purpose, is, that if we are compelled to abandon the system, in the gross, the quantity of variation in the line of Mr. Park’s travels, cannot be greatly different from what we have assumed. For, whether the line of 18° in the south Atlantic, be a continuation of that in the north Atlantic, or of that in the Indian sea, much the same result will follow: only that in the former case, the quantity will be somewhat greater.
Chart of the Lines of Magnetic Variation, in the Seas around Africa.
Published by James Rennell May 18th. 1798.
It would appear, that between the East Indies and South America, Europe and South Africa, there are four distinct sets of what may be termed concentric curves of variation lines, on the globe, and whose highest points of convexity are opposed to each other, within the great body of Northern Africa. The accompanying sketch will best explain it.[63] It would appear moreover, that from the place of opposition of these curves, in Africa, where the quantity of variation is 18°, it decreases with great rapidity, and finally to nothing, in going eastwards to India, or south-westward to south America: and that from the same point, it increases, in going NNW, towards Ireland, or to the opposite quarter, towards the coast of Caffraria. But the change is not in any proportion so rapid in the increase, in going north or south, as in the decrease, in going east or west. Such are the outlines of this system; by which, if a person was to set out from Cape Verd, or Cape Blanco, to traverse Africa from WSW to ENE, to Upper or Lower Egypt, he would find at setting out, from 15½ to 16½ degrees of westerly variation, which would increase to 18° about the centre of the continent, and afterwards decrease to a less quantity than at his setting out. Mr. Park’s travels being comprized between Cape Verd and the centre of Africa, will therefore be in the quarter that has from 16 to 18 degrees; and the part more particularly, where, from the want of observations of latitude, we must depend on his compass bearings, nearer 18° than 17°.[64] I shall now proceed to the detail of Mr. Park’s materials.
Leaving Pisania, Mr. Park proceeded eastward to Medina,[65] the capital of Woolli; and thence to the ENE, through the countries of Bondou, Kajaaga, and Kasson; the two latter of which are separated by the river of Senegal.
In his way, he took observations of latitude at Kolor, Koorkoorany, and Joag, on this side the river: by which means we are enabled to correct the parallels. The distance at present is left as it stands in the Journal, with a view of correcting it afterwards. The result of these bearings and distances (the particulars of which appear below),[66] places Joag at 247 miles east of Pisania; and the latitude, by two different observations, was found to be 14° 25′.[67]
At Joag Mr. Park was informed that Dramanet, which is about 2½ miles to the eastward of Fort St. Joseph, was 10 miles to the westward of him. That fort is said to be in 14° 34′, or 9 min. north of Joag; so that it ought to lie to the north-west, rather than to the west: and accordingly, there is found, in the map in Labat,[68] a place named Gacouva, no doubt meant for Joag, on the SE of St. Joseph; and in point of distance answering to it. Other places in the same map of Labat, are also recognized in the route of Mr. Park; so that the connection between it and the French settlement on the Senegal, is very clearly made out; which is a circumstance of some importance in this geography.
It will appear that the reckoning kept by Mr. Park gave 89 min. difference of latitude between Pisania and Joag, whilst the observation gave 50 only. About 9 minutes may be placed to the account of excess of distance, and 30 will then remain, on the whole distance of 253 geographic miles. In other words, the dead reckoning, corrected by allowing 17 degrees of westerly variation, gave a course of E 20°½ N; which, however, by the observations of latitude, is shewn to be E 11°¼ N (or E by N) making a difference of 9¼ degrees. But this I regard as coming very near, considering the circumstances under which the reckoning was kept.