10. The “Tortoise Marshes” of Ptolemy.

The passage in Ptolemy’s ‘Geographia’ (lib. IV. cap. 6, sect. 4) in which the position of the “Tortoise Marshes” is given may be translated as follows[37]:

There are two great rivers running into the Mediterranean; one of them is the Gir, joining Mount Usargala with the Garamantic narrows, from which, changing its course, the river is located in long. 42°, lat. 16°, and makes the Tortoise Marshes (Chelonitides Paludes), whose position is long. 49°, lat. 20°.

The information on which Ptolemy based this statement, at least as regards the river Gir, must have been very fragmentary, if not, indeed, grossly erroneous; for there is no river flowing to the Mediterranean anywhere near the positions he gives. But there can be little doubt that his names Gir and Chelonitides Paludes refer to real places, and there has been much speculation among geographers as to their identification.

Thus, for instance, Knoetel[38] suggested that the Gir may have been the Wadi Djedi, to the south of Biskra, and the Tortoise Marshes the modern Lake Melghir (lat. 34°, long. 6°)[39] Dr. William Smith[40] thought that the Gir was really a branch of the Niger, and the marshes the modern Lake Fittri (lat. 13°, long. 18°). On the map of Africa on the 2-million scale published by the Geographical Service of the French Army in 1899 the Tortoise Marshes are depicted as lying in about latitude 19° 20′, longitude 27°, with a note stating that this delineation is taken from an earlier map of the Nile Basin by Miani, who regarded the marshes as being connected with the “dry river” which was then supposed to run northwards through Dakhla Oasis. Colonel Tilho has lately suggested[41] the lowlands to the north-east of Lake Chad (lat. 18°, long. 17°) as a possible site for the marshes; while still more recently Mr. Harding King[42] has thought that they might perhaps be identified with the salt lake of Merga (lat. 19° 3′, long. 26° 19′).

Of the various localities which have been suggested as possible sites for the Tortoise Marshes, the only one which I have visited is Merga. I found the salt lake at that place to be very small, covering only some 10 acres; it lies at an altitude of 509 metres above sea-level, and though it is situated in a rather wide shallow depression, I saw no traces of any extensive salt-marshes around the lake, while the configuration of the surrounding country appeared to me to be such that the depression cannot possibly have formed part of a continuous drainage-channel.

I do not know whether it has hitherto been suggested that Kufra Oasis may be the site of the Tortoise Marshes; but on correcting Ptolemy’s figures for the errors in his adopted position for Alexandria and in the length which he assumed for a degree of latitude, I find Kufra is in very much closer agreement with them than any of the places named above. In Ptolemy’s day, even the latitudes of but few places had been astronomically observed (Alexandria, where he himself resided, was supposed by Ptolemy to be in latitude 31°, instead of its true 32° 12′), and as the only method at that time known for astronomically determining differences of longitude was by the observation of eclipses, the number of observed longitudes was smaller still. The process by which Ptolemy deduced his position for the Tortoise Marshes and other places in the interior of Libya was most probably that of first estimating their distances south and west of Alexandria from travellers’ itineraries, then converting these distances into degrees of latitude and longitude, and finally subtracting the differences thus found from the latitude and longitude of Alexandria. But Ptolemy made the great mistake of assuming the length of a degree of latitude (or of equatorial longitude) to be 500 stades, instead of the 700 stades which it really is. Thus all his dead-reckonings resulted in differences of latitude and longitude which were too great in the proportion of seven to five. If we correct Ptolemy’s position for Alexandria, and his dead-reckoning for the erroneous assumption which he made regarding the size of the Earth, as follows:

Latitude. Longitude.

Ptolemy’s position for Alexandria 31° 60° 30′

Ptolemy’s position for the Tortoise Marshes 20° 49°

Ptolemy’s difference of lat. and long. 11° 11° 30′

These differences reduced in the proportion of 5 to 7 become, in true degrees and minutes 7° 52′ 8° 12′

The true position of Alexandria is 32° 12′ 29° 53′

Whence the corrected position for the marshes in our modern system becomes 24° 20′ 21° 41′

we get lat. 24° 20′ and long. 21° 41′ for the Tortoise Marshes. Comparing this position with that of Kufra, we find that the latitude agrees very closely with the 24° 14′ which Hassanein observed at Taj, the principal village of that oasis; while the longitude, though more than a degree and a half west of that of the principal village, is almost exactly correct for Taiserbo, the north-western oasis of the Kufra group.

Besides this remarkably close agreement as regards position, Kufra presents several natural features which would tend to support the view that it may be the locality which Ptolemy meant by the “Tortoise Marshes.” Not only is Kufra an extensive tract of relatively low-lying ground with numerous lakes and salt-marshes of very considerable size, but it has distinctly the form of a valley. Hassanein Bey repeatedly refers to it as a valley in his description[43]; Mrs. Forbes also speaks of the “Wadi of Kufra,” and mentions that as a result of a ride westward from Taj the “wadi” was found to have no definite ending to the west.[44] What more natural, therefore, than that some ancient traveller should have imagined that Kufra was a series of marshes formed by a river coming from the south-west? And is it not possible that Ptolemy, in endeavouring to piece together the scraps of information he could get, may have mistakenly inferred that this river ultimately reached the Mediterranean, and also have confused the account of it with those of other streams further south, which may have been branches of the Niger?