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OCTOBER, 1884.]
PTOLEMY'S GEOGRAPHY.
315
correction both as to the method of delineating the sphere on a plane surface, and as to the com. putation of distances, which he generally exaggerated. He censures him likewise for having assigned to the known world too great a length from west to east, and too great a breadth from north to south.
Of Ptolemy's own system, the more prominent characteristics may now be noted. He assumed the earth to be a sphere, and adopting the estimate of Poseidônios fixed its circumference at 180,000 stadia, thus making the length of a degree at the equator to be only 500 stadia, instead of 600, which is its real length. To this fundamental miscalculation may be referred not a few of the most serious errors to be found in his work. With regard to the question of the length and the breadth of the inhabited part of the earth, a question of first importance in those days, he estimated its length as measured along the parallel of Rhodes which divided the then known world into two nearly equal portions at 72,000 stadia, and its breadth at 10,000. The meridian in the west from which he calculated his longitudes was that which passed through the Islands of the Bleet (Μακάρων Νήσοι) probably the Canary Islands, and his most eastern meridian was that which passed through the Metropolis or the Sinai, which he calls Sinai or Thinai, and places in 180° 40 E. Long. and 3° S. Lat. The distance of this meridian from that of Alexandria he estimated at 119degrees, and the distance of the first meridian from the same at 60 degrees, making together 180 de. grees, or exactly one-half of the circumference of the earth. His estimate of the breadth he obtained by fixing the southern limit of the inhabited parts in the parallel of 161 degrees of South Latitude, which passes through a point as far south of the Equator as Meroë is north of it. And by fixing the northern limit in the parallel of 63 degrees North Latitude, which passes through Thoulé (probably the Shetland Islands), a space of nearly 80 degrees was thus included between the two parallels, and this was equivalent in Ptolemy's mode of reckoning to 40,000 stadia.
| Having made these determinations he had next
to consider in what mode the surface of the earth with its meridians of longitude and parallels of latitude should be represented on a sphere and on a plane surface-of the two modes of delineation that on the sphere is the much easier to make, as it involves no method of projection, but a map drawn on a plane is far more convenient for use, as it presents simultaneously to the eye a far greater extent of surface. Marinos had drawn his map of the world on a plane, but his method of projection was altogether unsatisfactory. It is thus described by Ptolemy: Marinos, he says, on account of the importance of the countries around the Mediterranean, kept as his base the line fixed on of old by Eratosthenes, vix, the parallel through Rhodes in the 36th degree of north latitude. He then calculated the length of a degree along this parallel, and found it to contain 400 stadia, the equatorial degree being taken at 500. Having divided this parallel into degrees he drew perpendiculars through the points of division for the meridians, and his parallels of latitude were straight lines parallel to that which passed through Rhodes. The imperfections of such a projection are obvious. It represented the parts of the earth north of the parallel of Rhodes much beyond, and those south of it much below, their proper length. Places again to the north of the line stood too far apart from each other, and those to the south of it too close together. The projection, moreover, is an erroneous representation, since the parallels of latitude ought to be circular arcs and not straight lines.
Ptolemy having pointed out these objections to the system of Marinos proceeds to explain the methods which he himself employed. We need say nothing more regarding them than that they were such as presented a near approximation to some of those which are still in use among modern Geographers.
Ptolemy's treatise is divided into 8 books. In the 1st or introductory book he treats first of Geography generally-he then explains and and Rhodes, which had been determined by direct observation."-Bunbury, Hist. of Anc. Geog., vol. II,
• The Island of Ferro-the westernmost of the Group of the Canaries, which was long taken as the prime meridian, and is still so taken in Germany-is really situated 18° 20' west of Greenwich, while Cape St. Vincent (called anciently the sacred Cape) is just about 9.
o that the real difference between the two amounted to 9° 20' instead of only 21'. Two corrections must there. fore be applied to Ptolemy's longitudes-one-sixth must be deducted because of his under-estimate of the length of a degree along the Equator, and 6° 50' must be added beOse Forro was so much further west than he supposed. Subject to these corrections his longitudes would be fairly accurate, provided his caloulations of distances were otherwise free from error.
• The Olympic stadium, which was in general use throughout Groeco, contained 600 Greek feet, which were equal to 625 Roman feet, or 6061 English feet. The Roman mile contained 8 stadia, or about hall #stadium less than an English mile. A stadium of 600 Greek feet was very nearly the 600th part of a degree, and 10 stadia are therefore just about equal to a Nautical or Geographical mile. According to Eratosthenes, a degree at the Equator Was equal to 700 stadia, but according to Poseidônios it was equal to only 500. The truth lay between, but Ptolemy unfortunately followed Poseidonios in his error.
3 "The equinoctial line was of course porfestly fixed and definite in Ptolemy's mind, as an astronomical line; but he had no means of assigning its position on the Map of the World, except with reference to other parallele, such as the tropic at Syene, or the parallels of Aletandria
