Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/104

Rh 92 PTOLEMY means of determination are not available. The greater part of the treatise of Marinus was occupied with the discussion of these authorities, and it is impossible for us, in the absence of the original work, to determine how far he had succeeded in giving a scientific form to the results of his labours ; but we are told by Ptolemy himself that he considered them, on the whole, so satisfactory that he had made the work of his predecessor the basis of his own in regard to all the countries bordering on the Mediterranean, a term which would comprise to the ancient geographer almost all those regions of which he had really any definite know- ledge. With respect to the more remote regions of the world, Ptolemy availed himself of the information imparted by Marinus, but not without reserve, and has himself ex- plained to us the reasons that induced him in some instances to depart from the conclusions of his predecessor. It is very unjust to term Ptolemy a plagiarist from Marinus, as has been done by some modern authors, as he himself acknowledges in the fullest manner his obligations to that writer, from whom he derived the whole mass of his materials, which he undertook to arrange and present to his readers in a scientific form. It is this form and ar- rangement that constitute the great merit of Ptolemy's work and that have stamped it with a character wholly distinct from all previous treatises on geography. But at the same time it possesses much interest, as showing the greatly increased knowledge of the more remote por- tions of Asia and Africa which had been acquired by geo- graphers since the time of Strabo and Pliny. It will be convenient to consider separately the two different branches of the subject, (1) the mathematical portion, which constitutes his geographical system, properly so termed ; and (2) his contributions to the progress of positive knowledge with respect to the Inhabited World. See Plate 1. Mathematical Geography. As a great astronomer, Ptolemy VII., vol. was of course infinitely better qualified to comprehend and explain xv. the mathematical conditions of the earth and its relations to the celestial bodies that surround it than any preceding writers on the special subject of geography. But his general views, except oil a few points, did not differ from those of his most eminent precursors Eratosthenes and Strabo. In common with them, he assumed that the earth was a globe, the surface of which was divided by certain great circles the equator and the tropics parallel to one another, and dividing the earth into five great zones, the relations of which with astronomical phenomena were of course clear to his mind as a matter of theory, though in regard to the regions bordering on the equator, as well as to those ad- joining the polar circle, he could have had no confirmation of his conclusions from actual observation. He adopted also from Hip- parchus the division of the equator and other great circles into 360 parts or " degrees " (as they were subsequently called, though the word does not occur in this sense in Ptolemy), and supposed other circles to be drawn through these, from the equator to the pole, to which he gave the name of "meridians." He thus conceived the whole surface of the earth (as is done by modern geographers) to be covered with a complete network of "parallels of latitude" and "meridians of longitude," terms which he himself was the first ex- tant writer to employ in this technical sense. Within the network thus constructed it was the task of the scientific geographer to place the outline of the world, so far as it was then known by experience and observation. Unfortunately at the very outset of his attempt to realize this conception he fell into an error which had the effect of vitiating all his subsequent conclusions. Eratosthenes was the first writer who had attempted in a scientific manner to determine the cir- cumference of the earth, and the result at which he arrived, that it amounted to 250,000 stadia or 25,000 geographical miles, was generally adopted by subsequent geographers, including Strabo. Posidonius, however, who wrote about a century after Eratosthenes, had made an independent calculation, by which he reduced the circumference of the globe to 180,000 stadia, or less than three- fourths of the result obtained by Eratosthenes, and this computa- tion, on what grounds we know not, was unfortunately adopted by Marinus Tyrius, and from him by Ptolemy. The consequence of this error was of course to make every degree of latitude or longi- tude (measured at the equator) equal to only 500 stadia (50 geo- graphical miles), instead of its true equivalent of 600 stadia. Its effects would indeed have been in some measure neutralized had there existed a sufficient number of points of which the position was determined by actual observation ; but we learn from Ptolemy himself that this was not the case, and that such observations for latitude were very few in number, while the means of determining longitudes were almost wholly wanting. l Hence the positions laid down by him were really, with very few exceptions, the result of computations of distances from itineraries and the statements of travellers, estimates which were liable to much greater error in ancient times than at the present day, from the want of any accurate mode of observing bearings, or portable instruments for the measure- ment of time, while they had no means at all of determining dis- tances at sea, except by the rough estimate of the time employed in sailing from point to point. The use of the log, simple as it appears to us, was unknown to the ancients. But, great as would naturally be the errors resulting from such imperfect means of cal- culation, they were in most cases increased by the permanent error arising from the erroneous system of graduation adopted by Ptolemy in laying them down upon his map. Thus, if he had arrived at the conclusion from itineraries that two places were 5000 stadia from one another, he would place them at a distance of 10 apart, and thus in fact separate them by an interval of 6000 stadia. Another source of permanent error (though one of much less im- portance) which affected all his longitudes arose from the errone- ous assumption of his prime meridian. In this respect also he followed Marinus, who, having arrived at the conclusion that the Fortunate Islands (the Canaries) were situated farther west than any part of the continent of Europe, had taken the meridian through the outermost of this group as his prime meridian, from whence he calculated all his longitudes eastwards to the Indian Ocean. But, as both Marinus and Ptolemy were very imperfectly acquainted with the position and arrangements of the islands in question, the line thus assumed was in reality a purely imaginary one, being drawn through the supposed position of the outer island, which they placed 2^ west of the Sacred Promontory (Cape St Vincent), which was regarded by Marinus and Ptolemy, as it had been by all previous geographers, as the westernmost point of the continent of Europe, while the real difference between the two is not less than 9 20'. Hence all Ptolemy's longitudes, reckoned east- wards from this assumed line, were in fact about 7 less than they would have been if really measured from the meridian of Ferro, which continued so long in use among geographers in modern, times. The error iu this instance was the more unfortunate as the longitude could not of course be really measured, or even calculated, from this imaginary line, but was in reality calculated in both directions from Alexandria, westwards as well as eastwards (as Ptolemy himself has done in his eighth book) and afterwards re- versed, so as to suit the supposed method of computation. It must be observed also that the equator was in like manner placed by Ptolemy at a considerable distance from its true geo- graphical position. The place of the equinoctial line on the sur- face of the globe was of course well known to him as a matter of theory, but as no observations could have been made in those remote regions he could only calculate its place from that of the tropic, which he supposed to pass through Syene. And as he here, as elsewhere, reckoned a degree of latitude as equivalent to 500 stadia, he inevitably made the interval between the tropic and the equator too small by one-sixth ; and the place of the former on the surface of the earth being fixed by observation he necessarily carried up the supposed place of the equator too high by more than 230 geographical miles. But as he had practically no geographical acquaintance with the equinoctial regions of the earth this error was of little importance. With Marinus and Ptolemy, as with all preceding Greek geo- graphers, the most important line on the surface of the globe for all practical purposes was the parallel of 36 of latitude, which passes through the Straits of Gibraltar at one end of the Mediter- ranean, and through the Island of Rhodes and the Gulf of Issus at the other. It was thus regarded by Dicsearchus and almost all his successors as dividing the regions around the inland sea into two portions, and as being continued in theory along the chain of Mount Taurus till it joined the great mountain range north of India ; and from thence to the Eastern Ocean it was regarded as constituting the dividing line of the Inhabited World, along which its length must be measured. But it sufficiently shows how inaccurate were the observations and how imperfect the materials at his command, even in regard to the best known portions of the earth, that Ptolemy, following Marinus, describes this parallel as passing through Caralis in Sardinia and Lilybseum in Sicily, the one being really in 39 12' lat., the other in 37 50'. It is still more strange that he places so important and well known a city as Carthage 1 20' south of the dividing parallel, while it really lies nearly 1 to the north of it. 1 Hipparchus had indeed pointed out long before the mode of de- termining longitudes by observations of eclipses, but the instance to which he referred of the celebrated eclipse before the battle of Arbela, which was seen also at Carthage, was a mere matter of popular obser- vation, of no scientific value. Yet Ptolemy seems to have known of no other. -