424 



NATURE 



[September 3, 189] 



clearness and perspicuity not attainable by any other metho \. 

 You will not, therefore, think it strange if I deal at considerable 

 length with the development of cartography, more especially as 

 my own labours have in a large measure been devoted to that 

 department of geographical work. An inspection of the interest- 

 ing collection of maps of all ages which I am able to place 

 before you will serve to illustrate what I am about to say on this 

 subject. . . . 



Ptolemy, like all great reformers, stood upon the shoulders of 

 the men who had preceded him, for before a map like his could 

 be produced much preliminary work had been accomplished. 

 Parmenides of Elea (460 B.C.) had demonstrated that our earth 

 was a globe, and Eratosthenes (276-196 B c.) had approxi- 

 mately determined its size. Hipparchus (190-120), the greatest 

 astronomer of antiquity, the discoverer of the precession of the 

 equinoxes, and the author of a catalogue of stars, had transferred 

 to our earth the auxiliary lines drawn by him across the heavens. 

 He had taught cartographers to lay down places according to 

 their latitude and longitude, and how to project a sphere upon a 

 plane. It is to him we are indebted for the stereographic and 

 orthographic projections of the sphere. Ptolemy himself in- 

 vented the tangential conical projection. 



The gnomon or sun-dial, an instrument known to the Chinese 

 600 years before Christ, had long been used for the determina- 

 tion of latitudes, and the results were relatively correct, although 

 uniformly subject to an error of 16 minutes, which was due to 

 the observers taking the altitude of the upper limb of the sun, 

 when measuring the shadow cast by their dial, instead of that of 

 the sun's centre. 



It was known, likewise, that differences of longitude could be 

 determined by the simultaneous observation of eclipses of the 

 sun or moon, or of occultations of stars, and Hipparchus actually 

 calculated ephemerides for six years in advance to facilitate 

 computations. • Ptolemy himself suggested the use of lunar 

 distances. But so imperfect were the astrolabes and other 

 instruments used by the ancient astronomers, and especially 

 their time-keepers, that precise results are quite out of the 

 question. 



Ptolemy, in fact, contented himself with accepting eight lati- 

 tudes determined by actual observation, of which four were in 

 Egypt, whilst of the three longitudes known to him he only 

 utilized one in the construction of his map. Unfortunately, the 

 one selected proved the least accurate, being erroneous to the 

 extent of 32 per cent., whilst the error of the two which he 

 rejected did not exceed 13 per cent.^ This want of judgment — 

 pardonable, no doubt, under the circumstances— vitiated 

 Ptolemy's delineation of the Mediterranean to a most deplorable 

 extent, far more so than did his assumption that a degree 

 only measured five hundred stades, when in reality it measures 

 six hundred. For whilst the breadth of his Mediterranean, 

 being dependent upon the relatively correct latitudes of Alex- 

 andria, Rhodes, Rome, and Massilia, fairly approximates the 

 truth, its length is exaggerated to the extent of nearly 50 per 

 cent., measuring 62° instead of 41° 40'. This capital error of 

 Ptolemy is due therefore to the unfortunate acceptance of an 

 incorrect longitude, quite as much as to an exagL;eration of 

 itinerary distances. It is probable that Ptolemy would have 

 presented us with a fairer likeness of our great inland sea had he 

 rejected observed latitudes and longitudes altogether, and 

 trusted exclusively to itineraries and to such bearings as the 

 mariners of the period could have supplied him with. 



No copy of Ptolemy's original set of maps has reached us, for 

 the maps drawn by Agathodjemon in the fifth century are, under 

 the most favourable circumstances, merely reductions of Ptolemy's 

 originals, or they are compiled from Ptolemy's "Geography," 

 which, apart from a few explanatory chapters, consists almost 

 wholly of lists of places, with their latitudes and longitudes. I 

 am almost inclined to adopt the latter view— firstly, because of 

 the very crude delineation of Egypt, for which country an accu- 

 rate cadastral survey was available ; and secondly, on account 

 of the cylindrical projection on which these maps are drawn, 

 although from Ptolemy's own statements we are justified in 

 believing that he made use of a conical projection in the con- 

 struction of his maps. 



' The three longiiudes are the foHowi g : — 



Result of ancient Adopted by Actual differ- 



observations. Pt.lemy. ence of longitude. 



Arbela ... 45° E. of Carthage ... 45° ... 34° 



Babylon ... 12" 30' E. ot Alexandria ... J 8° 30' ... 14° 18' 



•Home ... 20" E. of Alexandria ... 23' 50' ... 17° 24' 



NO. I 1 40, VOL. 44] 



An examination of Ptolemy's maps shows very clearly that 

 they were almost wholly compiled from itineraries, the greater 

 number of which their author borrowed from his predecessor 

 Marinus. It shows, too, that Ptolemy's critical acumen as a 

 compiler cannot be rated very high, and that he failed to utilize 

 much information of a geographical nature which was available 

 in his day. His great merit consisted in having taught carto- 

 graphers to construct their maps according to a scientific method. 

 This lesson, however, they were slow to learn, and centuries 

 elapsed before they once more advanced along the only correct 

 path, which Ptolemy had been the first to tread. 



During the "Dark Ages" which followed the dismember- 

 ment of the Roman Empire there was no lack of maps, but they 

 Were utterly worthless from a scientific point of view. The 

 achievements of the ancients were ignored, and the principal 

 aim of the map-makers of the period appears to have been to 

 reconcile their handiwork with the orthodox interpretation of 

 the Holy Scriptures. Hence those numerous "wheel maps," 

 upon which Jerusalem is made to represent the hub, whilst the 

 western half of the disk is assigned to Europe and Africa, and 

 the eastern to Asia. 



As it is not my intention to introduce you to the acchjeo- 

 logical curiosities of an uncritical age, but to give you some 

 idea of the progress of cartography, I at once pass on to the 

 Arabs, 



The Arabs were great as travellers, greater still as astronomers, 

 but contemptible as cartographers. Their astronomers, fully 

 possessed of the knowledge of Ptolemy, discovered the error of 

 the gnomon ; they improved the instruments which they had in- 

 herited from the ancients, and carefully fixed the latitudes of 

 quite a number of places. Zarkala, the Director of the Observa- 

 tory of Toledo, even attempted to determine the difference of 

 longitude between that place and Bagdad ; and if his result 

 differed to the extent of 3° from the truth, it nevertheless 

 proved a great advance upon Ptolemy, whose map exhibits 

 an error amounting to 18°. Had there existed a scientific 

 cartographer among the Arabs, he would have been able, 

 with the aid of these observations, and of the estimates of 

 distances made by careful ob.-ervers like Abul Hasan, to effect 

 most material corrections in the map of the known world. 

 If Edrisi's map (1154) is better than that of others of his 

 Arab contemporaries, this is simply due to his residence at 

 Palermo, where he was able to avail himself of the knowledge 

 of the Italians. 



Quite a new epoch in the history of cartography begins with 

 the introduction of the magnetic needle into Europe. Hitherto 

 the seaman had governed his course by the observation of the 

 heavens ; thenceforth an instrument was placed in his hands 

 which made him independent of the state of the sky. The 

 property of the magnet or "loadstone" to point to the north 

 first became known in the eleventh century, and in the time of 

 Alexander Neckam (1185) it was already poised upon a pivot. 

 It was, however, only alter Flavio Gioja of Amalfi (1302) had 

 attached to it a compass-card, exhibiting the direction of the 

 winds, that it became of such immediate importance to the 

 mariner. It is only natural that the Italians, who were the 

 foremost seamen of that age, should have been the first to avail 

 themselves of this new help to navigation. At quite an early 

 date, as early probably as the twelfth century, they made use of 

 it for their maritime surveys, and in course of time they pro- 

 duced a series of charts upon which the coasts frequented by 

 them, from the recesses of the Black Sea to the mouth of the 

 Rhine, are delineated for the first time with suprising fidelity to 

 nature. The appearance of these so-called compass- charts, 

 with gaily coloured roses of the winds and a bewildering number 

 of rhumb-lines, is quite unmistakable. A little consideration will 

 show you that if the variation of the compass had been taken into 

 account in the construction of these charts, they would actually 

 have developed into a picture of the world on Mercator's projec- 

 tion. But to deny them all scientific value because they do not 

 fulfil this condition, is going too far. As correct delineations of 

 the contours of the land they were a great advance upon Ptolemy's 

 maps, and it redounds little to the credit of the "learned" 

 geographers of a later time that they rejected the information so 

 laboriously collected and skilfully combined by the chart makers, 

 and returned to the deformities of Ptolemy. The adjustment of 

 these charts to positions ascertained by astronomical observations 

 could have been easily effected. An inspection of my diagrams 



