The reign of Ptolemy Euergetes was equally distinguished, with, those of his predecessors, by attention to commerce, and a desire to extend it. As the navigation of the Red Sea had now become a source of great wealth to his subjects, he deemed it necessary to free it as much as possible from the pirates that infested it's coasts; for this purpose, as well as to preserve a communication between Egypt and the countries which extended to its mouth, he established governors from the isthmus of Suez, along the Arabian and African coasts, as far as the straits of Babelmandeb; and planted colonies of Greeks and Egyptians to carry on the commerce, and protect the interests of his subjects. But the most extraordinary instance of his enterprising spirit is to be found in his conquest (evidently for the purpose of facilitating and securing the commerce of the Red Sea) of part of Abyssinia. The proof of this, indeed, rests entirely on an inscription found at Aduli, which there can be no doubt is the harbour and bay of Masuah; the only proper entrance, according to Bruce, into Abyssinia. The inscription to which we have alluded was extant in the time of Cosmas (A.D. 545), by whom it was seen. From it, Ptolemy appears to have passed to the Tacazze, which he calls the Nile, and to have penetrated into Gojam, in which province the fountains of the Nile are found. He made roads, opened a communication between this country and Egypt, and during this expedition obliged the Arabians to pay tribute, and to maintain the roads free from robbers and the sea from pirates; subduing the whole coast from [Leucke->Leuke] Come to Sabea. The inscription adds: "In the accomplishment of this business I had no example to follow, either of the ancient kings of Egypt, or of my own family; but was the first to conceive the design, and to carry it into execution. Thus, having reduced the whole world to peace under my own authority, I came down to Aduli, and sacrificed to Jupiter, to Mars, and to Neptune, imploring his protection for all who navigate these seas."
Ptolemy Euergetes was particularly attentive to the interests of the library at Alexandria. The first librarian appointed by Ptolemy the successor of Alexander, was Zenodotus; on his death, Ptolemy Euergetes invited from Athens Eratosthenes, a citizen of Cyrene, and entrusted to him the care of the library: it has been supposed that he was the second of that name, or of an inferior rank in learning and science, because he is sometimes called Beta; but by this appellation nothing else was meant, but that he was the second librarian of the royal library at Alexandria. He died at the age of 81, A.C. 194. He has been called a second Plato, the cosmographer and the geometer of the world: he is rather an astronomer and mathematician than a geographer, though geography is indebted to him for some improvements in its details, and more especially for helping to raise it to the accuracy and dignity of a science. By means of instruments, which Ptolemy erected in the museum at Alexandria, he ascertained the obliquity of the ecliptic to be 23° 51' 20". He is, however, principally celebrated as the first astronomer who measured a degree of a great circle, and thus approximated towards the real diameter of the earth.
The importance of this discovery will justify us in entering on some details respecting the means which this philosopher employed, and the result which he obtained.
It is uncertain whether the well at Syene, in Upper Egypt, which he used for this purpose, was dug by his directions, or existed previously. Pliny seems to be of the former opinion; but there is reason to believe that it had a much higher antiquity. The following observations on its structure by Dr. Horsley, Bishop of Rochester, are ingenious and important. "The well, besides that it was sunk perpendicularly, with the greatest accuracy, was, I suppose, in shape an exact cylinder. Its breadth must have been moderate, so that a person, standing upon the brink, might safely stoop enough over it to bring his eye into the axis of the cylinder, where it would be perpendicularly over the centre of the circular surface of the water. The water must have stood at a moderate, height below the mouth of the well, far enough below the mouth to be sheltered from the action of the wind, that its surface might be perfectly smooth and motionless; and not so low, but that the whole of its circular surface might be distinctly seen by the observer on the brink. A well formed in this manner would afford, as I apprehend, the most certain observation of the sun's appulse to the zenith, that could be made with the naked eye; for when the sun's centre was upon the zenith, his disc would be seen by reflection on the water, in the very middle of the well,--that is, as a circle perfectly concentric with the circle of the water; and, I believe, there is nothing of which the naked eye can judge with so much precision as the concentricity of two circles, provided the circles be neither very nearly equal, nor the inner circle very small in proportion to the outer."
Eratosthenes observed, that at the time of the summer solstice this well was completely illuminated by the sun, and hence he inferred that the sun was, at that time, in the zenith of this place. His next object was to ascertain the altitude of the sun, at the same solstice, and on the very same day, at Alexandria. This he effected by a very simple contrivance: he employed a concave hemisphere, with a vertical style, equal to the radius of concavity; and by means of this he ascertained that the arch, intercepted between the bottom of the style and the extreme point of its shadow, was 7° 12'. This, of course, indicated the distance of the sun from the zenith of Alexandria. But 7° 12' is equal to the fiftieth part of a great circle; and this, therefore, was the measure of the celestial arc contained between the zeniths of Syene and Alexandria. The measured distance between these cities being 5000 stadia, it followed, that 5000 X 50 = 250,000, was, according to the observations of Eratosthenes, the extent of the whole circumference of the earth.
If we knew exactly the length of the stadium of the ancients, or, to speak more accurately, what stadium is referred to in the accounts which have been transmitted to us of the result of the operations of Eratosthenes, (for the ancients employed different stadia,) we should be able precisely to ascertain the circumference which this philosopher ascribed to the earth, and also, whether a nearer approximation to the truth was made by any subsequent or prior ancient philosopher. The circumference of the earth was conjectured, or ascertained, by Aristotle, Cleomedes, Posidonius, and Ptolemy respectively, to be 400, 300, 240, and 180 thousand stadia. It is immediately apparent that these various measures have some relation to each other, and probably express the same extent; measured in different stadia; and this probability is greatly increased by comparing the real distances of several places with the ancient itinerary distances.
The observation of Eratosthenes respecting the obliquity of the ecliptic (though undoubtedly not so immediately or essentially connected with our subject as his observation of the circumference of the earth) is too important to be passed over entirely without notice. He found the distance between the tropics less than 53° 6', and greater than 52° 96', which gives a mean of 23° 51' for the obliquity of the ecliptic. The observations of Hipparchus (who flourished at Alexandria about 140 years before Christ, and whom we shall have occasion to mention more particularly afterwards) coincided with those of Eratosthenes. Plutarch, however, who died A.D. 119, informs us, that, in his time, the gnomons at Syene were no longer shadowless on the day of the summer solstice. As the interval between Eratosthenes and Plutarch was only about 512 years, Bishop Morsley has very naturally expressed his doubts of the accuracy of Plutarch's assertion. He says, that the change in the obliquity of the ecliptic in this interval was only 2' 36". "A gnomon, therefore, at Syene, of the length of twelve inches, if it cast no shadow on the day of the solstice in the time of Eratosthenes, should have cast a shadow in the time of Plutarch of the length only of 9/1000th, or not quite 1/100th part of an inch. The shadow of a perpendicular column of the height of 100 feet would have been 9/10ths of an inch." As, however, the ancients do not appear to have constructed gnomons of such a size, and as gnomons of inferior size would have given a shadow scarcely perceptible, it is probable that Plutarch is mistaken in his assertion; or, at any rate, that the very small variation which did take place between his time and that of Eratosthenes (if it were observed at all) was ascertained by means of the well itself, which would point it out much more distinctly and accurately than any gnomon the ancients can be supposed to have used.
We are also indebted to Eratosthenes for the first regular parallel of latitude, and also for tracing a meridian. His parallel of latitude began at the Straits of Gibraltar, and passed eastward through Rhodes to the mountains of India; the intermediate places being carefully set down. His meridian line passed through Rhodes and Alexandria, as far as Syene and Meroe. Meroe, on this account, became an object of the greatest interest and importance to all the succeeding ancient geographers and astronomers, and they have taken the utmost labour and care to ascertain its latitude accurately. Strabo informs us, that Eratosthenes constructed a map of the world; but he does not give such particulars as will enable us to trace the extent of his geographical knowledge. At the extremity of the world to the east, bounded by the ocean, Thina was placed in the map of Eratosthenes, in the parallel of Rhodes; a parallel which passes through the empire of China, within the Great Wall. Eratosthenes, according to Strabo, (to whom we are indebted for nearly all we know respecting this philosopher,) asserts that Thina had been, previously to the construction of his map, incorrectly placed in the more ancient maps. His information respecting Meroe or Abyssinia, is most probably derived from Dalion, Aristocreon, and Bion, who had been sent by Ptolemy Philadelphus and his successors into that country, or from Timosthenes, who sailed down the coast of Africa as low as Cerne. His information on the subject of India (which, however, as far as regards oriental commerce, is very confused) must have been derived from the Macedonians. There is little doubt that the library of Alexandria afforded him access to all the knowledge which then existed respecting the various countries of the globe; but the turn of his mind led him rather to astronomical than geographical studies; or rather, perhaps, he directed his labours and his talents to the discovery of the figure and circumference of the earth, thinking, that till this was effected, the delineation of the habitable world, and the relative position of different countries, must be very inaccurate as well as incomplete. This opinion regarding Eratosthenes, that he was more of a geometrician than a geographer, seems to be confirmed by the testimony of Marcian of Heraclia, who informs us, that Eratosthenes took the whole work of Timosthenes, preface and all, as it stood, and in the very same words. If this account be accurate, it is probable that Eratosthenes' knowledge of Thina, and his being able to correct the erroneous position of this country in more ancient maps, was derived from Timosthenes, who had commanded the fleet of Ptolemy Philadelphus on the Indian Ocean.
If we reflect on the rude and imperfect state of science at this period, the paucity and inadequacy of the instruments by means of which it might be improved, and the superstitions and prejudices which opposed the removal of error or the establishment of truth, we shall not be disposed to question the justice of the panegyric pronounced by Pliny on Eratosthenes. This author, after detailing all that was then known on the subject of the circumference of the earth, and on the distances which had been ascertained by actual admeasurement, or approximated by analogy or probable conjecture, between the most remarkable places on its surface, adds, that Eratosthenes, whose acuteness and application had advanced him far in every branch of knowledge, but who had outstripped all his predecessors or contemporaries in that particular branch which was connected with the admeasurement of the earth, had fixed its circumference at 250,000 stadia; a bold and almost presumptuous enterprize, (improbum ausum) but which had been conducted with so much judgment, and on such sound principles, that it commanded and deserved our credit. Hipparchus, who was distinguished for his correctness and diligence in every part of geometrical and astronomical science, and who had specially exerted those qualities in his endeavours to correct the errors of Eratosthenes, had been able to add only the comparatively small extent of 25,000 stadia to the computation of Eratosthenes.--Plin. Nat. Hist. lib. ii. c. 108.
Eratosthenes seems, from the nature of his studies, not to have availed himself so much as he might have done of the treasures contained in the Alexandrian library under his care, to correct or extend the geographical knowledge of his contemporaries. The same observation will not apply to Agatharcides, who was president of the library after Eratosthenes. The exact time at which he flourished is not known: according to Blair, he was contemporary with Eratosthenes, though younger than him, and flourished 177 A.C., Eratosthenes having died at the age of eighty-one, in the year 194 A.C. Dodwell, however, fixes him at a later period; viz. 104 A.C.; but this date must be erroneous, because Artemidorus of Ephesus, who evidently copies Agatharcides, undoubtedly lived 104 A.C. Agatharcide's was born at Cnidus in Caria: no particulars are known respecting him, except that he was president of the Alexandrian library, in the reign of Ptolemy Philometor, if he flourished 177 A.C.; and in the reign of Ptolemy Lathyrus, if, according to Dodwell, he did not flourish till 104 A.C.