[39] Thesprotis, in Epirus, opposite Corfu.
[40] Afterwards named Temsa. This town was in Citerior Calabria. Some think Torre de Nocera stands on the ancient site.
[41] This is a misstatement, as before remarked.
[42] This writer occupies so prominent a position in Strabo’s work, that no apology I think will be needed for the following extract from Smith’s dictionary of Greek and Roman Biography and Mythology.
“Eratosthenes of Cyrene was, according to Suidas, the son of Aglaus, according to others, the son of Ambrosius, and was born B. C. 276. He was taught by Ariston of Chius, the philosopher, Lysanias of Cyrene, the grammarian, and Callimachus, the poet. He left Athens at the invitation of Ptolemy Euergetes, who placed him over the library at Alexandria. Here he continued till the reign of Ptolemy Epiphanes. He died at the age of eighty, about B. C. 196, of voluntary starvation, having lost his sight, and being tired of life. He was a man of very extensive learning: we shall first speak of him as a geometer and astronomer.
“It is supposed that Eratosthenes suggested to Ptolemy Euergetes the construction of the large armillæ, or fixed circular instruments, which were long in use at Alexandria; but only because it is difficult to imagine to whom else they are to be assigned, for Ptolemy the astronomer, though he mentions them, and incidentally their antiquity, does not state to whom they were due. In these circles each degree was divided into six parts. We know of no observations of Eratosthenes in which they were probably employed, except those which led him to the obliquity of the ecliptic, which he must have made to be 23° 51′ 20″, for he states the distance of the tropics to be eleven times the eighty-third part of the circumference. This was a good observation for the times. Ptolemy the astronomer was content with it, and according to him Hipparchus used no other. Of his measure of the earth we shall presently speak. According to Nicomachus, he was the inventor of the κόσκινον, or Cribrum Arithmeticum, as it has since been called, being the well-known method of detecting the prime numbers by writing down all odd numbers which do not end with 5, and striking out successively the multiples of each, one after the other, so that only prime numbers remain.
“We still possess under the name of Eratosthenes a work, entitled Καταστερισμοί, giving a slight account of the constellations, their fabulous history, and the stars in them. It is however acknowledged on all hands that this is not a work of Eratosthenes.... The only other writing of Eratosthenes which remains, is a letter to Ptolemy on the duplication of the cube, for the mechanical performance of which he had contrived an instrument, of which he seems to contemplate actual use in measuring the contents of vessels, &c. He seems to say that he has had his method engraved in some temple or public building, with some verses, which he adds. Eutocius has preserved this letter in his comment on book ii. prop. 2, of the sphere and cylinder of Archimedes.
“The greatest work of Eratosthenes, and that which must always make his name conspicuous in scientific history, is the attempt which he made to measure the magnitude of the earth, in which he brought forward and used the method which is employed to this day. Whether or no he was successful cannot be told, as we shall see; but it is not the less true that he was the originator of the process by which we now know, very nearly indeed, the magnitude of our own planet. Delambre says that if it were he who advised the erection of the circular instruments above alluded to, he must be considered as the founder of astronomy: to which it may be added, that he was the founder of geodesy without any if in the case. The number of ancient writers who have alluded to this remarkable operation (which seems to have obtained its full measure of fame) is very great, and we shall not attempt to combine their remarks or surmises: it is enough to say that the most distinct account, and one of the earliest, is found in the remaining work of Cleomedes.
“At Syene in Upper Egypt, which is supposed to be the same as, or near to, the town of Assouan, (Lat. 24° 10′ N., Long. 32° 59′ E. of Greenwich,) Eratosthenes was told (that he observed is very doubtful) that deep wells were enlightened to the bottom on the day of the summer solstice, and that vertical objects cast no shadows. He concluded therefore, that Syene was on the tropic, and its latitude equal to the obliquity of the ecliptic, which, as we have seen, he had determined: he presumed that it was in the same longitude as Alexandria, in which he was out about 3°, which is not enough to produce what would at that time have been a sensible error. By observations made at Alexandria, he determined the zenith of that place to be distant by the fiftieth part of the circumference from the solstice, which was equivalent to saying that the arc of the meridian between the two places is 7° 12´. Cleomedes says that he used the σκάφη, or hemispherical dial of Berosus, in the determination of this latitude. Delambre rejects the idea with infinite scorn, and pronounces Cleomedes unworthy of credit; and indeed it is not easy to see why Eratosthenes should have rejected the gnomon and the large circular instruments, unless, perhaps, for the following reason. There is a sentiment of Cleomedes which seems to imply that the disappearance of the shadows at Syene on the day of the summer solstice was noticed to take place for 300 stadia every way round Syene. If Eratosthenes took his report about the phenomenon (and we have no evidence that he went to Syene himself) from those who could give no better account than this, we may easily understand why he would think the σκάφη quite accurate enough to observe with at his own end of the arc, since the other end of it was uncertain by as much as 300 stadia. He gives 500 stadia for the distance from Alexandria to Syene, and this round number seems further to justify us in concluding that he thought the process to be as rough as in truth it was. Martianus Capella states that he obtained this distance from the measures made by order of the Ptolemies (which had been commenced by Alexander): this writer then implies that Eratosthenes did not go to Syene himself.
“The result is 250,000 stadia for the circumference of the earth, which Eratosthenes altered into 252,000, that his result might give an exact number of stadia for the degree, namely, 700; this of course should have been 694-4/9. Pliny calls this 31,500 Roman miles, and therefore supposes the stadium to be the eighth part of a Roman mile, or takes for granted that Eratosthenes used the Olympic stadium. It is likely enough that the Ptolemies naturalized this stadium in Egypt; but nevertheless, it is not unlikely that an Egyptian stadium was employed. If we assume the Olympic stadium, (202¼ yards,) the degree of Eratosthenes is more than 79 miles, upwards of 10 miles too great. Nothing is known of any Egyptian stadium. Pliny asserts that Hipparchus, but for what reason he does not say, wanted to add 25,000 stadia to the circumference as found by Eratosthenes. According to Plutarch, Eratosthenes made the sun to be 804 millions of stadia from the earth, and the moon 780,000. According to Macrobius, he made the diameter of the sun to be 27 times that of the earth. With regard to the other merits of Eratosthenes, we must first of all mention what he did for geography, which was closely connected with his mathematical pursuits. It was Eratosthenes who raised geography to the rank of a science; for previous to his time it seems to have consisted, more or less, of a mass of information scattered in books of travel, descriptions of particular countries, and the like. All these treasures were accessible to Eratosthenes in the libraries of Alexandria; and he made the most profitable use of them, by collecting the scattered materials, and uniting them into an organic system of geography, in his comprehensive work entitled Γεωγραφικά, or as it is sometimes but erroneously called, γεωγραφούμενα or γεωγραφία. It consisted of three books, the first of which, forming a sort of Introduction, contained a critical review of the labours of his predecessors from the earliest to his own times, and investigations concerning the form and nature of the earth, which, according to him, was an immoveable globe, on the surface of which traces of a series of great revolutions were still visible. He conceived that in one of these revolutions the Mediterranean had acquired its present form; for according to him it was at one time a large lake covering portions of the adjacent countries of Asia and Libya, until a passage was forced open by which it entered into communication with the ocean in the west. The second book contained what is now called mathematical geography. His attempt to measure the magnitude of the earth has been spoken of above. The third book contained the political geography, and gave descriptions of the various countries, derived from the works of earlier travellers and geographers. In order to be able to determine the accurate site of each place, he drew a line parallel with the equator, running from the Pillars of Hercules to the extreme east of Asia, and dividing the whole of the inhabited earth into two halves. Connected with this work was a new map of the earth, in which towns, mountains, rivers, lakes, and climates were marked according to his own improved measurements. This important work of Eratosthenes forms an epoch in the history of ancient geography; but unfortunately it is lost, and all that has survived consists in fragments quoted by later geographers and historians, such as Polybius, Strabo, Marcianus, Pliny, and others, who often judge of him unfavourably, and controvert his statements; while it can be proved that in a great many passages they adopt his opinions without mentioning his name. Marcianus charges Eratosthenes with having copied the substance of the work of Timosthenes on Ports, (περὶ λιμένων,) to which he added but very little of his own. This charge may be well-founded, but cannot have diminished the value of the work of Eratosthenes, in which that of Timosthenes can have formed only a very small portion. It seems to have been the very overwhelming importance of the geography of Eratosthenes, that called forth a number of opponents, among whom we meet with the names of Polemon, Hipparchus, Polybius, Serapion, and Marcianus of Heracleia.... Another work of a somewhat similar nature, entitled Ἑρμῆς, was written in verse, and treated of the form of the earth, its temperature, the different zones, the constellations, and the like.... Eratosthenes distinguished himself also as a philosopher, historian, grammarian, &c.”