March 26, 1896] 



NATURE 



489 



prived of his dominions by Sargon, the greatest of the 

 Assyrian monarch s. 



Ptolemy describes his method of attempting to deter- 

 mine the distances of the moon and of the sun. To the 

 former he obtained a very fair approximation, making it 

 equal to about fifty-nine semidiameters of the earth ; but 

 the sun's distance he thought to be only about 1210 

 of these semidiameters, or about twenty times the dis- 

 tance of the moon, which is in that unit only about the 

 square root of its true amount. The eighth book of the 

 Almagest contains the earliest extant catalogue of stars, 

 founded upon that of Hipparchus. Six stars are marked 

 red or reddish, one of which, Sirius, has ceased to be so, 

 furnishing a remarkable instance of change of colour ; the 

 reading in Ptolemy was contested, but there is no real 

 reason (as has been pointed out by Dr. See) to doubt its 

 genuineness, and the red colour of Sirius in ancient times 

 is confirmed by the testimony of several classical writers. 



We must now turn from Ptolemy as an astronomer to 

 Ptolemy as a geographer. If his work in the former de- 

 partment is founded principally upon that of Hipparchus, 

 so does he take for the basis of his geographical system 

 the work of Marinus the Tyrian, which in its latest 

 form but little preceded his own. Those only who 

 are ignorant of both can accuse Ptolemy of plagiarism 

 in this, seeing how fully he recognises his obligations to 

 his predecessor whilst pointing out the necessity of modi- 

 fying some of his conclusions with regard to the most 

 distant known regions of the world. But for Ptolemy, 

 indeed, we, in these days, should never have heard of 

 Marinus at all. " It is clear," says the late Sir E. H. 

 Bunbury, "that he did not attempt to present his 

 readers with a complete body of descriptive geo- 

 graphy such as was furnished by the compre- 

 hensive work of Strabo. His object, like that of 

 Eratosthenes at an earlier period, was simply to correct 

 and reform the map of the world so as to adapt it both 

 to the increased knowledge of distant countries and to 

 the improved state of mathematical science, which were 

 possessed in his day." Much more was then known 

 than in the time of preceding geographers of the extent 

 of Africa towards the south and of Asia towards the 

 east ; but, in applying this increased knowledge, Marinus 

 exaggerated the extensions so greatly as to distort his 

 map of the world almost as much as theirs, though in the 

 opposite way. This led him to what was, to some extent, 

 a retrograde step — the idea that Asia had an indefinite 

 e.xtension towards the east, similar to that entertained by 

 Columbus when he expected to reach the Indies by a 

 voyage to the west, little dreaming of another isolated 

 continent between. 



Ptolemy refers to his astronomical in his geographical 

 work, so that the latter must have been composed subse- 

 quently to the former, and its date was probably near the 

 end of the reign of Antoninus Pius, who died in A.D. 

 161. He fully appreciated the necessity, if positions in 

 the world were to be accurately laid down, of determining 

 their latitudes and longitudes and mapping them thereby. 

 But, unfortunately, in his time the number of places for 

 which this had been done was so small that he was 

 obliged, to a ver>' great extent, to rely upon results 

 obtained from itineraries by the old rough method. 

 These he places before us, both on his map and in exten- 

 sive tables, as if they had been really founded upon scienti- 

 fic determinations. To quote Bunbury again : " He saw 

 clearly the true principles upon which geography should 

 be based, and the true mode in which a map should be 

 constructed. But the means at his command did not 

 enable him to carry his ideas into execution ; the sub- 

 stance did not correspond to the form, and the 

 specious edifice that he reared served, by its external 

 symmetry, to conceal the imperfect character of its 

 foundations and the rottenness of its materials." Some 

 of the exaggerated conclusions of Marinus, particularly 



NO. 1378, VOL. 53] 



with reference to the distances of places in the east 

 of Asia, he rejects, but can only suggest conjectural 

 reductions of them. But even in lands within the bounds 

 of the Roman Empire, few indeed were the places of 

 which even the latitudes, still less the longitudes, had 

 been scientifically determined. Hipparchus had sug- 

 gested the observation of lunar eclipses at different 

 stations as a means of finding the difference of longitude 

 of these stations ; but even in the time of Ptolemy, no 

 such determinations had been actually made, though he 

 refers to one which took place on September 20, B.C. 331, 

 shortly before the battle of Arbela, or rather Gaugamela, 

 which was observed, but not with sufficient accuracy to 

 make it the basis of calculation. In inquiries of this 

 nature, a remark made by the late Sir George Airy often 

 comes into one's mind : " The first man who made good 

 astronomical observations was the first man who made 

 good clocks" — a graphic way of saying how essential 

 was an accurate means of measuring time. 



We ha\e reserved but a short space to speak of what 

 may be called Ptolemy's minor works. The principal of 

 these are his Tetrabiblos or Quadripartite, which is in 

 fact a treatise on astrology in the modern sense of the 

 word, and his Harmonics, in which he gives an account 

 of the theory of music. The former (as well as the 

 Centiloquy, or hundred aphorisms, which forms a sort of 

 supplement or summary to it) has been translated into 

 English, the last time by J. M. Ashmand, whose version 

 was published in 1822, and dedicated (like the Prince 

 Regent's famous bumper in Scott's presence) to the 

 author of Wa\erley. There does not appear any good 

 ground for doubting its genuineness, though many have 

 wished to do so from their admiration of Ptolemy, and 

 feeling that it was unworthy of him. Great astronomers, 

 howe\er, in later times than that of Ptolemy, have 

 believed in the delusive and imaginary "science" of 

 judicial astrology, which still serves to charm some of 

 the ignorant and foolish, and e.xcite apprehensions in 

 others of that still large family. The translator of the 

 Tetrabiblos (who appeared before the public only in that 

 capacity) had no feeling of this kind, and endeavours in 

 a note to parry an objection to astrological predictions as 

 old as the time of Cicero by citing a case of a man who, he 

 tells us, was born within a few moments of George III., 

 and in the same parish, went into business in the 

 same month (October 1760) in which that king came 

 to the throne, was married, like him, on September 

 8, 1761, and died, like him, on January 29, 1820, 

 "coincidences," we are told, "highly remarkable." 

 However, if any one can derive amusement from astro- 

 logy? "e need not object. Flamsteed drew the horoscope 

 of the Royal Observatory, Greenwich, at the moment of 

 its foundation, though he aflixed a line from Horace ask- 

 ing who could forbear laughter. So we may take a few 

 of Ptolemy's notions, one of which was that natural 

 characteristics in different regions and climates was 

 caused by the planets and constellations holding sway 

 there, whence the inhabitants of Britain, for instance 

 (and let us remember that by the argument this still 

 applies), are, he tells us, wilder, bolder, and more ferocious 

 than others. 



Ptolemy's Harmonics was edited, with a Latin 

 translation, by Dr. Wallis, in 1682, and an abstract of it 

 is given in Rees's Cyclopaedia. We cannot describe 

 here his proposals for the refomiation of the musical 

 scale ; but we cannot help regretting to find him, in the 

 third book, going off into fanciful musical relations 

 amongst the celestial spheres in a way which reminds us 

 of some portions of his book on astrology. 



Our author wrote a work on Optics, which is only known 

 to us through Latin versions made from incomplete 

 Arabic manuscripts. He is said to have discovered 

 the fact of tlie refraction of the rays of light by pass- 

 ing through substances of different density ; but, like 



