314 FIGURE OF THE EARTH. 



tion of instruments, the want of precision on tlie part of observers, or from 

 liavinj^ reached us in obscure expressions or in units vagui,'ly understood, have 

 been of no service to modern geometers. Five of these final results ai-e cited 

 by Bailly in his Uistory of Ancient Astronomy, and these, doubtless, are not all 

 that might have been cited; it is sufficient to compare them with one another, 

 to perceive how little guarantee of exactness either of them affords a priori, or 

 without subsequent corroboration. According to Aristotle, the opinions received 

 in his time assigned 400,000 stadia as the circumference of the earth; Ptolemy 

 adopted 180,000 ; Eratosthenes and Posidonius respectively 250,000 and 

 240,000; Cleomedes 300,000. The learned historian above mentioned explains 

 these enormous discrepancies, which could not have resulted from the ignorance 

 or dullness of the observers, in a suHiciently natural maimer, by assigning a 

 different value in each case to the stadium ; assuming, as the state of knowl- 

 edge at his time respecting ancient measures seemed to indicate, four kinds of 

 stadia, approximately of 100, 136, 170 and 230, metres each, he arrives at the 

 conclusion that these results, so discordant in appearance, are in the main iden- 

 tical, and not remote from those obtained by modern investigation. But Bailly 

 himself, one of the most enthusiastic defenders of ancient science, agrees with 

 us in thinking that the geodesic Libors of the astronomers but little anterior to 

 his own epoch, as well as those of his cotemporaries, were conducted in complete 

 independence of the investigations of remote ages and Avithout reference to a 

 coincidence of numbers. Leaving to himself, therefore, the responsibility of his 

 ideas, which we shall neither attempt to defend nor contravene, and judging 

 this to be no occasion for reporting the earnest arguments adduced by highly 

 respectable authors both for and against his views, let us concede, not to anti- 

 quity in general, but to a part of its philosophers, a knowledge, however loosely 

 approximate, of the dimensions of the earth ; and with this concession, let us 

 pass to an exposition of the geodesic labors of times nearer our own and of 

 more authentic character, though not all marked by an undoubted stamp of 

 exactness.* 



Towards the year 830 of our era, the Arabian astronomers measured, by 

 order of the wise Caliph Almamon, an arc of the meridian in the plain of 8ind- 

 giar, near the coasts of the Red Sea ; but the result of this operation made but 

 little approach to the truth, or was either confusedly expressed at first or has 

 been corrupted in the transmission. 



In the year 1490, as Martin de Navarette relates in his compendious History 

 of the Spanish Marine, our learned countryman Antonio de Nebrija, determined 

 by various measurements and observations the quantity of a terrestrial degree, 

 and obtained a number more near the truth than those before deduced. Subse- 

 quently, Glareans, in Switzerland, and Oroncio Fineo, in France, undertook and 

 accomplished a labor analogous to that of Nebrija ; and the same thing, with 

 even better success, was effected by the French physician Fernel, who founded 

 his estimate on the number of revolutions made by the wheel of a carriage 

 in its transit from Paris to Amiens, cities situated under nearly the same me- 

 ridian. t 



In 1617, the Dutch astronomer Schnell revived the method of Eratosthenes, 

 and applied it, with better means and more accuracy than had yet been observed, 



*The reader who may desire to know the slight or deficient foundations on which rest the 

 conjectures of the authors who maintain the profound astronomical science of the ancients 

 may consult the treatise of Sr. Vasquez Queipo, entitled. Essay on the Metric and Monetary 

 Systems of Ancient Nations, tome J, ])p. (i5, 6(3, and note 10, corresponding thereto. 



t Upon the points licre treated of, and other analogous ones not less deserving to be known, 

 the reader will find critical notices of great value in the discourse relative to the progress of 

 geodesy, read by Sr. Saavedra Meneses, at the beginning of the present year, on his recep- 

 tion into the Academy of Sciences. 



