On the History o/* Fernel's Measure of a Degree. 445 



agency of which I have proved much to my satisfaction in 

 some experiments conducted in East Pool copper mine. 

 November 4, 1841. 



LXVI. On the History of FernePs Measure of a Degree. 

 By Professor De Morgan. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



[ BEG to call the attention of mathematicians to a very 

 -*- strange circumstance in the account of Fernel's measure 

 of a degree, which will show how the history of science is 

 sometimes written. 



Montucla (vol. ii. p. 231, first edition ; vol. ii. p. SI 6, second 

 edition) gives the usual account of Fernel's measuring by- the 

 revolutions of a wheel from Paris towards Amiens, and finding 

 a degree to be 56746 Paris toises. In the second edition he 

 adds, that the details are found in Fernel's Cosmotheoria. 



L,a\ande{BibliographieAstro?i. i p. 46) gives the Cosmotheoria 

 as having 46 feuillets, and as published in 1528: he adds, 

 that it is remarkable as containing the first determination of 

 the true magnitude of the earth, and refers to his own ac- 

 count of it in the Memoirs of the Academy for 1787, page 

 216. 



Delambre (Astronomy, vol. iii. p. 516) repeats the story, 

 but makes the result 57070 toises: he does not refer to the 

 Cosmotheoria. 



On looking over the work of Fernel for another purpose, 

 I was surprised to see that he himself states a very different 

 result, and piques himself on its close agreement with that of 

 Almseon (Al Mamun he means). The story about the wheel 

 is correctly told ; it should have been added, that he observed 

 his solar altitudes with Ptolemy's rules. His result is, that 

 the degree of latitude consists of 68 Italian miles and 96 

 paces, which he makes 68 miles 95^ paces, to avoid fractions 

 in the resulting diameter of the earth. I quote three different 

 places where he has expressed this result, premising that my 

 copy has 46 leaves, and was published in 1528 ; so that all 

 doubt of its being the work described by Lalande may be im- 

 possible. 



Leaf 2, page 1. " ..., idipsum experimento comprobans, 

 deprehendi accurata supputatione,cuique gradui circuli majoris 

 tam in terrae quam in maris convexo 68 Italica miliaria, passus 

 95 cum una quarta respondere." 



