168 DISSERTATION SECOND. [part i. 



that of Snellius, according to which, the distance between 

 Amiens and Malvoisine was found from a series of triangles, 

 and a base of 5663 1-6 toises. He determined the differ- 

 ence of latitude by means of a zenith sector of ten feet radi- 

 us, and found it to be 1° 22' .55". The whole distance was 

 78,850 toises, whence the degree came out 57,060 toises. 

 This was the first measurement of a degree of the meridian, 

 on which perfect reliance could be placed. 



Hitherto no doubt had been entertained of the spherical 

 figure of the earth, and, of consequence, of the equality of 

 all the degrees of the meridian, so that if one was known, 

 the whole circumference was determined. Men, with the 

 precipitation which they so often manifest, of assuming, 

 without sufficient evidence, the conclusion which appears 

 most simple, were no sooner satisfied that the earth was 

 round, than they supposed it to be truly spherical. An 

 observation soon occurred, which gave reason to suspect, 

 that much more must be done before its figure or its magni- 

 tude were completely ascertained. 



With a view of observing the sun's altitude in the vicinity 

 of the equator, where the distance from the zenith being 

 inconsiderable, the effects of refraction must be of small 

 account, it was agreed, by the same academy, to send an 

 astronomer, M. Richer, to make observations at the island 

 of Cayenne, in South America. 



Richer observed the solstitial altitude of the sun at that 

 place in 1672, and found the distance of the tropicks to be 

 46° 57' 4" ; and, therefore, the obliquity of the ecliptick 

 23° 28' 32", agreeing almost precisely with the determina- 

 tion of Cassini. 



The most remarkable circumstance, however, which oc- 

 curred in the course of this voyage, was, that the clock, 

 though furnished with a pendulum of the same length which 

 vibrated seconds at Paris, was found, at Cayenne, to lose 



