786 



GEOGRAPHICAL LATITUDE. 



them separately the value of a degree would vary, according to Cassini, 

 from 56,130 toises to 62,000.^ Riccioli's own calculations, combining all 

 the elements, gave 02,650 toises for the value of a degree, an error of 10 

 per cent.^ He seems not only not to have observed all three angles of 

 each triangle, but also to have employed very acute angles, even as 

 small as two degrees, which practice, as before remarked, increases 

 greatly the liability to error. 



The year 1669 is memorable as that of the first survey which is used 

 as a starting point for the best of modern work. It was conducted by 

 JeanPicard, a French physicist, who, by improvements in instruments 

 and methods, showed a decided advance over any of his predecessors. 

 The line extended from Malvoisine to Sourdou in Picardy, which points 

 were connected by a series of thirteen triangles, two of which were 

 principally for verification.^ Cassini complains of him that he ob- 

 served only two angles in each two triangles, and only one angle of a 

 third triangle.* This serves to show that up to that period even the 

 most careful savant had not arrived at the recent conception of extreme 

 accuracy. Cassini himself, as we shall see later, made a survey which 

 was by no means faultless. Picard based his calcuhitions on the 

 measurement of two base lines, each of which he measured twice. The 

 principal one was on the highway from Villejuive to Juvisy, the ad- 

 vantage of which for the base line was one of the reasons for choosing 

 this region for the survey.^ Here he measured with two wooden rules, 

 each 4 toises long, a line which, according to the first measurement, 

 was 5,662 toises 5 feet long, the second, 5,063 toises 1 foot. He 

 adopted the mean of 5,663 toises. His second base had a length of 

 3,902 toises.^ His calculations gave as a result 57,057 toises for the 

 length of a degree; but he adoi)ted the more convenient number of 

 57,000.'' Later he remarks that on the ocean the length would be 8 

 feet less, but thinks this unworthy of consideration.' It may be well 

 to add that though he was acquainted with the fact of refraction, and 

 discovered the influence of temperature thereon, he takes no notice of 

 it in his calculations, nor did he make allowance for the precession of 

 the equinoxes or the aberration of light, and was still ignorant of the 

 flattening of the earth at the poles ; consequently calculated on the 

 basis of the absolute sphericity of the earth. Later Maupertuis made 

 a calculation of the length of a degree, based on Picard's geodetic work, 

 but taking into account the precession of the equinoxes and the ab- 

 erration of light, and found it to be 57,183 toises;^ or according to 

 another calculation, including the effect of refraction also, he finds the 

 value of a degree to be 56,925 toises.'" As Picard was the first to ap- 



' Cassini, Grandeur, etc., 365-8. 

 * Jordan, Vermessungskuude, ii, 4. 

 ^ Picard, Mesure de la terre, 7. 

 ••Cassini, Grandeur, etc., 331. 

 ' Picard Mesure de la terre, 3. 



^Ibid., 3. 



■'Ibid., 22. 



» Ibid., 23. 



* Maupertuis, CEuvres, iv, 330. 



"»/6id. Ill, 167. 



