THE SIZE OF THE EARTH. 



21 



The determination of the figure of the earth by the meas- 

 urement of degrees of longitude on different parallels requires 

 very r^reat accuracy in fixing the longitudes of different places. 

 Casshii de Thury and Lacaille employed, in 1740, powder 

 signals to determine a perpendicular line at the meridian of 

 Paris. In more recent times, the great trigonometrical sur- 

 vey of England has determined, by the help of far better in- 

 struments and with greater accuracy, the lengths of the arcs 

 of parallels and the differences of the meridians between 

 Beachy Head and Dunnose, as well as between Dover and 

 Falmouth. Thesfc determinations were, however, only made 

 for differences of longitude of 1 26' and 6 22'.* By far 

 the most considerable of these surveys is the one that was 

 carried on between the meridians of Marennes, on the west- 

 ern coast of France, and Fiume. It extends over the west- 

 ern chain of the Alps, and the plains of Milan and Padua, 

 in a direct distance of 15 32' 27 X/ , and was executed under 

 the direction of Brousseaud and Largeteau, Plana and Car- 

 lini, almost entirely under the so-called mean parallel of 45. 

 The numerous pendulum experiments which have been con- 

 ducted in the neighborhood of mountain chains have con- 

 firmed in the most remarkable manner the previously-recog- 

 nized influences of those local attractions which were inferred 

 from the comparison of astronomical latitudes with the re- 

 sults of geodetic measurements, f 



* Airy, Figure of the Earth, in the EncycL Metrop., 1849, p. 214- 

 216. 



t Biot, Astr. Physique, t. ii., p. 482, and t. iii., p. 482. A very no 



