GEOGRAPHICAL LATITUDE. 791 



least a strong desire for a more correct determination of the size of the 

 earth than had yet been made, in order to deduct therefrom a stand- 

 ard of measurement founded in nature, so that if ever lost it could be 

 recovered; and further that there might be a standard which was in 

 its character not national, but universal. This was one of those plans 

 for universal improvement so rife at the beginning of the French Revo- 

 lution, and the one perhaps of all which has been most permanent 

 and widespread in its results. The work of carrying out the project 

 was intrusted to Delambre and Mechain, two prominent scientists of 

 the day. They surveyed a meridian line extending from Duukerque 

 to Barcelona, with an amplitude of 9° 40' 24". 75, and a length of 

 551,584.72 toises. The line was later extended to Formentera in the 

 Balearic Islands, but too late to change the result, which had been 

 accepted for the standard of measurement. For this survey the most 

 careful preparations were made to secure the utmost accuracy. Instead 

 of the old-fashioned quadrants and sectors, with their unwieldly bulk 

 and subject to a variety of changes from temperature, position, their 

 own weight, etc., the then newly invented repetition circles of Borda 

 were used with excellent result. The same expert also provided rules 

 for measuring the base line, which were of a pattern entirely new and 

 capable of an accuracy hitherto impossible. The measuring part was 

 formed of platinum, whose relations to the toises of Peru and Lapland 

 were accurately determined. Upon this a rule of copper 18 inches 

 shorter was fastened securely at one end. The ratio of expansion of the 

 two metals and the difference of length at a fixed temperature being 

 known, an observation of the temperature and of the difference of their 

 lengths by means of a vernier provided for that purpose, which was 

 fixed to the metal and protected from the sun's rays, gives the amount 

 of expansion at the moment of observation. Here was also adopted 

 for the first time a plan which afterwards became universal in such 

 operations, namely in measuring the base, to place the rules at a dis- 

 tance from each other to prevent the effect of the shock of contact, and 

 measure carefully the interval. For this purpose there was attached 

 at one end of each rule a small slide, accurately divided into hundred- 

 thousandths for measuring minute distances, and provided with a 

 microscope for reading them. After both rules were placed in line this 

 was moved forward with the greatest care till it covered the interval 

 between the rules, and the distance was at once read off and noted. 



In reference to the actual labor in determining a meridian line, De- 

 lambre remarks:' "De toutes les operations qui concourent a la 

 mesure des degr6s du m6ridieu, les observations de latitude sont celles 

 qui demandent plus de precautions, plus de soins et plus de temps." 

 As an example of the extreme care taken in this work, may be cited the 

 fact that to determine the latitude of the Pantheon at Paris, Mechain 

 and Delambre each made eighteen hundred astronomical observations, 



' Base du syst^me, ii, 158. 



