three Degrees of the Meridian. g5 



and the Pantheon at Paris, from the data published by the 

 Chevalier Delambre in the third vohanie of the Measure- 

 ment of the Meridian. I employed the same elements and 

 similar calculations to those made on the English arc. 

 The oblateness of -gJ-^. gives the difference between the pa- 

 rallels equal to 7883,615 seconds by the easlern series of 

 triangles, and 7883,617 seconds by the weslern series. The 

 mean of these 7 883,6 16 may be taken as ihe true extent of 

 the total arc. 



The two other elements' give for this quantitv 78B3'',6-21 

 and 7863",493, or 2° 1 1' 23",6 and 23",49,'as the cal- 

 culated extent of the arc. But the arc observed was 

 Z° 1 i' 19", S3, according to M. Dclainbre, and 2° 1 T iJ0",S5 

 according to M. Mcchain; so that the least difference be- 

 tween the calculation and the observations will be 2",64. 

 M. Delambre is of opipion, thai the latitude of Dunkirk, 

 which is supposed to be 51° 2' 9",20, should be dimi- 

 nished ; and in fact the distance between the parallels of 

 Dunkirk and Greenwich, which is 25r24 1,9 toises, gives 

 by the mean of the three assumed cllipticities 26' 32'',3 

 for the difference of latitude. After deducting this quantity 

 from 51° 28' 40", the supposed latitude of Greenwich, 

 there remains 51° 2' 7",7 or 8", for that of the tower at 

 Dunkirk. If from this again we deduct the calculated arc 

 2° 1 1' 23",5, we have 48^ 50' 4i",5 for the latitude of the 

 Pantheon, while, according to the observations of M. De- 

 lambre, it is 49",37, or 48",35 by those of M. Mechain. 

 If various circumstances, with regard to unfavourable 

 weather, and also others of a different kind connected with 

 the revolution, and of which M. Delambre complains with 

 much reason, have occasioned some uncertainty with re- 

 spect to the observations ^t Dunkirk, still the numerous 

 observations made at Paris, both by him and by M.Mechaiu, 

 at a more favourable seasv)n, and in times of perfect tran- 

 quillity, render the su])position of an error of four seconds 

 in the latitude of the Pantheon wholly inadmissible. It is 

 however too true, that such errors are possible, and it is only 

 by careful perseverance, and by repeated verification, that 

 they are to be discovered and removed, as we have seen to 

 be highly probable with respect to the station at Arburv 

 Hill. ^ 



But the same celebrated observer, M. Mechain, who 

 handled instruments with great delicacy, and was possessed 

 of peculiar talents for this species of observation, has given 

 us an instance of singular irregularity in the observfitions 

 made at Moiitjui and at Barcclyna. 



The 



