CHAP. III., 1.] 



ASTRONOMY, MASKELYNE DELAMBRE. 



39 



were the principal means of extricating him from his 

 difficulties : but his danger was often imminent, and 

 he appears to have sometimes heard the dreadful words 

 which, as an eloquent author has expressed it, were 

 the last sounds that vibrated in the ear of many an 

 unhappy victim." The operations were actually sus- 

 pended for a time by a decree of Robespierre and his 

 colleagues, who deposed Delambre, along with Laplace, 

 Lavoisier, Borda, and others, from the Commission 

 of Weights and Measures, as being deficient in " re- 

 publican virtues and their hatred of kings." They 

 were, however, resumed, and Delambre had finished 

 his share of the work long before his colleague Me- 

 chain, whose shorter task was conducted amidst a 

 people rude and uneducated, indeed, yet far more to 

 be trusted than were then those of the north. Me- 

 chain was apparently wayward and impracticable, 

 somewhat too aged for so great a work, yet a really 

 good astronomer. The want of agreement to within 

 3" of two sets of observations for latitude at Barcelona, 

 the southern end of the arc at that time, led him to 

 the suppression of one of them, and he was tormented 

 ever after by the consciousness of the evasion, which 

 deprived him of the tranquillity necessary to resume 

 and complete his work, which was done chiefly by 

 Delambre after vexatious delays. 1 The error, which 

 may be said to have cost Mechain his life, was pro- 

 bably owing to the instrument employed on this sur- 

 vey, the repeating circle of Borda, only fourteen 

 inches diameter, with a rather weak telescope. The 

 opinion generally entertained in Britain is, that the 

 repeating circle was quite inadequate to the prodi- 

 gious accuracy required of it, especially in the deter- 

 mination of latitudes. The errors of mere division 

 are often trivial compared to those inherent in other 

 parts of an instrument. Of these a deficiency of op- 

 tical power, and the want of absolute security of the 

 clamps, upon which the entire success of the princi- 

 ple of repetition depends, are amongst the most ob- 

 vious. The arc was finally prolonged from Barcelona 

 to Formentera by Biot and Arago in 1806. The 

 conclusion of the survey was not destitute of the ad- 

 venturous character of its commencement. The 

 French astronomers ran many risks, underwent much 

 suffering, and Arago narrowly escaped finishing his 

 days in the dungeons of Spain. 



The English survey carried on by Roy and Mudge 

 has been also noticed in the previous Dissertation, 

 The arc from Dunnose to Burleigh Moor amounts 

 to 3 57' 13"-1, the measured length to 1442953 

 feet. An arc of parallel was also measured from 

 Dover to Falmouth. We shall say something of its 

 later progress in the concluding part of this essay, 

 but we have still to regret the postponed publication 

 of the British Arc of the Meridian, which we have 

 no reason to doubt will bear a favourable compari- 

 son with the work of Delambre. The practical 



appliances, the three-feet theodolite of Ramsden-for 

 horizontal angles, and the eight-feet zenith sector 

 of the same artist for latitudes, were unequalled in- 

 struments, and contrasted in almost every respect 

 with the light and portable apparatus of the French. 

 By means of the former the spherical excess of terres- 

 trial triangles was first observed as a fact. The 

 results of the French and British arcs taken sepa- 

 rately concurred in showing a local curvature in this 

 part of the world altogether anomalous, the de- 

 grees rather shortening in the northern part of each 

 arc. This fact, which must be imputed either to 

 large local attractions, giving errors of several seconds 

 in the determination of latitudes, or (less probably) 

 to a local departure in our quarter of the world from 

 the general or mean figure of the earth, sufficiently 

 shows the futility of the proposed method of deter- 

 mining a natural, recoverable standard of length. 

 When combined with the measures of Bouguer in 

 South America and Lambton in India, and the revised 

 arc (measured in the beginning of this century) of 

 Svanberg and Melanderhjelm in Lapland, the French 

 and English measures give a general ellipticity some- 

 what under F ^, which is probably as near the truth 

 as local inequalities admit of the determination being 

 made. 



To Delambre was confided the drawing up of the 

 trigonometric formulas used in the calculations of the 

 survey, which were published in a separate work ; 

 De Prony conducting the laborious calculation of an 

 altogether new set of logarithmic tables, with the 

 aid of an immense staff of computers, the results of 

 whose labour (still in MS.) are preserved at Paris in 

 17 folio volumes. Delambre carried his personal 

 exertions so far as to compute his own triangles 

 which were also independently calculated by Le- 

 gendre, Van Swinden, and Tralles. 



As an acknowledgment of his merit, the highest 

 indeed in their power to bestow, the Institute of 

 France decreed to him in 1810 one of the Decennial 

 Prizes instituted by Napoleon. But the Emperor, 

 though professing to be the warm encourager of 

 science, suffered some meaner motive to interfere, 

 and refused to ratify the decision. " Ce fut," writes 

 Dupin, " un pas dans la route qui le menait & sa 

 chute." After the siege of Paris in 1814 Delambre 

 wrote a characteristic letter to his friend Moll. The 

 tranquil spirit which had braved the horrors of the Re- 

 volution was not to be moved by the sounds of the 

 artillery of the allied armies. In spite of the can- 

 nonade which he heard from his library, he laboured 

 from eight in the morning until midnight ; and, con- 

 scious of rectitude, he feared little the revolution of 

 circumstances, which changing dynasties might call 

 forth. " Labour," he says, " occupies all my time 

 and all my faculties." 



As Secretary to the Academy of Sciences for the 



(168.) 

 French 

 trigono- 

 metric for- 

 mulae and 

 tables. 



(169.) 

 Prize 



awarded to 

 Uelambre. 



(170.) 



1 The history is minutely given by Delambre himself in his Biography of Mechain. Astron. du XVIII. Siecle. 



