56 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



count of it, published at the East India Company's 

 expense in 1847. The heats and the rains, the 

 dull opake atmosphere of the Doab, with its bound- 

 less and densely-wooded flats, the jungle fever and 

 the wild animals, were natural impediments enough. 

 But Colonel Everest, whose conscientious anxiety 

 reached almost a nervous pitch, seems not to have 

 been satisfied with any one instrument which the 

 mechanicians of London could produce ; but to 

 have metamorphosed most of them, partly with his 

 own hands, partly by native aid. It says much for 

 his ability in this respect, that the results appear 

 entitled to compete with all the most exquisite of the 

 kind in Europe. His bases, for instance, about 7J 

 miles in length, when checked by intermediate trian- 

 gulation above 400 miles in extent, differed in one 

 instance by 4, in another by 7 inches from the pri- 

 mary measurement. The whole extent of Lambton's 

 and Colonel Everest's operations includes a continu- 

 ous arc of 21 21' (1477 miles), by far the greatest 

 at that time executed. 



(252.) I* is indeed only rivalled in this respect by a vast 

 Russian arc operation very lately executed in Russia and other 

 northern countries of Europe, by which an arc of 25 

 20', extending from the banks of the Danube to the 

 shores of the Arctic Sea, near the North Cape, has 

 been measured under the general superintendence 

 and direction of that able astronomer M. Struve, 

 whose meritorious labours in other departments of 

 astronomy will be specified in a succeeding section. 

 The results are still incomplete, though the opera- 

 tions in the field were happily concluded just in time 

 to prevent their total frustration by the unhappy war 

 in which Russia has since engaged. 



(253.) The arcs of India and of Russia include a space 

 Its extent. from Lat 3 to Lat 7^ ^^ the exce ption of only 



about sixteen degrees, and are unquestionably the 

 most important which exist for the determination of 

 the earth's figure. When to them we add the French 

 arc of 12 22' in a medium latitude, it will scarcely 

 be necessary to take into account any other, at least 

 for the Northern Hemisphere. 



(254.) The following brief details of the Russian arc are 

 taken from the provisional report of M. Struve (1852). 

 (255.) The southern extremity of the Russo-Scandinavian 

 the^Russ^ arc is Ismail on the Danube (Lat. 45 20'), the north- 

 Scandina- ern extremity is Fuglenaes on the island of Qualoe 

 vian Arc. in Finnmarken (Lat. 70 40'). The interval from 

 Tornea to Fuglenaes (4 4 9') was measured by Swedish 

 and Norwegian engineers ; all the remainder by those 

 of Russia, and, in particular, by M. Von Tenner, who, 

 with M. Struve, has since 1816 directed the whole 

 operation. The whole line is remarkably free from con- 

 siderable inequalities of ground, and from mountain 

 ranges, so that local attractions are probably incon- 

 siderable. 1 On the other hand, extensive forests ex- 



tending over dead flats have, in the southern part of 

 the arc (as in India), occasioned great difficulties, and 

 compelled the erection of numerous temporary struc- 

 tures to overlook the country. In the north the 

 extraordinary refractions have, as usual, created some 

 difficulties. Ten different base lines, all at a small 

 height above the sea, form part of the operations. 

 It is a very satisfactory circumstance, that by the 

 care of M. Struve and Mr Airy, the standards of 

 length used in the Indian and Russian arcs have 

 been directly compared. 



The calculation of the figure of the earth from the (256.) 

 completed Russian arc, in combination with others, General re- 

 has not yet been made, but it is believed that it * e Earth's 

 will indicate an ellipticity somewhat greater than ellipticity 

 that generally received. The results obtained by Co- from g e - 

 lonel Everest, on the other hand, by comparing his desy ' 

 arc with those of Europe, give generally small ellip- 

 ticities, that is under ^ . The French and Indian 

 arcs, for instance, give 7 $ T . Now Mr Airy had de- 

 duced 20 years ago from the best existing observa- 

 tions -z^-g > Bessel, a few years later, obtained the al- 

 most identical result of ^^; and Schmidt of ^$ r . 

 The determinations by means of tjje pendulum are and from 

 somewhat larger. The extensive observations of Co- pendulum 

 lonel Sabine and Captain Foster concur in giving ex P e ^ 

 an ellipticity of $%$ ; but the French experiments by 

 Duperrey and Freycinet lead to a result considerably 

 greater. The discrepancy between the geodetical and 

 pendulum results may of course be a real one de- 

 pending on local variations of density. The astro- 

 nomical determination from the lunar inequalities, 

 which might be expected to concur with the results 

 of the pendulum, gives (as we have seen in the chap- 

 ter on Physical Astronomy, Art. 63) 7 r at a mean. 



I regret that my limits do not permit me to speak (257.) 

 of the measure of degrees of longitude or arcs of pa- British arc 

 rallel, as another test of the earth's figure. The re- 

 suits, however, cannot compete in point of certainty 

 with those from arcs of the meridian. It may be sa- 

 tisfactory to add, that the British arc of parallel from 

 Greenwich to Valentia (west coast of Ireland) sen- 

 sibly accords with the earth's figure obtained inde- 

 pendently. 





In connection with the subject of the pendulum (258.) 

 treated in the earlier part of this section, I shall here M - F p u * 

 mention a very remarkable experimental observation ^^ 

 communicated to the Paris Academy of Sciences, on periment, 

 the 3d February 1851, by M. Leon Foucault, to 

 whom we also owe some ingenious experiments on 

 the velocity of light. It is not indeed connected with 

 the determination of the earth's figure, but it has a 

 connection with astronomy, inasmuch as it affords the 

 most remarkable and direct proof of the earth's rota- 

 tion round its axis. 



1 In India, on the contrary, the local attractions of the Himalaya must be very sensible ; indeed Mr Pratt has calculated them 

 to be so considerable, as to have given rise to a curious speculation by Mr Airy as to the circumstances which may tend to 

 diminish the attraction of mountain ranges. (PhiLTrans. 1855.) 



