$56 Variations of (he 7'crnslrhl MagnctiMU 



Wc do not give this determination as rigorously exact : 

 some corrections might no doubt be made to it, had we 

 a areattr number ot observations equally precise; but 

 \ve~^are of opinion that these corrections would be very 

 small ; and it will be seen hereafter that, independently of 

 the conlidcnce which the two observations we have em- 

 plovcd deserve, we have other reasons for entertaining, 

 this opinion *. 



It is very remarkable that this determination of the mag- 

 netic equator agrees almost perfectly with that given long 

 airo bv Wilke and Lemonnier. The latter in particular, 

 who for want of direct observations had discussed a great 

 number of corresponding observations, indicates the mag- 

 netic equator in Peru towards 7"-^ of south latitude; and 

 M. Humboldt found it in the same place at 7'79<53° (7° V) ; 

 besides, Lemonnier's chart, as well as that of M. Wilke, 

 indicates for the inclination of the magneiic meridian 12-2^2'» 

 (about 1 !°), and they place the node about 155" 56' (140°) 

 of west loncritude, reckoned from the meridian of Paris. 



Can it be by chance, then, that these elements, found 

 more than 40 years ago, should accord so well with ours 

 founded on recent observations ? or does the inclination of 

 the nlacuetic equator experience only very small variations, 

 wliile all the other symptoms of terrestrial magnetism 

 change so rapidly ? We should not be far from admitting 

 the latter opinion, when we consider that the inclination of 

 the ma<^netic needle has changed at Paris 3° during 6o years 

 since it has been observed ; and that at London, according 

 to the observations of Mr. Graham, it has not changed S'^ 

 in 200 years; while the declination has varied more than 

 20° in the same interval, and has passed from east to west : 

 but, on the other hand, the observation of the inclination is 

 so difiicult to be made with exactness, and it is so short a 

 time since the art of measuring it with precision was known, 



and wc shall have 



sin. r sin. ^ 



^ .sin. (r-^) 



By these equations we may find »\ and then y, by any of the first two. 



* Since this memoir was read, we have collected new information whicli 

 con^iniis these first results-. Lapcy rouse, after having doubled Cape Horn, 

 fell in a second time whh the magnetic equator in \W north lat. and 119" 7' 

 of longitude west from Paris. He was therefore very near the node of the 

 magnetic equator, such as we have deduced it from observations. This 

 fact establishes in a positive manner two important eonsequenccs : first, that 

 the preceding determinations require only very slight corrections; and the. 

 fccond, that the n.agnetic equator is really a great circle of the earth, if not 

 exactly at least very nearly. — Note of the AhUjots. 



that 



