VARIATION. 



Of the Curia of no Variation. — In the preceding part of 

 this article, we have principally alluded to the variation eaft 

 or weil, at different times and in different places ; but it is 

 no lefs interefting to trace the curve in which no variation is 

 obferved. We have feen, that there are certain points in the 

 northern and fouthern hemifpheres where the needle points 

 to the true poles of the world ; thefe points, however, are not 

 all iituated on the fame meridian, but form an irregular 

 curve, iiiflefted different ways, and in perpetual motion. 



In the northern hemifphere, a curve of no variation 

 moved from weft to eaft during two centuries prior to 

 1662. This curve firft paffed the Azores, then the meridian 

 of London, and, after a certain number of years, the meri- 

 dian of Paris. But in the fouthern hemifphere, there was an- 

 other curve of no variation moving from eaft to weft. This 

 curve firft paffed Cape Aguillas, and then the Cape of Good 

 Hope ; the wefterly variation following the eafterly, the fame 

 as in the northern hemifphere, but in a contrary direftion. 

 And from the obfervations that have fmce been made, it ap- 

 pears that the curve of no variation in the northern hemi- 

 fphere, after paffmg the meridian of London and Paris, has 

 difcontinued its eaftern progrefs ; while the curve of no va- 

 riation in the fouthern hemifphere, ftill continues its courfe 

 north-weft ward. 



The variation on the eaft fide of the curve of no variation, 

 which paffed the meridian of Aguillas in 1600, and extended 

 north and fouth to a high degree of latitude in both hemi- 

 fpheres, being wefterly ; and the curve which paffed the me- 

 ridian of London in 1662 being eafterly ; it follows, that the 

 curve which paffed London could not reach beyond the 

 1 8th degree of eaft longitude, as the denomination of the 

 variation was the fame eaft ward of that meridian in 1600 that 

 it now is, namely, wefterly. The exaft point where the 

 fouthern curve of no variation paffed the northern curve 

 cannot be fatisfaftorily afcertained ; but it is known, that 

 w^ile the northern curve paffed London eaftward, the 

 fouthern curve paffed weftward in nearly the fame lon- 

 gitude. 



Dampier, in his voyage to the Eaft Indies in 1669, found, 

 that from 6° fouth latitude and 25° weft longitude, to the point 

 where the 37 th degree of fouth latitude was interfefted by the 

 meridian of Greenwich, the variation was eafterly, but never 

 exceeding 1 3° ; at which latter point the eafterly variation was 

 o, and thence became wefterly, and continued to increafe to 

 47° E. long., and 35° S. lat., where it appears to have 

 attained its maximum; i, e. 25° or 26° W.; and from this 

 point the variation gradually diminilhed, till it again affumed 

 another denomination in S. lat. 10°, and E. long. 125°: and 

 from this laft point, as far north of the equator as 10°, the 

 variation appears to have then been, as it ftill is, influenced 

 by local attraftions ; the quantity being always fmall, fome- 

 timeseaft, and at others weft. Captain Cook, in I772> in 

 S. lat. 6°, and W. long. 16°, found 9" 30' wefterly varia- 

 tion ; and in S. lat. 24°, and W. long. 23° 5 1', found only 39' 

 weft variation. From this point the wefterly variation gra- 

 dually increafed, as the fouthern latitude and eaftern longi- 

 tude augmented to 96° of eaft longitude, and 60" 49' fouth 

 latitude, where it attained its maximum ; ;. e. 43° 45' weft. 

 From this laft point it rapidly decreafed, and became eaft in 

 S. lat. 58° 53', and E. long. 143" 40'. But as the trad of 

 D'Entrecaileaux, in 1791, nearly coincides with Dampier's 

 in 1669, *^^ change in the pofition of the curve of no varia- 

 tion, will be beft illuftrated by a comparifon of their re- 

 fpeAive obfervations. 



D'Entrecafteaux found the variation in S. lat. 6", and 

 W. long. 25°, to be 7° 15' weft; and in S. lat. zf, and W. 

 long. 25°, only 1° 56' weft ; from which laft point )t again 

 Vol. XXXVI. 



increafed as the weft longitude diminifhed, till the meridian of 

 Greenwich interfefted the 33d degree of fouth latitude, where 

 the variation was 15° weft ; and it attained its maximum of 

 30° 48' 9" in S. lat. 34'=' 52', and E. long. 38° 14' 18'' from 

 Paris ; and from hence, to S. lat. 44°, and W. long. 133°, the 

 variation continued wefterly, but it there changed and be- 

 came eaft. 



The fpace between the two curves obferved by Dampier 

 in 1669, reached from the meridian of Greenwich to 130° E. 

 long.; and the diftanee between them, according to D'En- 

 trecafteaux, in 1 79 1, muft be 155°, Dampier having cut 

 the curve in 80° E. long., and the other in 25° W. The 

 curve of no variation muft therefore have advanced from the 

 weftward annually about 16', in the parallel of 34° fouth 

 latitude, to have ftiifted its place 25° in ninety-two years, 

 and at a yearly rate of 14' 8" from the time it paffed Cape 

 Aguillas in 1600 to 1791. This curve is now known to ex- 

 tend acrofs the magnetic and terreftrial equators, to a point 

 in N. lat. 37° 27', and W. long. 70° 44'. 



From a comparifon of the above refults, and others drawn 

 from the obfervations of captain Flinders, and the Ruffian 

 navigator Krufenftern, it appears, that a curve of no varia- 

 tion bending weftward, extends from the higheft degree of 

 fouthern latitude, in about 144° E. long, to 52° N. lat. fn 

 the fame parallel, interfefting the equator in 130" E. long. 

 Now from the obfervations of La Peroufe and Krufenftern, 

 the wefterly variation ceafed, and the curve of no variation 

 terminated in about 52" N. lat., and 153° E. long. ; for the 

 variation north-eaft of this point affumes another character ; 

 and 18° farther north, and 52° to the eaft, captain Cook 

 found near 36° of eafterly variation. 



It appears alfo, from a comparifon of obfervations made in 

 Perfia and the frontiers of China by Schubert, with thole 

 of Peroufe, Krufenftern, and others, made on board his 

 majefty's ftiip Sybille by Mr. Bain, that a curve of no va- 

 riation ag£un takes its rife in about 52° N- lat., and 143° E. 

 long., and terminates a little eaft from Spitzbergen, extend- 

 ing in a direftion nearly eaft by fouth, and weft by north, 

 through 6780 miles of longitude. 



Biot, in fpeaking of this fubjeft, fays, " that a curve of 

 no variation feems to take its rife in the great Southern ocean, 

 paffmg through the weftern part of New Holland, traverfes 

 the Indian ocean, enters the continent of Afia at Cape Co- 

 morin, and thence paffes through Perfia and Weft Siberia, 

 and proceeds towards Lapland. But what is more re- 

 markable, that line divides itfelf into two in the great 

 Afian archipelago, and gives rife to another branch, which, 

 pointing direftly from the fouth to the north, paffes that 

 archipelago, traverfes China, and is again found in Eaftern 

 Siberia. The exiftence of this branch, and its feparation 

 from the former, are clearly indicated by the obfervations 

 made in the Chinefe feas ; but I am able to offer a ftill farther 

 confirmation of it, by the obfervations made in Ruffia and the 

 frontiers of China, by the celebrated aftronomer Schubert, 

 who has been kind enough to communicate them to me ;" 

 which latter are the obfervations fpoken of above. 



We have feen that a curve of no variation, extending 

 from 60° of fouth latitude to 52° of north latitude, in about 

 143° eaft longitude, but taking a fmall bend to the weft- 

 ward, is interfefted by the meridian of Amboyna : and by 

 comparing v/ith each other different obfervations made by 

 Mr. Wales, in his voyage with captain Cook, and thofe cf 

 Vancouver in 1791, and others made by fir Home Popham, 

 and by Humboldt, it appears, that from the wefterly part 

 of the curve of no variation, found in about 130° eaft of the 

 meridian of Greenwich, where the wefterly changes to an 

 eafterly variatior., to the meridian where the eafterly varia- 



4 1 tion 



