VARIATION. 



tion was found to terminate by captain Cook, S. lat. 58° 

 27'. W. long. 13° 10', the diftance eaft and well will be 

 216° 50'. 



To the meridian of Peroufe, S. lat. 20= 39'. W. long. 

 28' 38', the diilanceis 201° 22'. 



To the meridian of Vancouver, S. lat. 35^ W. long. 28', 

 the diftance is 202°. 



To the meridian of fir H. Popham, S. lat. 30^. W. long. 

 26°, the diftance is 204°. 



To the meridian of Humboldt, N. lat. 1 3°. W. long. 59°, 

 the diftance ( taking the mean ) is 171°. 



And if the curve of no variation extends to N. lat. 37° 

 27'. W. long. 70^ 40', the eafterly variation in that parallel 

 1V1II only meafure 158° 16'. 



Thefe different diftances point out, under different paral- 

 lels of latitude, the direftion which the curve of no variation at 

 prefent afliimes ; and we may perhaps conclude, that the va- 

 riation of the magnetic needle is caufed by two different and 

 diftinft fyftems of magnetic forces ; the one producing a 

 wefterly variation in the northern hemifphere, over the fpace 

 of 200° 44' ; and in the fouthern hemifphere, in the fame 

 parallel of latitude, of 143° 10': and the other an eafterly 

 variation in the northern hemifphere, over a fpace of 159° 

 16' ; and on the fouthern, of 216° 50'. 



From thefe fafts it would feem, that the north end of the 

 curve, which pafted Cape Aguillas in 1 600 north-weft ward, 

 was in 1804 found to extend from S. lat. 60^. W. long. 13° 

 to', to N. lat. 37° 27'. W. long. 70^40', interfering the 1 3th 

 degreeof north latitude, and the 21ft, 30th, and 35th of fouth 

 latitude ; whence the medium rate of its motion is found 

 ;;bout 265' annually : but it is extremdy probable, that the 

 iouth end of this curve has during the fame period remained 

 flationarv ; and this difference in the motion of the two ex- 

 tremes of the curve in the fouthern hemifphere, may perhaps 

 point out to us the reafon why the quantity of variation 

 fhould have continued the fame off Cape Horn during 133 

 years, while at the Cape of Good Hop'^, Paris, and London, 

 it Ihould have increafed 25° or 27° in the fame period. 



The curve of no variation which paffed through London 

 eaftward, about the fame time the curve in the fouthern 

 hemifphere paffed the meridian of Greenwich weftward, ap- 

 pears to have been loft among the fmallcr magnetic powers 

 at prefent found in the continent of Europe and Afia. 



The view which we have given of this perplexing fubjeft, 

 deduced from aftual obfervation, renders it obvious that all 

 attempts, by theory, to fix on charts the exaft polltions of the 

 curves of no variation, or lines of given variation, muft prove, 

 as they have always liitherto done, entirely abortive. Tliere 

 are indeed places in the world, fuch as Spitzberger, Cape 

 Horn, Chili, and Mexico, the great Afian archipelago, the 

 coail of Coromandel, Peru, Brafil, &c., where the curve of 

 variation, and the variation itfelf, has not undergone any per- 

 ceptible change fince firft obferved ; but we have good rea- 

 fons to beheve, that at all the above-mentioned places the 

 variations are regulated by incidental magnetic attraftions, 

 which are loft or merged in the two great powers already 

 defcribed, at a little diftance from their refpettive fpheres. 

 In all other places of the globe, it is continually undergoing 

 a regular and progreflive change ; but it is impofllble accu- 

 rately to determine when this change may ceafe, or to forefee 

 what quantity our prefent wefterly variation may attain, 

 though there is fome reafon to believe it has very nearly or 

 entirely arrived at its maximum. 



At prefent, we have fpoken only of thofe great and in 

 fome meafure continued changes in the variation which takes 

 place, and become very obvious after a long period ; but 

 there are others of a more minute quantity, and of daily, 



and we might add of hourly occurrence, to which it will be 

 proper to refer in the prefent article. • 



Mr. George Graham made feveral obfervations of this 

 kind in the years 1722 and 1723, profefling himfelf altoge- 

 ther ignorant of the caufe of the variation which he ob- 

 ferved. Phil. Tranf. N^ 383, or Abr. vol. vii. p. 290, &c. 



About the year 1 750, Mr. Wargentin, fecretary of the 

 Royal Academy of Sciences in Sweden, took notice both 

 of the regular diurnal variation of the needle, and alfo of its 

 being diif urbed at the time of the aurora borealis, as record- 

 ed in the Phil. Tranf. vol. xlvii. p. 126, &c. 



About the year 1756, Mr. Canton commenced a feries of 

 obfervations, amounting to nearly 4000, with an excellent 

 variation-compafs, of about nine inches diameter. The num- 

 ber of days on which thefe obfervations were made was 603, 

 and the diurnal variation on 574 of them was regular ; ;. e. 

 the abfolute variation of the needle weftward was increafing 

 from about eight or nine o'clock in the morning till about 

 one or two in the afternoon, when the needle became fta- 

 tionary for fome time ; after that, the abfolute variation 

 weftward was decreafing, and the needle came back again 

 to its former fituation, or near it, in the night, or by the 

 next morning. The diurnal variation is irregular when the 

 needle moves flowly eaftward in the latter part of the morn- 

 ing, or weftward in the latter part of the afternoon ; alfo 

 when it moves much either way after night, or fuddenly both 

 ways in a fhort time. Thefe irregularities leldom happen 

 more than once or twice in a month, and are always accom- 

 panied, as far as Mr. Canton obferved, with an aurora 

 borealis. 



Mr. Canton lays down and evinces by experiment the io. ■ 

 lowing principle, w«. that the attractive power of the maL; 

 net (whether natural or artificial) will decreafe while t'rj 

 magnet is heating, and increafe while it is coohng. He then 

 proceeds to account for both the regular and irregular varia- 

 tion. It is evident, he fays, that the magnetic parts of the 

 earth in the north on the eaft fide, and the magnetic parts of 

 the earth in the north on the weft fide of the magnetic meri- 

 dian, equally attraft the north end of the needle. If then 

 the eaftern magnetic parts are heated fafter by the fun in the 

 morning than the weft;rn, the needle will move weftward, and 

 the abfolute variation will increafe : when the attrafling pans 

 of the earth on each fide of the magnetic meridian have 

 their heat increafing equally, tiie needle will be ftationary, 

 and the abfolute variation will then be greateft ; but when 

 the weftern magnetic parts are either heating fafter, or cool- 

 ing (lower than the eaftern, the needle will move eaftward, or 

 the abfolute variation will decreafe ; and when the eaftern and 

 weftern magnetic parts are coohng equally faft, the needle 

 will again be ftationary, and the abfolute variation will then 

 be least. 



By this tlieory, the diurnal variation in the fummer ought 

 to exceed that in winter ; and accordingly it is found by ob- 

 fervation, that the diurnal variation in the months of June 

 and July is almoft double that of December and January. 



The irregular diurnal variation muft arife from fome other 

 caufe than that of heat communicated by the fun ; and here 

 Mr. Canton had recourfe to fubterranean heat, which is ge- 

 nerated without any regularity as to time, and which will, 

 when It happens in the north, affeft the attraftive power of 

 the magnetic parts of the earth on the north end of the 

 needle. That the air neareft the earth will be moft warmed 

 by the heat of it, is obvious ; and this has been frequently 

 taken notice of in the morning, before day, by means of 

 thermometers at different diftances from the ground. Phil. 

 Tranf. vol. xlviii. p. 526. 



Mr. Canton has annexed to his paper on this fubjeft a 



complete 



