120 



NOTES TO PRECEDING SECTION. 



consequence in connection with the boundaries of property : 

 •' The whole mass of West India property," says Sir John 

 Herschel, " has been saved from the bottomless pit of end- 

 less litigation by the invariability of the magnetic declina- 

 tion in Jamaica and the surrounding archipelago during the 

 whole of the last century ; all surve.ys of property there 

 having been conducted solely by the compass." Vide Rob- 

 ertson, in the Phil. Trans, for 1806, pt. ii. p. 348, On the 

 Permanency of the Compass in Jamaica since 1660. In the 

 parent country (England) the magnetic declination has va- 

 ried by 14° in the same period of time. 



121 (p. 56.) — I have elsewhere shovyrn that from the docu- 

 ments which have come down to us in connection with the 

 voyages of Columbus, we can with great certainty fix upon 

 three places in the Atlantic line of no variation for the 13th 

 September, 1492, the 21st May, 1496, and the 16th August, 

 1498. This line ran at these dates from North-East to 

 South-West. It touched the American continent some- 

 what to the east of Cape Codera, whilst at present the con- 

 junction is observed on the north coast of Brazil. (Hum- 

 boldt, Examen critique de I'hist. de la G6ogr. tom. iii. p. 

 44 — 48.) From Gilbert's Physiologia nova de Magnete, we 

 see plainly (and this fact is very remarkable) that in the 

 year 1600 the variation was still nil in the region of the 

 Azores (lib. iv. cap. 1), precisely as in Columbus's time. I 

 believe that, from documents in my Examen critique (tom. 

 iii. p. 54), I have demonstrated that the celebrated line of 

 demarcation, by means of which Pope Alexander VI. divi- 

 ded the western hemisphere between Spain and Portugal, 

 was not drawn through the most western of the Azores, be- 

 cause Columbus wished to turn a physical division into a 

 political one. He indeed laid great stress upon the zone 

 (raya), "on which the compass showed no variation, where 

 the air and the ocean, the latter covered with sea-weed, 

 show themselves differently constituted, where cooling 

 winds begin to blow, and (for so erroneous observations of 

 the polar star made him imagine) where the figure (the 

 sphericity) of the earth is no longer the same." 



123 (p. 56.)— It is a question of the highest interest in the 

 problem of the physical cause of the terrestrial magnetism, 

 whether the two oval systems of isogonal lines, so singular- 

 ly included each within itself, will continue to advance for 

 centuries in the same form, or will resolve themselves and 

 expand. In the eastern Asiatic coil, the variation increases 

 from without inwards ; in the coil or oval of the South Sea, 

 the opposite holds good ; at present, indeed, no line without 

 variation is known in the whole Southern Oceair; to the 

 east of the meridian of Kamtschatka, no line has less varia- 

 tion than 2° (Erman, in Poggend. An. b. xxi. s. 129). Yet 

 Cornelius Schoutenappears, on Easter-day of the year 1616, 

 somewhat to the south of Mukahiva, in 15° S. Lat., 132° 

 W. Long., in the middle of the present closed isogonal sys- 

 tem, consequently, to have found the variation nil (Hansteen, 

 Magnetism, der Erde, 1819, S. 28). It must not be forgot- 

 ten, that in all these considerations we can only follow the 

 direction of the magnetic lines in their advances as they 

 are projected upon the surface of the earth. 



123 (p. 56.)— Arago, in Annuaire, 1836, p. 284 ; and 1840, 

 p. 330-338. 



124 (p. 56,) — Gauss, Allg. Theorie des Erdmagnetismus, 

 «31. 



125 (p. 56.) — Duperrey, de la configuration de l'6quateur 

 inagn6tique, in the Annales de Chemie, tom. xlv. p. .371 

 and 379 (see also Morlet, in M6moires pr6sent6s par di- 

 Ters savans A I'Acad. roy. des Sciences, tom. iii. p. 132). 



126 (p. 57.) — See the remarkable mass of isoclinal lines 

 in the Atlantic Ocean for the years 1825 and 1837, in Sa- 

 bine's Contrii)utions to Terrestrial Magnetism, 1840, p. 139. 



127 (p. 57.) — Humboldt, iiber die seculSre Verfinderung 

 der magnetischen Inclination, in Poggend. Annalen, Bd. 

 XV. S. 322. , 



128 (p. 57.) — Gauss, Resultate der Beob. des magn. Ve- 

 xeins im Jahr. 1838, ^ 21 ; Sabine, Report on the Variations 

 of the Magnetic Intensity, p. 63. 



129 (p. 57.)— The following is the history of the discovery 

 of the law of the (general) increase of intensity in the mag- 

 netic force with magnetic latitude. When in 1798 I was 

 anxious to attach myself to the expedition of Captain Bau- 

 din, fitting out for a voyage round the world, I was request- 

 ed by Borda, who took a warm interest in my project, in 

 different latitudes of both hemispheres, to observe the swing 

 of the vertical needle in the magnetic meridian, with a 

 yiew to determine whether the intensity of the force was 

 the same or different in different places. This investiga- 

 tion I made one of the principal points in the course of my 

 voyage to the tropical countries of America. I observed 

 that the same needle which in Paris performed 245, in Ha'' 

 vannah 246, in Mexico 242 oscillations, in the course often 

 minutes ; at San Carlos, Rio Negro (10° 53' N. lat., 8(1° 40' 

 W. long.), in the same interval of time, performed 216 oscil- 

 lations ; on the magnetic equator, i. e. the line on which 

 the inclination is = 0, m Peru (7© 1' S. lat., 80° 40' W. 

 long.), it performed only 211 oscillations; in Lima (12© 2' 

 S. lat.) it again gerforined 219 oscillations. I found further, 



from 1799 to 1803, that the whole force taken at 1,0000 on 

 the magnetic meridian in the Peruvian Andes, betweea 

 Micuipampa and Caxamarca, at Paris will be represented 

 by 1,3482 ; in Mexico by 1,3155 ; in San Carlos by 1,0480 : 

 in Lima by 1,0773. When I made known this law of the 

 variable intensity of the terrestrial magnetic force, and ad- 

 duced the numerical value of observations made in 104 dif- 

 ferent places, in illustration of the conclusions, in a paper 

 which was read before the Parisian Institute at its sitting 

 of the 26th Frimaire, An. xiii., and of which the mathe- 

 matical portion belongs to M. Biot, the subject was regarded 

 as entirely new. It was only after the reading of this pa- 

 per, as Biot himself says expressly, (Lam^therie, Journ. de 

 Physique, t. lix. p. 446, note 2,) and as I repeat the state- 

 ment in my Relation Historique (t. i. p. 262, note 1), that 

 M. de Rossel communicated to M. Biot his observations on 

 oscillation made six years previously in Van Dieman's 

 Land, Java, and Amboyna ; from these observations was 

 deduced the same law of declining intensity in the Indiaa 

 Archipelago. It is almost to be supposed that this excel- 

 lent man, in his own work, was not aware of the regularity 

 of the increase and decrease of the intensity, as before the 

 reading of my paper he never mentioned this certainly not 

 unimportant physical law to our common friends. La Place, 

 Delambre, Pnmy, and Biot. It was only in 1808, four years 

 after my return from America, that the observations "made 

 by M. de Rossel were published in the Voyage de I'Entre- 

 casteaux, t. ii. p. 287, 291, 321, 480, 644. Up to the pres- 

 ent time it has still been usual in all the tables of magnetic 

 intensity that have been published in Germany by Hansteen, 

 Magnet, der Erde 1819, s. 71 ; Gauss, Beob. des magnet. Ve- 

 reins 1838, S. 36—39 ; Erman, Physikal. Beob. 1841. S. 529 — 

 579 ; in England (Sabine, Report on Magnet. Intensity, 1838, 

 p. 43—62 ; Contributions to Terrestrial Magnetism, 1843,) 

 and in France ^Becquerel, Trait6 d'electr. et de magnet, t. 

 vii. p. 354 — 367), to reduce the oscillations observed in any 

 part of the earth to the measure of the force which I found on 

 the magnetic equator in North Peru ; so that from the unity 

 thus arbitrarily assumed, the intensity of the magnetic force 

 at Paris is always set down at 1,348. Still older than the ob- 

 servations of Admiral Rossel, however, are those that were 

 made in the unfortunate expedition of La Ptirouse by Lama- 

 non, during the stay at Teneriffe ( 1785) and to the arrival at 

 Macao (1787), and which were sent to the Academy of Scien- 

 ces. It is known for certain that these papers were in the 

 hands of Condorcet in the July of 1787 (Becquerel, t. vii. p. 

 320). In spite of searching, however, they have not again 

 been found ; hut from the copy of a letter of Larnanou, now 

 ill the possession of Ad. Duperrey, addressed to the then 

 perpetual secretary of the Academy of Sciences, which has 

 been omitted in the account of the Voyage of La Perouse, 

 it is stated expressly, " Que la force attractive de I'aimant 

 est moindie dans les tropiques qu'en avancant vers les poles, 

 et que I'lntensite magn6tique deduite du*nombre des oscil- 

 lations de I'aiguille de la boussole d'inclinaison change et 

 augmente avec la latitude." Had the Academy of Sciences, 

 still anticipating the return of La Perouse, felt itself at lib- 

 erty, in the course of 1767, to publish an account of obser- 

 vations made by three different individuals unknown to one 

 another, the theory of terrestrial magnetism would have 

 been extended by a new class of observations eighteen years 

 sooner than it was. This simple statement of facts will 

 perhaps justify the assertion which the third volume of my 

 Relation historique (p. 615) contains: "Les observations 

 sur les variations du magnttisme terrestre auxquelles je me 

 suis livre pendant 32 ans, au inoyen d'iiistruniens compar- 

 ables entre eux en Am6rique, en Europe et en Asie, em- 

 brassent, dans les deux luimispheres, dcpuis les frontieres 

 de la Dzoungarie chinoise jusque vers I'ouest i la Mer du 

 Sud qui baigne les cfltes du Mexique et du Perou, un es- 

 pace de 188° de longitude, depuis les 60° de latitude nord 

 jusqu'aux 12° de latitude sud. J'ai regani^ la loi du d6- 

 croissemeut des forces magnfetiques, du pole a I'^quateur, 

 comme le r6sultat le plus important de mon voyage Am6ri- 

 cain." It is not certain, but extremely probable, that Con- 

 dorcet read the letter of Lamanon of July, 1787, at a meet- 

 ing of the Academy of Sciences of Paris ; and such a sim- 

 ple reading 1 myself regard as a sufficient act of publication 

 (Annuaire du Bureau des Longit. 1842, p. 463). The first 

 recognition of the law, therefore, belongs indisputal)Iy to 

 the companion of La Perouse ; but, long unheeded or forgot- 

 ten, I believe that the knowledge of the law of the varia- 

 tion in the intensity of the magnetic force with the latitude, 

 fir.st acquired^ a scientific existence with the publication of 

 my observations from 1798 to 1804. The subject, and the 

 length of this note, will not appear indifferent to him who 

 is familiar with the recent history of magnetism, and the 

 doubts that have been started in connection with it, and 

 who from personal experience knows that we are apt to at- ^ 

 tach some value to that which has been the object of our 

 uninterrupted attention for five long years, under the press- 

 ure of tropical climates, and engaged in hazardous mountain 

 expeditions. 

 130 (p. 57.) — The maximun; intensity for the whole »uf 



