[footnote] *See the remarkable chart of isoclinic lines in the Atlantic Ocean for the years 1825 and 1837, in Sabine's 'Contributions to Terrestrial Magnetism', 1840, p. 134.
It would be extremely important to know whether the opposite pole, near the Gilbert Islands, in the South Sea, has aproached the meridian of the Carolinas in a westerly direction. These general remarks will be sufficient to connect the different systems of isoclinic non-parallel lines with the great phenomenon of equilibrium which is manifested in the magnetic equator. It is no small advantage, in the exposition of the laws of terrestrial magnetism, that the magnetic equator (whose oscillatory change of form and whose nodal motion exercise an influence on the inclination of the needle in the remotest districts of the world, in consequence of the altered magnetic latitudes)* should traverse the p 185 ocean throughout its whole course, excepting about one fifth, and consequently be made so much more accessible, owing to the remarkable relations in space between the sea and land, and to the means of which we are now possessed for determining with much exactness both the declination and the inclination at sea.
[footnote] *Humboldt, 'Ueber die seculäre Veränderung der Magnetischen Inclination' (On the secular Change in the Magnetic Inclination), in Pogg. 'Annal.', bd. sv., s. 322.
We have described the distribution of magnetism on the surface of our planet according to the two forms of 'declination' and 'inclination'; it now, therefore, remains for us to speak of the 'intensity of the force' which is graphically expressed by isodynamic curves (or lines of equal intensity). The investigation and measurement of this force by the oscillations of a vertical or horizontal needle have only excited a general and lively interest in its telluric relations since the beginning of the nineteenth century. The application of delicate optical and chronometrical instruments has rendered the measurement of this horizontal power susceptible of a degree of accuracy far surpassing that attained in any other magnetic determinations. The isogonic lines are the more important in their immediate application to navigation, while we find from the most recent views that isodynamic lines, especially those which indicate the horizontal force, are the most valuable elements in the theory of terrestrial magnetism.*
[footnote] *Gauss, 'Resultate der Beob. des Magn. Vereins', 1838, 21; Sabine, 'Report on the Variations of the Magnetic Intensity', p. 63.
One of the earliest facts yielded by observation is, that the intensity of the total force increases from the equator toward the pole.*
[footnote] *The following is the history of the discovery of the law that the intensity of the force increases (in general) with the magnetic latitude. When I was anxious to attach myself, in 1798, to the expedition of Captain Bandin, who intended to circumnavigate the globe, I was requested by Borda, who took a warm interest in the success of my project, to examine the oscillations of a vertical needle in the magnetic meridian in different latitudes in each hemisphere, in order to determine whether the intensity of the force was the same, or whether it varied in different places. During my travels in the tropical regions of America, I paid much attention to this subject. I observed that the same needle, which in the space of ten minutes made 245 oscillations in Paris, 246 in the Havana, and 242 in Mexico, performed only 216 oscillations during the same period at St. Carlos del Rio Negro (1 degree 53' north lat. and 80 degrees 40' west long. from Paris), on the magnetic equator, i.e., the line in which the inclination =0; in Peru (7 degrees 1' south lat. and 80 degrees 40' west long. from Paris) only 211;while at Lima (12 degrees 2' south lat.) the number rose to 219. I found, in the years intervening between 1799 and 1803, that the whole force, if we assume it at 1.0000 on the magnetic equator in the Peruvian Andes, between Micuipampa and Caxamarca, may be expressed at Paris by 1.3482, in Mexico by 1.3155, in San Carlos del Rio Negro by 1.0480, and in Lima by 1.0773. When I developed this law of the variable intensity of terrestrial magnetic force, and supported it by the numerical value of observations instituted in 104 different places, in a Memoir read before the Paris Institute on the 26th Frimaire, An. XIII. (of which the mathematical portion was contributed by M. Biot), the facts were regarded as altogether new. It was only after the reading of the paper, as Biot expressly states (Lametherie, 'Journal de Physique', t. lix., p. 446, note 2) and as I have repeated in 'the Relation Historique', t. i., p. 262, note 1, that M. de Rossel communicated to Biot his oscillation experiments made six years earlier (between 1791 and 1794) in Van Diemen's Land, in Java, and in Amboyna. These experiments gave evidence of the same law of decreasing force in the Indian Archipelago. It must, I think be supposed, that this excellent man, when he wrote his work, was not aware of the regularity of the augmentation and diminution of the intensity as before the reading of my paper he never mentioned this (certainly not unimportant) physical law to any of our mutual friends, La Place, Delambre, Prony, or Biot. It was not till 1808, four years after my return from America that the observations made by M. de Rossel were published in the 'Voyage de l'Entrecasteaux', t. ii., p. 287 , 291, 321, 480, and 644. Up to the present day it is still usual, in all the tables of magnetic intensity which have been published in Germany (Hausteen, 'Magnet. der Erde', 1819, s. 71; Gauss, 'Beob. des Magnet. Vereins', 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, 'Traite de Electr. et de Magnet.', t. vii., p. 354-367), to reduce the oscillations observed in any part of the Earth to the standard of force which I found on the magnetic equator in Northern Peru, so that, according to the unit thus arbitrarily assumed, the intensity of the magnetic force at Paris is put down as 1.348. The observations made by Lamanon in the unfortunate expedition of La Perouse, during the stay at Teneriffe (1785), and on the voyage to Macao (1787), are still older than those of Admiral Rossel. They were sent to the Academy of Sciences, and it is known that they were in the possession of Condorcet in the July of 1787 (Becquerel, t. vii., p. 320); but, notwithstanding the most careful search, they are not now to be found. From a copy of a very important letter of Lamanon, now in the possession of Captain Duperrey, which was addressed to the then perpetual secretary of the Academy of Sciences, but was omitted in the narrative of the 'Voyage de La Perouse', it is stated "that the attractive force of the magnet is less in the tropics than when we approach the poles, and that the magnetic intensity deduced from the number of oscillations of the needle of the inclination-compass varies and increases with the latitude." If the Academicians, while they continued to expect the return of the unfortunate La Perouse, had felt themselves justified, in the course of 1787, in publishing a truth which had been independently discovered by no less than three different travelers, the theory of terrestrial magnetism would have been extended by the knowledge of a new class of observations, dating eighteen years earlier than they now do. This simple statement of facts may probably justify the observations contained in the third volume of my 'Relation Historique' p. 615): "The observations on the variation of terrestrial magnetism, to which I have devoted myself for thirty-two years, by means of instruments which admit of comparison with one another, in America, Europe, and Asia, embrace an area extending over 188 degrees of longitude, from the frontier of Chinese Dzoungarie to the west of the South Sea bathing the coasts of Mexico and Peru, and reaching from 60 degrees north lat. to 12 degrees south lat. I regard the discovery of the law of the decrement of magnetic force from the pole to the equator as the most important result of my American voyage." Although not absolutely certain, it is very probable that Condorcet read Lamanon's letter of July, 1787, at a meeting of the Paris Academy of Sciences; and such a simple reading I regard as a sufficient act of publication. ('Annuaire du Bureau des Longitudes', 1842, p. 463.) The first recognition of the law belongs, therefore, beyond all question, to the comparison of La Perouse; but, long disregarded or forgotten, the knowledge of the law that the intensity of the magnetic force of the Earth varied with the latitude, did not, I conceive, acquire an existence in science until the publication of my observations from 1798 to 1804. The object and the length of this note will not be indifferent to those who are familiar with the connection with it, and who, from their own experience, are aware that we are apt to attach some value to that which has cost us the uninterrupted labor of five years, under the pressure of a tropical climate, and of perilous mountain expeditions.
p 186 The knowledge which we possess of the quantity of this increase, and of all the numerical relations of the law of intensity p 187 affecting the whole Earth, is especially due, since 1819, to the unwearied activity of Edward Sabine, who, after having observed the oscillations of the same needles at the American north pole, in Greenland, at Spitzbergen, and on the coasts of Guinea and Brazil, has continued to collect and arrange all the facts capable of explaining the direction of the isodynamic system in zones for a small part of South America. These lines are not parallel to lines of equal inclination (isoclinic line), and the intensity of the force is not at its minimum at the magnetic equator, as has been supposed, nor is it even equal at all parts of it. If we compare Erman's observations in the southern part of the Atlantic Ocean, where a faint zone (0.706) extends from Angola over the island of St. Helena to the Brazilian coast, with the most recent investigations of the celebrated navigator James Clark Ross, we shall find that on the surface of our planet the force increases almost in the relation of 1:3 toward the magnetic south pole, where Victoria Land extends from Cape Crozier toward the volcano Erebus, which has been raised to an elevation of 12,600 feet above the ice.*
[footnote] *From the observations hitherto collected, it appears that the maximum of intensity for the whole surface of the Earth is 2.052, and the minimum 0.706. Both phenomena occur in the southern hemisphere; the former in 73 degrees 47' S. lat., and 169 degrees 30'E. long. from Paris, near Mount Crozier, west-northwest of the south magnetic pole, at a place where Captain James Ross found the inclination of the needle to be 87 degrees 11' (Sabine, 'Contributions to Terrestrial Magnetism', 1843, No. 5, p. 231); the latter, observed by Erman at 19 degrees 59' S. lat., and 37 degrees 24' W. long. from Paris, 320 miles eastward from the Brazilian coast of Espiritu Santo (Erman, 'Phys. Beob.', 1841, s. 570), at a point where the inclination is only 7 degrees 55'. The actual ratio of the two intensities is therefore as 1 to 2.906. It was long believed that the greatest intensity of the magnetic force was only two and a half times as great as the weakest exhibited on the Earth's surface. (Sabine, 'Report on Magnetic Intensity', p. 82.)
If the intensity near the magnetic south pole p 188 be expressed by 2.052 (the unit still employed being the intensity which I discovered on the magnetic equator in Northern Peru), Sabine found it was only 1.624 at the magnetic north pole near Melville Island (70 degrees 27' north lat.), while it is 1.803 at New York, in the United States, which has almost the same latitude as Naples.