MAGNETISM. 185 



ocean throughout its whole course, excepting ahout 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 determin- 

 ing with much exactness both the declination and the inclina- 

 tion at sea. 



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 investi- 

 gation 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 measure- 

 ment of this horizontal power susceptible of a degree of accu- 

 racy far surpassing that attained in any other magnetic de- 

 terminations. The isogenic hues 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 ele- 

 ments in the theory of terrestrial magnetism.* One of the 

 earliest facts yielded by observation is, that the intensity of 

 the total force increases from the equator toward the pole.f 



clination (On the secular Change in the Magnetic Inclination), in Fogg. 

 Annul., bd. xv., s. 322. 



* Gauss, ResuUate der Beob. des Magn. Vereins, 1838, $ 21; Sabine, 

 Report on the Variations of the Magnetic Intensity, p. 63. 



t 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, iu 1798, to the expedition of 

 Captain Baudin, who intended to circumnavigate the globe, I was re- 

 quested by Borda, who took a warm intei'est in the success of my proj- 

 ect, to examine the oscillations of a vertical needle in the magnetic me- 

 ridian 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° 53' north lat. and 80^ 

 40 west long, from Paris), on the magnetic equator, i. e., the line in 

 which the inebriation =0 ; in Peru (7^ 1 south lat. and 80° 40' west 

 long, from I'ansj only 211 ; while at Lima (12° 2' south lat.) the num- 

 ber 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, 



