96 On the Laws of Terrestrial Magnetism 



The researches of M. Huiiiboldt on this subject have brought 

 to Hght a verv remarkable phaenomeuon, which is, the general 

 increu'ie of I he intensity in proceeding from the magnetic equator 

 towards the poles. 



The same compass which at the departure of M. Humboldt 

 gave 245 oscillations in ten minutes at Paris, gave only 211 at 

 Peru; and it has constantly varied in the same direction, that is to 

 say, the number of oscillations has always diminished in ap- 

 proaching the magnetic equator, and always increased in re- 

 ceding from it towards the north. These differences cannot be 

 attributed to a diminution of the magnetic strength of the com- 

 pass ; nor can we suppose it would be weakened by the effect 

 of time and heat : for, after a residence of three years in the 

 warmest country of the earth, this compass again gave as rapid 

 oscillations at Mexico as at Paris. Lastly, M. Humboldt neg- 

 lected nothing in his observations to ensure exactness: and they 

 have since been confirmed by the results which he has found in 

 making the needle oscillate successively in the magnetic meri- 

 dian, and in the plane at right angles to it: for the dip obtained 

 from these data, agrees perfectly with that by experiment, al- 

 though he did not then know the connexion between these ele- 

 ments which has since been pointed out by M. Laplace. The 

 exactness of these observations never having been called in ques- 

 tion, the truth of the consequences derived from them must also 

 be admitted: and this is the increase of the terrestrial magnetic 

 force, in proceeding from the magnetic equator towards its poles. 

 The experiments made by M. Rossel, at Brest and in New Hol- 

 land, lead also to the same conclusion. 



The explanation just given of our knowledge of the magnetism 

 of the earth, is sufficient to show hovf very imperfect it is. Ig- 

 norant as we are of an immensity of essential data, principally 

 with respect to the variation, we cannot yet expect to arrive at 

 its true causes. We can therefore only seek for some empiric 

 laws, which, by embracing the greatest possible number of facts, 

 may bring before us their numeric relations, and point out the 

 principal elements on which to rest the observations. 



I have already mentioned, that a great portion of the observed 

 dips, especially in those parts of the globe where the magnetic 



The mean of the first 50 vibrations of all the thirteen experiments is 

 2' 49",4. The mean of the second 50 is 2' 32",? : consequently those 

 that receded farthest from zero required 16,"7 of time more tlian the last, 

 and each 100 vibrations on an average were made in 5' '23", 1. The dif- 

 ference between these times, shows liow necessary it is to begin counting 

 the vibrations when the needle reaches some stated number of degrees on 

 each side the magnetic north: for otherwise no comparison can be made 

 bctv/een the number of vibrations in different latitudes. 



equator 



