MAGNETIC INCLINATION. 105 



of Bab-el-Mandeb, namely, 1 south of the island of Soco- 

 tora, in 8 40' N. lat. There was, therefore, an alteration 

 of 1 21 '' lat. for 49 years, while the corresponding altera- 

 tion in the longitude was determined by Arago and Duper- 

 rey to have been 10 from east to west. The direction of 

 the secular variation of the nodes of the magnetic equator on 

 the eastern coasts of Africa, toward the Indian Ocean, was 

 precisely similar to that on the western coast. The quanti- 

 ty of the motion must, however, be ascertained from much 

 more accurate results than we at present possess. 



The periodicity of the alterations of the magnetic inclina- 

 tion, whose existence had been noticed at a much earlier pe- 

 riod, has only been established with certainty and thorough 

 completeness within the last twelve years, since the erection 

 of British magnetic stations in both hemispheres. Arago, to 

 whom the theory of magnetism is so largely indebted, had 

 indeed recognized, in the autumn of 1827, " that the dip was 

 greater at 9 A.M. than at 6 P.M. ; while the intensity of 

 the magnetic force, when measured by the oscillations of a 

 horizontal needle, attained its minimum in the first, and its 

 maximum in the second period."* In the British magnetic 



* In a letter from Arago to myself, dated Mayence, 13th of Decem- 

 ber, 1827, he writes as follows : "I have definitely proved during the 

 late Auroras Boreales, which have been seen at Paris, that this phe- 

 nomenon is always accompanied by a variation in the position of the 

 horizontal and dipping needles, as well as in intensity. The changes 

 of inclination have amounted to T or 8'. To effect this change, after 

 allowing for every change of intensity, the horizontal needle must 

 oscillate more or less rapidly, according to the time at which the ob- 

 servation is made, but in correcting the results by calculating the 

 immediate effects of the inclination there still remained a sensible 

 variation of intensity. On repeating by a new method the diurnal 

 observation of inclination, on which I was engaged during your late 

 visit to Paris, I found a regular variation, not for the means but for 

 each day, which was greater in the morning at nine than in the even- 

 ing at six. You are aware that the intensity, measured with the hori- 

 zontal needle, is, on the contrary, at its minimum at the first period, 

 while it attains its maximum between six and seven in the evening. 

 The total variation being very small, one might suppose that it was 

 merely due to a change of inclination ; and, indeed, the greatest por- 

 tion of the apparent variation of intensity depends upon the diurnal 

 alteration of the horizontal component, but, when every correction 

 has been made, there still remains a small quantity as an indication 

 of a real variation of intensity" In another letter, which Arago wrote 

 to me from Paris on the 20th of March, 1829, shortly before my Sibe- 

 rian expedition, he expressed himself as follows : " I am not surprised 

 that you should have found it difficult to recognize the diurnal change 

 of inclination, of which I have already spoken to you, in the winter 

 months, for it is only during the warmer portions of the year that this 



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