MAGNETIC INCLINATION. 107 



Arago, to whom the theory of magnetism is so largely in- 

 debted, had indeed recognised, in the autumn of 1827, " that 

 the dip was greater at 9 A.M. than at 6 P.M., whilst the inten- 

 sity of the magnetic force, when measured by the oscilla- 

 tions of a horizontal needle, attained its minimum in the 

 first, and its maximum in the second period." 34 In the 



w 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 Aurorpe boreales, which have been seen at Paris, that this pheno- 

 menon is always "accompanied by a variation in the position of the hori- 

 zontal and dipping needles, as well as in intensity. The changes of 

 inclination have amounted to 1' 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 obser- 

 vation is made, but in correcting the results by calculating the imme- 

 diate 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, 1 

 found a regular variation, not for the means but for each day, which 

 was greater in the morning at nine than in the evening at six. You 

 are aware that the intensity, measured ivith the horizontal 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 iuclination ; and, indeed, the greatest portion of the apparent 

 variation of intensity depends upon the diurnal alteration of the hori- 

 zontal component, but, when every correction has been made, there 

 still remains a small quantity as an indication of a real variation of in- 

 tensity." In another letter, which Arago wrote to me from Paris on the 

 20th of March, 1829, shortly before my Siberian expedition, he expressed 

 himself as follows : " I am not surprised that you should have found 

 it difficult to recognise 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 variation is sufficiently sen- 

 sible to be observed with a lens. I would still insist upon the fact, 

 that changes of inclination are not sufficient to explain the change of 

 intensity, deduced from the observation of a horizontal needle. An 

 augmentation of temperature, all other circumstances remaining the 

 same, retards the oscillations of the needles. In the evening, the tem- 

 perature of my horizontal needle is always higher than in the morning ; 

 hence the needle must on that account make fewer oscillations in a given 

 time in the evening than in the morning; in fact it oscillates more fre- 

 quently than we can account for by the change of inclination, and hence 

 there must be a real augmentation of intensity from morning till evening 

 in the terrestrial magnetic force." Later and more numerous observa- 

 tions at Greenwich, Berlin, St. Petersburg, Toronto, and Hobarton, 

 have confirmed Arago's assertion (in 1827) that the horizontal intensity 



