110 



COSMOS. 



found 66° 35' — all these observers adopting similar methods 

 and using similar instruments. This entire period, which 

 extends over more than half a century (from 1798 to 1851), 

 gives a mean annual diminution of the inclination at Paris 

 of 3 /, 69. The intermediate periods stood as follows : 



From 1798 to 1810 at 5'-08 

 « 1810 to 182G " 3-37 



From 1826 to 1811 at 3'-13 

 " 1811 to 1851 " 3-10 



The decrease between 1810 and 1826 has been strikingly 

 though gradually retarded ; for an observation which Gay- 

 Lussac made with extreme care (69° 12'), after his return in 

 1806 from Berlin, whither he had accompanied me after our 

 Italian expedition, gave an annual diminution of 4 / -S7 since 

 1798. The nearer the node of the magnetic equator ap- 

 proaches to the meridian of Paris in its secular, progression 

 from east to west, the slower seems to be the decrease, rang- 

 ing in half a century from about 5 / -08 to 3 /, 40. Shortly 

 before my Siberian expedition in April, 1829, I laid before 

 the Academy of Berlin a memoir, in which I had compared 

 together the different points observed by myself, and which, 

 I believe I may venture to say, had all been obtained with 

 equal care* Sabine, more than twenty-five years after me, 

 measured the inclination and intensity of the magnetic force 

 at the Havana, which, in respect to these equinoctial regions, 

 affords a very considerable interval of time, while he also de- 

 termined the variation of two important elements. Han- 

 steen, in 1831, gave the result of his investigations of the an- 

 nual variation of the dip in both hemispheres,! in a very ad- 



* Humboldt, in Poggend., Annahn, bd. xv., s. 319-336, bd. xix., s. 

 357-391 ; and in the Voyage aux Regions Equinox., t. iii., p. 616-625. 



f Hansteen, Ueber jahrliche Veranderung der Inclination, in Pog- 

 gend., Ann., bd. xxi., s. 403-129. Compare also, on the influence of 

 the progression of the nodes of the magnetic equator, Sir David Brew- 

 ster, Treatise on Magnetism, p. 217. As the great number of observa- 

 tions made at different stations have opened an almost inexhaustible 

 field of inquiry in this department of special investigation, we are 

 constantly meeting with new complications in our search for the laws 

 by which these forces are controlled. Thus, for instance, in the course 

 of a series of successive years we see that the dip passes in one of the 

 turning hours — that of the maximum from a decrease to an absolute 

 increase, while in the turning hour of the minimum the progressive 

 annual decrease continued the same. Thus, at Greenwich, the mag- 

 netic inclination in the maximum hour (9 A.M.) decreased in the 

 years 1811 and 1815, while it increased at the same hour from 1815 

 to 1816, and continued in the turning hour of the minimum (3 P.M.) 

 to decrease from 1811 to 1816 (Airy, Magn. Obscrv. at Greenwich, 

 1816, p. 113). 



