112 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) 

 giyes a mean annual diminution of the inclination at Paris 

 of 3'. 69. The intermediate periods stood as follows : 



From 17981810 at . . . . 5',08 



18101826 .. .. 3.37 



18261811 .. .. 3.13 



18411851 .. .. 3.40 



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 '.87 since 

 1798. The nearer the node of the magnetic equator approaches 

 to the meridian of Paris in its secular progression from east 

 to west, the slower seems to be the decrease, ranging 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. 38 

 Sabine more than 25 years after me measured the inclination 

 and intensity of the magnetic force at the Havanah, which in 

 respect to these equinoctial regions, affords a very considerable 

 interval of time, while he also determined the variation of 

 two important elements. Hansteen, in 1831, gave the 

 result of his investigations of the annual variation of the 

 dip in both hemispheres, 39 in a very admirable work which 

 is of a more comprehensive nature than my own. 



38 Humboldt, in Poggend. Annalen, Bd. xv, s. 319336, Bd. xix, 

 s. 357 391, and in the Voyage aux Regions Equinox, t. iii, pp. 616 

 625. 



39 Hansteen, Ueber jdlirliche Verdnderung der Inclination, in Poggend. 

 Ann. Bd. xxi, s. 403 429. Compare also, on the iofluence of the pro- 

 gression of the nodes of the magnetic equator, Sir David Brewster, 

 Treatise on Magnetism, p. 247. As the great number of observations 

 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 



