68 



COSMOS. 



1827-1839. Quetelet, Etat du Magnetisme Terrestre (Brux- 

 elles) pendant douze annees. Very accurate observations. 



1827. Sabine, On the determination of the relative in- 

 tensity of the magnetic terrestrial force in Paris and London. 

 An analogous comparison between Paris and Christiana was 

 made by Hansteen in 1825-1828 (Meeting of the British As- 

 sociation at Liverpool, 1837, p. 19-23). The many results 

 of intensity which had been obtained by French, English, 

 and Scandinavian travelers now first admitted of being-; 

 brought into numerical connection with oscillating needles, 

 which had been compared together at the three above- 

 named cities. These numbers, which could, therefore, now 

 be established as relative values, were found to be for Paris, 

 1*348, as determined by myself; for London, 1-372, by Sa- 

 bine ; and for Christiana, 1 -423, by Hansteen. They all 

 refer to the intensity of the magnetic force at one point of 

 the magnetic equator (the curve of no inclination), which in- 

 tersects the Peruvian Cordilleras between Micuipampa and 

 Caxamarca, in south latitude 7° 2 7 , and western longitude 

 78° 48', where the intensity was assumed by myself as= 

 1*000. This assumed standard (Humboldt, Hecueil oV Ohserv. 

 Astr., vol. ii., p. 382-385 ; and Voyage aux Regions Equin., 

 t. iii., p. 622) formed the basis, for forty years, of the reduc- 

 tions given in all tables of intensity (Gay-Lussac, in the 

 Mem. de la Societe d? Arcueil, t. i., 1807, p. 21 ; Hansteen, 

 On the Magnetism of the Earth, 1819, p. 71; Sabine, in the 

 Report of the British Association at Liverpool, p. 43-58). It 

 has, however, in recent times been justly objected to on ac- 

 count of its want of general applicability, because the line 

 of no inclination* does not connect together the points of 



* "Before the practice was adopted of determining absolute values, 

 the most generally used scale (and which still continues to be very fre- 

 quently referred to) was founded on the time of vibration observed by 

 M. de Humboldt, about tbe commencement of the present century, at 

 a station in the Andes of South America, where the direction of the 

 dipping-needle was horizontal, a condition which was for some time 

 erroneously supposed to be an indication of the minimum of magnetic 

 force at the earth's surface. From a comparison of the times of vibra- 

 tion of M. de Humboldt's needle in South America and in Paris, the 

 ratio of the magnetic force at Paris to what was supposed to be its 

 minimum was inferred (1-348), and from the results so obtained, com- 

 bined with a similar comparison made by myself between Paris and 

 London in 1827, with several magnets, the ratio of the force in Lon- 

 don to that of M. de Humboldt's original station in South America 

 has been inferred to be 1372 to 1*000. This is the origin of the num- 

 ber 1*372, which has been generally employed by British observers. 



