68 COSMOS. 



(Bruxell.es) pendant douze annees. Very accurate observa- 

 tions. 



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

 sity of the magnetic terrestrial force in Paris and London. 

 An analogous comparison between Paris and Christiana was 

 made by Hansteen in 18251828 (Meeting of the British 

 Association at Liverpool 1837, pp. 19 23). The many 

 results of intensity, which had been obtained by French, 

 English, and Scandinavian travellers, 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 Sabine, 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 intersects the Peruvian Cordilleras between Micui- 

 pampa and Caxamarca, in south latitude 7 2' and western 

 longitude 78 48', where the intensity was assumed by 

 myself as = 1.000. This assumed standard (Humboldt, 

 Mecueil d'Observ. Astr. vol. ii, p. 382 385, and Voyage aux 

 Regions Equin., t. iii, p. 622) formed the basis, for forty 

 years, of the reductions 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 Hep. of the British Association at Liverpool, 

 pp. 43 58). It has, however, in recent times been justly 

 objected to on account of its want of general applicability, 

 because the line of no inclination 71 does not connect together 



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

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

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

 Mr. de Humboldt, about the 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 vibration of Mr. 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, combined with a 

 similar comparison made by myself between Paris and London, in 1827, 

 with several magnets, the ratio of the force iu London to that of Mr. 



