REPORT ON THE MAGNETISM OF THE EARTH. 123 



changes in their direction and intensity, it would only be re- 

 quired to show, that electrical phaenomena may be excited, in 

 such bodies as the earth and the atmosphere, by a disturbance 

 in their tempei'ature when in contact. As I consider that if 

 such an experiment were carefully made it must give conclusive 

 results, I would strongly suggest to the Council of the British 

 Association the importance of having it made. 



It has been a question whether the intensity of terrestrial 

 magnetism is the same at the surface of the sea and at heights 

 above that surface to which we can attain. MM. Gay-Lussac 

 and Biot, in their aerostatic ascent, could detect no difference 

 at the height of 4000 metres *. Saussure had, however, con- 

 cluded, from the observations which he made at Geneva, Cha- 

 mouni, and on the Col du Geant, that the intensity was consi- 

 derably less at the latter station than at either of the former, 

 the difference in the levels being in the one case about 10,000 

 feet, in the other about 7800 f. 



M. Kupffer J also considers that his observations in the vi- 

 cinity of Elbours, in which the difference of elevation of his two 

 stations was 4500 feet, show clearly that the hoi'izontal intensity 

 decreases as we ascend above the surface ; and he accounts for 

 this decrease not having been observed by MM. Biot and Gay- 



• Biot, Traite de Physique. 



t Voyages dans les Alpes, torn. iv. p. 313. — I take for granted that, admit- 

 ting the accuracy of Saussure's observations, they warranted the conclusions 

 he drew from them ; but some unaccountable errors must have crept in, either in 

 transcribing or in printing them ; for not only the means which be deduces do not 

 result from the observations, but the numbers which he employs contradict his 

 conclusions. I transcribe the passage from the only edition I can consult, pub- 

 lished at Neufchatel, 1796. "A' Geneve ces vingt oscillations employ erent 

 5m 2'; 4m 50»; 5m; 4™ 40'; dont la moyenne etoit 5™ 0'-4; le thermometre 

 6tant a 6 degres. A' Chamouni 5™ 33* ; 5"" 34' ; moyenne 5"" 33'-5 ; thermometre 

 12 deg. Au Col du G6ant 5™ 30'-3 ; 5" 30'-5 ; 5" 31'-4; 5™ 34'-6, moyenne 

 fim 32'-45; thermometre 12-4 degres." 



" Or les forces magnetiques sont inversement comme les quarres des terns. 

 Mais, a Geneve, le terns etoit .'5™ 0'-4 ou 300".4, dont le quarre = 111155-56 ; 

 a Chamoimi 5"" 33'-5 = 333''-5, dont le quarre = 111223. Au G^ant 5™ 

 32'-45 = 332'-45, dont le quarre = 11523-0025; d'ou il suivroit que la plus 

 grande force etoit dans la piaine, et la plus petite sur la plus haute montagne, 

 a peu pres d'une cinquieme : observation bien importante, si elle 6toit confirmee 

 par des experiences repetees, et faites a la mSme temperature." 



The means of the above observations are 4™ 53' = 293', 5"" 33'-5 = 333'-5, 

 and 5™ 31'-7 = 331*-7; and the squares of these numbers are 85849, 111222*25, 

 110024-89. So that, according to this, the force was greatest at Geneva, and 

 least at Chamouni. Taking Saussure's numbers, 300' 4, 333'-5, 332'-45, their 

 squares are 90240-16, 111222-25, 110523-0025; so that still the general con- 

 clusions are the same. 



t Voyage dans les Environs du Mont Elbroiiz. Rapport fait a V Academic 

 Imperiale des Sciences de St. Petersbourg, p. 88. 



