so Questions for Solution relating to Navigation, 



Bonite, for at present the theorist is arrested at every step by 

 the want of exact measurements. 



The aerial excursions of MM. Biot and Gay Lussac, former- 

 ly undertaken under the auspices of the Academy, were in a 

 great measure designed for the examination of the following im- 

 portant question ; Has the magnetic force, which, on the surface 

 of the earth, directs the magnetic needle towards the north, ex- 

 actly the same intensity at every height to which it may be ele- 

 vated ? 



The observations of our two associates, those of M. de Hum- 

 boldt in mountainous countries, as well as the observations of 

 Saussure, of much older date, all seem to concur in shewing, 

 that at the greatest heights which man has yet reached, there is 

 no appreciable decrease in magnetic force. 



This conclusion has recently been disputed. Some have re- 

 marked, that, in the ascent of M. Gay-Lussac, for example, the 

 thermometer which indicated 87°.8 Fahr. (-}- 31 cent.) on the 

 ground at the time of departure, sunk as low as 15°.8 Fahr. 

 (-f- 9°.0 cent.) in the region of the atmosphere where the needle 

 was made to oscillate a second time. But it is now proved that 

 the same needle, occupying the same place, and under the in- 

 fluence of the same force, will oscillate so much the more quick- 

 ly, according to the lowness of the temperature. Thus, in or- 

 der to make the observations in the balloon and those on the 

 earth comparable, it would be necessary, on account of the state 

 of the thermometer, to make a certain diminution in the inten- 

 sity indicated by the higher observations. Without this correc- 

 tion, the needle would appear equally attracted above and be- 

 low ; in spite of appearances, there was therefore a real decrease. 

 This diminution of the magnetic force with the elevation seems 

 likewise to result from the observations made in 1829, on the 

 summit of Mount Elbrouz (in the Caucasus), by M. Kupffer. 

 In this case an exact account was taken of the effects of tem- 

 perature, and yet diverse irregularities in the inclination threw 

 some doubts on the result. 



We conceive, therefore, that the comparison of the magnetic 

 intensity, at the base and at the summit of a mountain, is a 

 matter which ought to be particularly recommended to the offi- 

 cers of the Bonite. Mowna-Roa, in the Sandwich Islands, seems 



