MAGNETISM, 



Academy of Sciences at Paris, in Nov. 

 1825, and of which an account is con- 

 tained in Pouillet's Elcmens de Phy- 

 sique *. 



7. Experiments on the Magnetic 

 Intensities at different Heights above 

 the Surface of the Earth. 



(345.) In the year 1804, Messrs. Gay- 

 Lussac and Biot undertook, at the de- 

 sire of the French government, an aero- 

 static voyage, expressly for the purpose 

 of ascertaining whether the magnetic 

 force experiences any perceptible dimi- 

 nution at considerable elevations above 

 the surface of the earth. De Saussure 

 had inferred, from some experiments 

 which he made on the Col du Geant, 

 near Mont Blanc, the height of which 

 is 3435 metres (about 11,270 feet), that 

 the magnetic force of the earth was re- 

 duced to four-fifths of what it was in the 

 plains below. The instrument with 

 which he made these experiments was 

 simply a magnetic needle, suspended by 

 a very fine silk thread. Messrs. Gay- 

 Lussac and Biot carried with them a 

 needle, carefully constructed by Fortin, 

 and magnetized by Coulomb, according 

 to ^Epinus's process. No iron was 

 allowed to "enter into the construction 

 of the car of the balloon ; the only arti- 

 cles of iron they carried with them, were 

 a few knives and a pair of scissors, 

 which were suspended in a basket below 

 the car, at a distance of from twenty-five 

 to thirty feet, so that they could have 

 no sensible influence on the magnetic 

 needle. The continual rotation of the 

 balloon on its axis, during its ascent, 

 seemed at first to present an insupera- 

 ble obstacle to their observing the oscil- 

 lations of the needle. But, by bringing 

 themselves in a line with terrestrial ob- 

 jects and the sides of the clouds, they 

 perceived that they did not always turn 

 round in the same direction, the rotatory 

 motion gradually decreasing, and then 

 taking place in a contrary direction. By 

 watching the short intervals during 

 which they remained stationary between 

 these opposite motions, they were ena- 

 bled to observe five, or at most ten 

 oscillations at a time ; they were obliged, 

 however, to be very careful not to agi- 

 tate the car, for the slightest motion, 

 such as that produced by letting the gas 

 escape, or even that of the hand in writ- 

 ing, was sufficient to turn the balloon 

 aside. With all these precautions, 

 which required a great deal of time, they 



Tom, i.,.p, 491. 



found means to make ten 

 in the course of the voyage, 

 ferent altitudes. The conclusion t 

 deduced from the average of all their 

 observations is, that the magnetic force 

 experiences no appreciable diminution 

 at any distance from the surface of the 

 earth, as far as 4000 metres, or 13,124 

 feet. 



(346.) Many important facts relating 

 to this question have lately been estab- 

 lished by M. Kupffer, in the course of 

 a journey to the neighbourhood of 

 Mount Elbrouz, in the Caucasus, under- 

 taken by order of the Emperor of Rus- 

 sia, in the year 1829, and of which an 

 account has been given in a paper pub- 

 lished in the Memoirs of the Imperial 

 Academy of Sciences of St.Petersburgh*. 

 M. Kupffer found that the intensity of ter- 

 restrial magnetism really decreased as he 

 rose above the level of the sea ; and that 

 this decrease was much more considerable 

 than is conformable with the commonly 

 received hypothesis of a focus of mag- 

 netic forces situated at the centre of the 

 globe. He even thinks that the experi- 

 ments of Gay-Lussac and Biot, in the 

 voyage just mentioned, should have led to 

 the same conclusion, because, although 

 they could not detect any difference 

 in the apparent intensity, yet since the 

 temperature of the elevated regions of 

 the atmosphere in which they made their 

 observations is exceedingly low, it is 

 probable that the magnetic power of 

 the needle itself was greater than in the 

 warmer atmosphere at the surface of 

 the earth ; so that, unless the terrestrial 

 force had really diminished, an increase 

 of magnetic intensity would have been 

 apparent, and would have been indicated 

 by the increased frequency of the vibra- 

 tions in a given time. As this increased 

 frequency was not observed to take 

 place, M. Kupffer concludes that the 

 force of terrestrial magnetism is, in fact, 

 less at the height to which they had 

 reached, than it is at the surface of the 

 earth. It is certain that, in all ex- 

 periments of this kind, the temperature 

 should be accurately noted, as consti- 

 tuting an essential element in the rea- 

 sonings to be founded on them t. 



CHAPTER VII. 

 Of the Magnetism of Bodies that are 



not ferruginous. 



(34 7.) It has long been suspected that 

 besides iron, other metallic substances 



* For 1830, p. 69. 



t See Journal of the Royal Institution, vol. i. p. 



