Account of an Aerostatic Voyage. 223 



upper parts of the atmosphere, and at greater heights it 

 follows a decreasing arithmetical progression. If we sup- 

 pose that from the surface of the earth, where tlie thermo- 

 meter was at 3-75^, to the height of 6977 metres, or 3.580 

 toises, where it fell to — Q'b"^, the heat decreased as the 

 heights increased, an elevation of 173'3 metres, or 88*9 

 toises, will correspond to each degree of the lowering of 

 temperature. 



The hygrometer had a very remarkable progress. At 

 the surface of the earth it was only bT'b°, while at the height 

 of 3030 metres it marked 62°. From this point it conti- 

 nually fell till the height of 5267 metres, where it indicated 

 only 27*5°, and thence to the height of 6884 metres it gra- 

 dually rose to 34-5". If we wish from these results to deter- 

 mine the law of the quantity of water dissolved in the air 

 at different elevations, it is evident that attention must be 

 paid to the temperature, and by adding this consideration 

 it will be seen that it follows an exceedingly decreasing pro- 

 gression. 



If we now consider the magnetic oscillations, it will be 

 remarked, that the time for ten oscillations, made at dif- 

 ferent heights, is sometimes above and sometimes below 

 that of 42' 16", which they require at the earth. Takins; a 

 mean of all these oscillations made in the atmosphere. Ten 

 oscillations will require 42-20", a quantity which differs 

 very little from the preceding 5 but if we consider only the 

 last observations made at greater heights, the time for ten 

 oscillations would be a litde below 4 2' 16", which would 

 indicate, on the other hand, that the magnetic force has a 

 little increased. Without wishing to draw anv consequence 

 from this slight apparent increase, which may arise from the 

 errors committed in experiments of this kind, I must con- 

 clude that the rf::ults 1 have presented confirm and extend 

 the fact obaerved by M. Biot and myself, and which, like 

 Viniversal gravitation, proves that the magnetic force does 

 not experience any sensible variation at the greatest heights 

 to which we can attain. 



The consequence we have deduced from our experiments 

 may appear a little too precipitate to those who reflect that 

 wc were not able to make experiments on the inclination of 

 the magnetic needle. Hut if it be recollected that the force 

 Wrliieh make* a horizontal needle oscillate, necossarilv de- 



f fends on »he intensity and direction of the macnetic force 

 iself, and ihat it is represented by the cosine of the angle 

 of the inclination of the latter force, no one can help con- 

 cludini^ with us, that, since the horizontal force did not 



varv, 



