376 Account of an Aerostatic Voyage 



first, in regard to the experiments of Sauss-ure, it appears to 

 ns, it' we dare say so, that some fault must have been com- 

 mitted. This is cieailv seen by the numbers he has men- 

 tioned. When he attempted to determine the magnetic 

 force of his needle at Geneva, he found for the time of 20 

 oscillations 302, 2<}0, 300, 280 seconds, — results which 

 cannot well be compared, as their difference amounts to 12. 

 On the other hand, in the preliminary experiments which 

 we made before we set out, we never found a difference of 

 half a second in the time of 20 oscillations. There is an 

 error also in the calculation made by Saussure to compare 

 the magnetic force on the mountain and in the plain ; and ; 

 therefore it needs excite no astonishment that his results 

 should be different from those obtained by us. But it ap- 

 pears to us that ours are preferable because they seem to 

 agree better, and because we ascended to a greater height. 



Jn regard to the other observation, made by some philo- 

 sophers, on the irregularities of the compass when one rises* 

 in the atmosphere, it appears to us that it may be easily 

 explained from the continual rotation of the machine already 

 mentioned. These observers, indeed, must have turned 

 round as we did, since the impulse of the gas alone, as it 

 escapes on opening the valve, is sufficient to produce that 

 effect. If they did not observe this circumstance, the needle 

 which turned' along with them must have appeared to them 

 uncertain and without any determinate direction. This 

 irregularity was therefore only an illusion produced by their 

 own motion. 



We have still to remove one doubt which maybe ex- 

 cited in regard to our experiments. It may be apprehended' 

 that our watches were deranged during the journey, so that 

 some variation might take place in the magnetic force with- 

 out our observing it; but since we perceived no difference, 

 the magnetic force in this supposition and the going of our 

 watches must have varied in a contrary direction, precisely 

 in the same ratio, and in such a manner as to compensate 

 exactly : an hypothesis exceedingly improbable' and alto- 

 gether inadmissible. * 



We were not able to observe with so much accuracy the 

 inclination of the magnetic needle, and we. therefore can- 

 not positively assert that it experiences no variation. It is* 

 however very probable it does not,, since tile horizontal 

 force is not altered : but we assured ourselves, at least, that 

 its variations, if they exist, are very inconsiderable; for our 

 magnetic bars, brought into equilibrium before our depar- 

 ture, constantly retained during the whole journey their 



horizontal 



