Account of an Aerostatic Voyage. 221 



When T reached the height of 3032 metres, or 1555 toises, 

 I hegan to make my horizontal needle oscillate, and I ob- 

 tained 20 oscillations in S3", while at the earth, under the 

 same circumstances, 83*33''' would have been necessary for 

 the same number*. Though my balloon was affected by the 

 rotarv motion which I experienced in niv first voyan;e, the 

 motion i)f tltt; needle allowed me to count twenty, thirty, 

 and even fortv oscillations. 



At the height of 3S63 metres, or 19,521 toises, I found 

 that the inclination of my needle, taking a mean of the am- 

 plitude of the oscillations, was sensibly 31°, as at the earth. 

 A great deal of time and patience was necessary to make 

 this observation, because, though carried away by the mass 

 of the atmosphere, I felt a light wind, which continually 

 deranged the compass ; and, after several fruitless attempts, 

 I was obliged to renounce making any more observations. 

 I am of opinion, however, that the observation I here pre- 

 sent deserves some confidence. 



Some time after I wished to observe the dippino- needle ; 

 the follov.'ing was the result : — Tha dryness, favoured, by 

 the action of the siui in a rarefied air, was so great, that the 

 compass was so far deranged as to nuike the metallic circle 

 on which the divisions were traced out to bend, and 

 become warped. The motions of the needle could not be* 

 performed u ith the same freedom ; but, independentiy oi 

 this disappointment, I remarked that it was very difficult 

 to observe the declination of the needle with this apparatus. 

 It happened, indeed, that when I placetl the compass iu 

 such a manner as to make the shadow of a horizontal thread, 

 which served as a style, coincide with a fixed line, the mo- 

 tion I gave the compass communicated one to the needle; 

 and, when the latter had altained nearly to a state of rest, 

 the shadow of the style no longer coincided with the fixed 

 line. It was still necessary to put the compass into a ho- 

 rizontal position ; and during the time which this operation 

 required, every thing was again deranged. Without per- 

 sisting to make observations in which I could pl^ce no cou- 

 lidence, I gave them up entirely; and, free from every other 

 care, 1 directed the whole of my attention to the oscillations 

 of the horizt.ntal needle. I am, however, convinced, iu 

 ackno\\ledging the faults of my compass, that it is possible 



• Though I here iii'licate hundredth parts of a second, it may be rea- 

 dily coiicovc-J tliat 1 was not :iilc to observe fractions so sniail ; but 

 tlity Were j^i^cfi to me by diviiion, bccSu'c at the earth I n.a'.lc com- 

 Iiionly thiity OJiilldUOns whish reuiMred ia6'5". 



to 



