AEROSTATIC VOYAGE. 



281 



quired much time and patience to make this obfervation; be 

 caufe, although borne by tiie mafs of the atmofphere, I per- 

 ceived a flight wind which continually deranged the compals 

 and after feveral unfuccefcful attempts, I was obliged to give 

 up making new ones. Neverthelels, I believe the obferva- 

 tion I have ftated above is entitled to foine confidence. 



Some time after I wifhed to oblerve the dipping-needle; The obfervationi 

 but behold what had happened. The drought, favoured by mut ^ com oafs 

 the aclion of the fun in a rarefied air, was fuel), that the com- not to be de- 

 pafs was fo much warped as to have bent the metallic circle fcn e UiK,n * 

 on which the divifions were traced, and to be itfelf crooked. 

 The motions of the needle therefore had not the fame free- 

 dom ; but independently of this difappointment, I had ob- 

 ferved that it was very difficult to obferve the dip of the needle 

 with this apparatus. In fhort, when I placed the compafs Caufes rfiti de«* 

 fo as to make the fhadow of the horizontal thread, which 

 ferved as a ftyle, coincide wilh a fixed line, the motion I had 

 given to the compafs communicated one to the needle, and 

 by the time this was nearly reftored to reft, the fhadow of 

 the ftyle no longer coincided with the fixed line. It was ne- 

 ceflary to place the compafs again in a horizontal pofition, 

 and during the lime which this operation required, every 

 thing was again deranged. Unwilling to peril ft in making 

 obfervations on which I could place no reliance, I abandoned 

 them entirely ; and diverted of every other care, I gave all 

 my attention to the ofcillations of the horizontal needle. I The experiment 

 am neverthelefs convinced, by the knowledge of the defe&s ftiJ1 F afticabk « 

 of our compafs, that it is pofiible to employ one more fuitable, 

 which would determine the dip with great precifion. In 

 trying this experiment, I lowered the other needles feparately 

 in linen bags, to 15 metres below the boat. 



That the aggregate of all the relults which I obtained may The aggregate 



be more eafily feen, I have collected them into a table which ° f the rcf " lts 

 J formed into a 



is at the end of this memoir ■ and they are placed there as table, 

 they occurred to me, with the correfponding indications of 

 the barometer, thermometer, and hygrometer. The heights Calculation of 

 have been calculated, according to the formula of M. Laplace, \ e eig " ts> y 



o ' * the barometer 



by M. Gouilly, engineer of bridges and highways, who has compared with 

 been fo kind as to take this trouble. The barometer 11ot one on the earth: 

 having varied fenhbly on the day of my afcent, from ten 

 o'clock to three, to calculate the different heights at winch I 



made 



