922 Account of an Aerostatic Voyage. 



to employ one fitter for tht purpose, which would detcf- 

 mine the declination with more precision, I shall ohserve> 

 that to attempt this experiment 1 had let down other needles, 

 separately, in linen bags to the distance of fifteen metres 

 below the car. 



That the whole of the results I obtained may be better 

 seen at one view, I have collected them in a table added to 

 the end of this memoir ; and thev are such as they occurred 

 to me, with the corresj)onding indications of the barometer, 

 the thermometer, and the hygrometer. The heights were 

 calculated, according to the formula of Laplace, by M. Gou- 

 iUy, engineer of bridges and causewavs, who was so kind 

 as to take this trouble. As the barometer did not sensibly 

 vary on the day of my ascent, from ten o'clock till three, 

 to calculate the different heights at which I made observa-' 

 tions, we took the height of the barometer, 70'56S centi- 

 metres, which was the height at the earth at three o'clock} 

 a height which, agreeably to the observations made by 

 M. Bouvard at the observatory, is greater bv 0*43 milli- 

 metres than that observed at the moment of our departure. 

 The heights of the barometer in the atmosphere were re- 

 duced to those which would have been indicated by a baro" 

 meter at a constant level placed under the saiT>e circum- 

 stances, and for each height was taken the mean between 

 the observations of two barometers. The temperature at 

 the earth, having varied between ten and three o'clock, it 

 was supposed constant and equal at SO* 75° of the centi- 

 grade ihermon^eter. 



If we now cast our eves on the table it will be seen that 

 the temperature follows an irregular law in regard to the 

 torresponding heights ; which no doubt arises from tl;is, — 

 that, having made observations sometimes in ascending and 

 sometimes in descending, the thermometer must have fol- 

 lowed these variations too slowly. But if we consider only 

 the degrees of the thermometer which form a decreasing 

 series, we shall iind a more regular law. Thus the tempe* 

 rature at the earth being 27*75, and at the height of 3(391 

 metres S-5", If v.'e divide the diflerence of the heights by 

 that of the temperatures we shall first obtain 191*7 metres, 

 or 9S'3 loiscs, of elevation for each lowering of tempera- 

 ture. Perforniing the same ojieration for the temperatures 

 .5-'25° and 0-5, as well as for those of 0*0" and — 9*5", wc 

 shall find in both cases 241*6 metres, or 72*6 toiscs of ele- 

 vation for each degree of lowering in the temperament, 

 which seems to indicate, that towards the surface of the 

 earth the licat follows a less decreasing law than in tha 



uppef 



