AfiROSfATIC! MACHINE^,. 1^5 



Tlie Committee of Public Safely, and afterwards the Exe- Experiments 

 cutive DIreaory, thought that the application of aeroftatS to ^"'^^"j^J^f 

 inilitary inquiries of every defcription ought to be fludied and the French 

 praaifed during peace. They were alfo defirous, that they government, 

 might be employed in the conftruclion of geographical plans. 

 Or at leaft in afcertainitig the intermediate particulars of the 

 territory between the points which had been geometrically de- 

 termined. Having been charged with the experiments re- 

 lative to thefe diiferent applications, I purpofe giving an ac- 

 count of the principal refults. 



The intention, from the firfi afcents, was to meafure the FIrft attempts at 

 angle formed by the vifual rays falling on the eye of the aerial ^!fie"of \he vi- 

 obferver, from feveral determinate points on the earth. The fualrays. 

 unavoidable motion of the aeroftat preventing the ufe of the 

 graphometer in this operation, a recipiangle was at firft fub- 

 ftituted, fufpended like a mariner's compafs, by the affiftanee 

 of which, it was hoped the meafure of the angles would be 

 eafily taken, and particularly that they would be obtained wiih 

 immediate relation to a horizontal plane. This attempt not 

 having fucceeded, it was neceflary in future to make ufe of a 

 fextant. 



This inftrument was every thing that could be defired for The ufe of the 

 celerity, as well as for the facility and precifion of the obfer- free^fVom "rror. 

 vations, but it has this inconvenience, in the cafe in hand, that 

 it only (liews the angle on a plane inclined to the horizon ; and 

 moreover, in its ordinary condruaion it furniflies no means of 

 noticing this inclination. The perpetual agitation of the ma- 

 chine is another fource of error; in faa, an aeroftat, kept ele- 

 vated and held by cords, is continually changing poiition; it 

 moves in fpace, defcribing alternate ellipfes, the curvature of 

 which is modified to infinity, according to tlie violence of the 

 wind, the elafticity of the cords, and the filuation of the places 

 to which it is fattened. It leaves then no trace of its variations, 

 and does not permit the obferver which it fupports, to add to 

 the meafure of any angle whalfoever, that of the two angles 

 neceflfary to conned the firft with the plane of the horizon. 



Neverthelefs, for plans relative to the generality of military But Is never- 

 inquiries, and in all cafes where a fl-cetch of the figure of the ^^^^f^' ^PP''^^^ 



^ . „.,... ble in many 



earth is fufficient without attendmg to flight inaccuracies m cafes. 

 diftances, fimple obfervations, made with the fextant, will 

 anfwer the purpofe, and furnifli the means of operating with 

 O 2 facilitjr 



