AEROSTATIC VOYAGE. 



53 



But one of the moft ftrikfng circumftances attending this Their dircaion 

 voyage is the rational means which thefe philofopers made "fe^^^J^^ ^^^^ 

 of to determine, and in a certain degree to regulate their courfe. 

 They made ufe of two inftrumenis, a log and a telefcope. The 

 log confifted of two ftieets of very thin paper, blacked, and— of paper, &c. 

 fixed at right angles to each other by a very light crofs of 

 wood. This was fufpended from their car by a ftring of fixty 

 feet in length, and affording a different refiftance to the air 

 from that of the balloon itfelf,t it was found to draw the ftring 

 out of the perpendicular direction, or as the narrators fay, to 

 follow ike balloon: (o that by its pofition determined by com- 

 pafs they could afcertain what direflion they were purfuing. 

 It alfo (hewed by its relative rife and fall whether the apparatus 

 was defcending or afcending, before their barometer had indi- 

 cated the flighted change. 



Their telefcope was applied to (liew the dire6lion of their Perpendicular 



courfe, and muft have been much lefs fubjea to doubt than ,^hlSi°^thek 



their log. Its application would be univerfal and perfed, if courfe and vclo- 



the earth could be (een at all times from the elevated regions of *^"^y ^^"^ ^^^^ 

 , . ^ ,. ^ , ,- , , . 11 on the ground, 



the air. Jt was directed perpendicularly downwards by means 



of a plumb-line, and having a confiderable magnifying power, 



the obje^s upon the furface of the earth were feen moving 



acrofs its field of view, and their diredion wouM moft clearly 



afcertain that of the car itfelf, and alfo its velocity. If, for 



example, the magnifying power were fifty times and the field Inveftlgationof 



of view one degree, the vifible fpace included in that field accuracrof^this 



from an elevation of two miles would be about 180 feet in di. method. 



ameter, in which objedls of fix or feven inches broad might be 



very well diftinguifiied through a favourable atmofphere; and 



at fo low a velocity as one mile an hour the whole field of view 



would be paffed over in about twelve feconds. Hence we fee 



that the method affords a confiderable degree of accuracy, 



and will not in general require any great power of magnifying 



or delicacy of obfervation. The computation would be founded 



on the following problem^ which will not prefent any difficulty 



to thofe who are acquainted with thefe fubjeds, if the phyfical 



allowances for temperature in barometrical admeafurements be 



admitted to be correal enough for this purpofe. 



t Or rather, perhaps, becaufe not exaflly in the fame current of 

 tl^e atmofphere. N» 



Given 



