1805. | 
620 feet, they defcended a little by the 
condensation of the gas 5 but rofe again 
by throwing out a little of the ballatt,— 
The ufual phenomenon of a flow rotation 
of the balloon prefented itfelf, which 
doubtlefs arofe from the unequal action of 
the air againft an irregularfurface, as we 
fee in moft other bodies rifing or falling in 
a fluid. 
But one of the moft ftriking circum. 
ftances attending this voyage, is the ra- 
tional means which thefe philofophers 
made ufe of to determine, and, in a cer- 
tain degree, to regulate their courfe.— 
They made ule of two inftruments, a log 
and a telefcope. The log confifted of two 
fheets of very thin paper, blacked, and 
fixed at right angles to each other by a 
very light crofs of wood. This was fuf- 
pended from their car by a ftring of fixty 
feet in length, and affording a different 
refiftance to the air from that of the ‘bal- 
Joon itfelf,* it was found to draw the 
ftring out of the perpendicular direétion, 
or, as the narrators fay, to follozw the bal- 
loon ; fo that by its pofition, determined 
by compas, they could afcertain what di- 
rection they were purfuing. It alfo thew- 
ed, by its relative rife and fall, whether 
the apparatus was defcending or -afcend- 
ing, before their barometer had indicated 
the flighteft change. 
Their telefcope was applied to thew the 
direction of their courfe, and muft have 
deen much lefs fubjeét to doubt than their 
log. Its application would be univerfal 
and perfect, if the earth could be feen at 
ail times from the elevated regions of the 
air. It was direéted perpendicularly 
downwards by means of a plumb-line, and 
having aconfiderable magnifying power, 
the objeéts upon the furface of the earth 
were feen moving acrofs its field of view, 
and their direétion would moft clearly af- 
eertain that of the car itfelf, and alfo its 
velocity. If, for example, the magnity- 
ing power were fifty times, and the field of 
view one degree, the vifible {pace included 
in that field, from an elevation of two 
miles, would be about 480 feet in diame- 
ter, tn which objects of fix or feven inches 
broad might be very well diftinguifhed 
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 wiil not in general require 
_ ™ Orrather, perhaps, becaufe not exactly 
Ww the fame current of the atmofphere. 
Monrury Mag. No. 129: 
Aeroftatic Voyage from Peterfourg ta Paris. 449 
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 fub- 
jects, if the phyfical allowances for tem- 
perature in barometrical admeafurements 
be admitted to be correct enough for this 
purpofe. 
Given the temperature on the earth and 
in the car, the height of the barometrical 
column, and the time employed in the ap- 
parent tranfit of an object an the earth 
through a given angle or field of view ; 
to find the velocity of the obferver. 
For practice, it would perhaps be fufh- 
ciently exact and convenient to compute a 
{mall table, to which, neglecting the tem- 
perature, the velocity in miles per hour 
might be had by infpection, when the 
height of the mercury, the time of tranft, 
and the magnifying power, were known. 
The aéronauts having noticed by thetic 
inftruments what were the direétion of the 
currents of air at different heights, found 
themfelves in one which carried them di- 
rectly towards the Baltic. ‘They theres 
fore defcended till they faw by the baro- 
meter that they had returned to a current 
which carried them inland; and after-~ 
wards again rofe much higher, and faw 
with great precifion- by their telefcope the 
inftant of their quitting the gulph. When 
the barometer ftood at 24 inches, they let 
go a pigeon, who flew with difficulty, and 
would not quit the balloon; but, upon 
being precipitated, he in vain endeavcured 
to regain it, and at length defcended 1a- 
pidly towards the earth. At ten at night 
the balloon had rifen to an height indicat- 
ed by 22 inches of the mercurial column, 
the thermometer ftanding at 42 degrees (I 
fuppofe centigrade). Here it was that M. 
Sacharoif carefully obferved a phenomenon 
which, had been before remarked by M. 
Robertfon, in his afcent from Hamburgh, 
but at a much greater elevation. Their 
dipping-needle was deranged ; but on in- 
fpecting the common compafs, its needle 
was found to be no longer horizontal, the 
north end being elevated near 10 degrees. 
Oa this phenomenon they remark, that the 
magnetic attraction probably diminifhing 
as the fquare of the diftance may afford ad- 
ditional means of direéting future ob- 
fervers in the atmof{phere, and even deter- 
mine the elevations independently of the 
barometer. From the prefent elevation, a 
pigeon being thrown down, fell io directly, 
that it was doubted whether he could 
have reached the earth alive. 
Daiknefs coming on, it became necef- 
3M fary 
