450 
fary to defcend, during which the obferver 
repeatedly made an experiment, which alfo 
promifes to be of great utility to voyagers 
in the air, as well as to enlighten our con- 
clufions refpecting the phenomena of 
found. When they fpoke through a 
trumpet directed towards the earth, the 
Voice was returned with extreme precifion, 
and without feeming to have loft any part 
of its intenfity. No repetition was made 
except when the trumpet was directed to 
the earth ; and the intervals of reflection 
were different according to the elevation of 
the obfervers. The percuffion imprefied 
on the air by the found, every time pro- 
duced a flight undulation in the aeroftat ; 
whence they deduce an inference in favour 
of the fuppofed efficacy of cannon in partly 
modifying or averting the difcharge of 
ftormy clouds. In one of their experi- 
mients, the found employed ten feconds in 
its return, which would give a diftance of 
about two miles out and home, if the 
fame law of the velocity of found were fup- 
pofed to prevail in the perpendicular 
courfe as along the furface of the earth, 
which, however, does not feem likely.— 
The barometer ftood then at 27 inches, 
and at their outfet it was at 30 inches on 
the ground. It would be eafy, and it is 
furely defirable, to make experiments with 
eannon and ftop-watches on the velocity of 
afcending, and, if poflible, defcending, 
found. 
This refleGtion of found or eeho, is a 
fubject of very great curiofity. There is 
perhaps no other inftance in nature where 
fo extended a wall of refleCtion can be had. 
I am difpofed to think, that the apparent 
Intenfity of the returned found may} in 
fome meafure have depended on the perfect 
filence in which the fpeakers were placed. 
In a ftill night the centinels on the ram- 
parts at Portfmouth may be heard at the 
Tfle of Wight, over a diftance of five 
miles ; and there are numerous inftances 
of low founds, fuch as the beating of a 
clock or watch, or the founds of footfteps, 
being heard to confiderable diftances, 
when other founds do not aét on the organ 
of fenfe. 
In their defcent to the earth, they paffed 
through various ftrata of vapours, all of 
different temperatures ; and at the inftant 
the earth came in fight, the thermometer 
ftarted up through feveral degrees, proba- 
bly becaufe they had quitted a cold mafs 
ef vapour which obfcured their view, or 
perhaps becaufe the radiant heat of the 
earth’s furface might at that moment have 
«eached them unimpaired. - 
x 
Aeroftatie Voyage from Peterfourg to Paris: 
[June 1, 
Such are the obfervations made by 
Mefirs. Robertfon arid Sacharoff : we now 
proceed to thofe of M. Guy Lufac, 
who fays, ‘* Scarcely had J rifen five 
hundred toifes above the earth, when I 
faw alight vapour difperfed throughout 
the atmofphere below me, and through 
which I obferved diftant objefts confu 
edly. When [I reached the height of 
1555 toifes, I began to make my horizon- 
tal-needle ofcillate, and obtained twenty 
ofcillations in 83”, while, under the fame 
circumftances, at the earth, $37.33 would 
have been neceflary for the fame number. 
At the height of 1982 toifes, I found that 
the inclination of my needle,. taking a 
mean of the amplitude of the ofcillations, 
was fenfibly 319, as at the earth. Time 
and patience were neceflary to make this 
obfervation, becaufe, though carried away 
by the mafs of the atmofphere, I felt a 
light wind, which continually deranged 
the compafs. oe 
Some time after, I wifhed to obferve 
the dipping-needle. The drynefs, favour= 
ed by the action of the fun in a rarefied air, 
was fo great, that the compafs was fo far 
deranged, as to caufe the metallic circle, 
on which the divifions were traced, to 
bend and become warped, by. which i 
was prevented from making any accurate 
obfervations on the declination of the 
needle.” 
M. Guy Lufac found, from the moft 
accurate experiments, that the temperature 
follows an irregular Jaw with regard to 
the correfponding heights, which partly 
muft have arifen from this, that he made 
his obfervations {emetimes mm afcending. 
and fometimes in defcending, and the 
thermometer probably followed thele va- 
riations too flowly. But by confidering 
the degrees of the thermometer, which 
formed a decreafing feries, there was 
found a more regular law, which feemed 
to indicate, that, towards the furface of 
the earth, the heat followed a lefs decreaf- 
ing law than in the upper regions of the 
atmofphere, and at greater heights it fol- 
lows a decreafing arithmetical progreflion. 
The hygrometer had a very remarkable 
progrefs. At the furface of the earth it 
was only 57°%.5, but at the height of 
about 1500 toifes it marked 62°. From 
this point it continually fell till the height 
of about 2600 toifes, where it indicated only 
27.5%, and thence, for nearly a thoufand 
toiles higher, it gradually rofe to 44.5°.— 
To determine, from thefe refults, the law 
of the quantity of water diffolved in the 
air at different elevations, it was neceflary 
” 
te 
