138 
has been published in the newspapers, 
&c.,1s perfectly true: viz. that; Monsieur 
Mongolfier hasl seme time before con- 
ceived the idea of flying up into the air 
by means of a very Jarge bag or balloon 
-of commogn, air, simply; rarified by fire or 
flame. This idea, being founded on a 
principle which has long been known, 
and which has no connexion with Mr. 
Cavendish’s discovery, it is only sur- 
prising that M. Mongolfier should not 
have put it sooner in practice. I sup- 
pose, therefore, that though he might 
have formed the projecta long time 
before, he never was roused. into an ex- 
ertion for making the trial until others 
began to talk of flying by means of in- 
flammable air. Who first thought of 
this method I cannot tell, for I confess I 
did not read the histories of these expe- 
riments; they never interested me in 
the least. ‘The hopes that means might 
be discovered for moving these machines 
at pleasure, and directing their course in 
the air, could only be formed by those 
who did not consider and understand 
their nature. The only circumstance 
which enables them to subsist in the air 
is, that however brisk the wind may be, 
as soon as they rise they are in effect 
perfectly becalmed; I mean that there is 
not the least perceptible motion or im- 
pulse of: the air against any part -of 
them. .This is perfectly evident from 
their situation; they float in mid air, 
and ‘have no hold on any thing else; 
the air must therefore carry them along 
with a velocity equal to its own, the 
machine of itself can make no resist- 
ance; but, while it moves along with the 
air, it cannot feel any motion of the air 
against it in any direction, it must, 
therefore, be in. effect, in a dead calm. 
the motion ef it cannot be perceived by 
them who are carried by it, except from 
the apparent motion with which the 
surface of the.earth seems to slide away 
from them, This tranquillity of the air 
with respect to them, is one cireum- 
stance which contributeg:to, that compo- 
sure of mind which miny of those have 
felt who have mounted. with balloons. 
I can imagine, however, that) ina hot 
climate, when the weather is perfectly 
calmed, the sun shining bright, a large 
balloon ascending with considerable ve- 
locity, may occasion a whirlwind, and 
may be involved in it, which might be at- 
tended with dangerous consequences to 
those who accompany it; you will easily 
understand how this may happen. But 
in other circumstances, there 1s no dan- 
ger of such an accident, the air will not 
Remains of eminent Persons. 
[Sept. 1, 
offer the smallest violence to the balloon, 
but, will carry it along like a feather, 
without making the least impression on 
any, part of it, and this, as I said above, 
is the reason why itis able to subsist in 
the air, for itis necessarily made of such 
flimsy materials, that it is quite unfit to 
sustain any, considerable shock or im- 
pression of the air on it; and this, 
among other things, shews the folly of 
attempting to give it motion at pleasure, 
or to command its motion and direction 
when it is up in the air; for, besides the 
impracticability of finding a power that 
can be applied to produce this effect, 
although the power were found, any 
attempt to make use of it would destroy 
the balloon, by impelling it against the 
air, and exposing it to such shocks and 
violence as it cannot bear; and, further, 
in attempting to use such a power, it 
must necessarily happen, on account of 
the great surface of the balloon and the 
resistance the air would make to its 
motion, that the men and machinery by 
which it were moyed, must of necessity 
go foremost, and the large bladder, &c., 
would be dragged after them in an hori- 
zontal direction, which would require 
an apparatus totally different from what 
has been contrived. ‘ 
Mrs. Young, Dr. Hutton, and, your 
other friends, are, well, and happy to 
hear that you find your situation agree- 
able. - Adieu, and believe me ever your 
affectionate Josreru, Brack. 
Dr. James Lind, Physician at Windsor, 
HELVETIUS. | 
Monsieur :—Je n’ai fait d’autre dé- 
marche sur votre derniére lettre que de 
parler a M. Marie pour l’engager a faire 
finir laffaire le plutdt qu’il sera possible 
a cause du cours, car je savais deja qu’il 
étoit arresté que vous auriez la place. 
Je ne doute pas que vous n’ayez votre 
brevet avant que M. Dargenson ne parte, 
du moins je le demanderai a M. Marie. 
Je vous fais mon compliment:sur le suc- 
cés de votre affaire et je le feray au 
publie quand vous commencez vos cours; 
et je vous prie d’étre persuadé que per- 
sonne dans le,monde n’est avec plus 
d’estime et d’attachement.—Monsieur, 
votre trés-humble et trés-obéissant ser- 
viteur, . oJ. HeLvetivs. 
- A Versailles, ce 30 Mai, 1750. 
CAR. HERSCHEL TO DR. LIND. 
Jan. 105.1790. 
Dear Sir,—If the. evening: (though 
unexpectedly) should turn out clear, 1. 
hope you will honour a comet. with a: 
peep 
