2 LETTER concernine 
is in its nature and of itfelf fpecifically lighter than air, 
and rifes in it for the fame reafon that cork rifes in water. 
Thefe fee no caufe why imoke fhould not rife in the chime 
ney, though the room be ever fo clofe. Others think 
_ there is a power in chimneys to draw up the {moke, and 
that there are different forms of chimneys which af- 
ford more or lefs of this power. Thefe amufe themfelves 
with fearching for the beft form. The equal dimenfions 
of a funnel in its whole length is not thought artificial 
enough, and it is made, for fancied reafons, fometimes ta- 
pering and narrowing from below upwards, and fome- 
times the contrary, &c. &c. A fimple experiment or two 
may ferve to give more correct ideas. - Having lit a pipe 
of tobacco, plunge the ftem to the bottom of a decanter 
half filled with cold water; then putting a rag over the 
bowl, blow through it and make the fmoke ferent in the 
ftem of the pipe, from the end of which it will rife in 
bubbles through the water; and being thus cooled, will 
not afterwards rife to go out through the neck of the de- 
canter, but remain f{preading itfelf and refting on the fur- 
face of the water. This fhows that fmoke is really hea- 
vier than air, and that it is carried upwards only when at- 
tached to, or aéted upon, by air that is heated, and there- 
by rarefied and rendered {pecifically lighter than the air in 
its neighbourhood. 
Smoke being rarely feen but in company with heated 
air, and its upward motion being vilible, though that of the 
rarefied air that drives it is not fo, has pateally given rife 
to the error. 
I need not explain to you, my learned athe what ts 
meant by rarefied air; but if you make the public ufe you 
propofe of this letter, it may fall into the hands of fome 
who are unacquainted with the term and with the thing. 
Thefe then may be told, that air is a fluid which has 
weight as well as others, though about eight hundred times 
lighter than water, That heat makes the particles of air 
recede 
