4 LET TER concerninc 
That this motion is produced merely by thedifference of 
fpecific gravity between the fluid within and that without 
the tube, and not by any fancied form of the tube itfelf, 
may appear by plunging it into water contained ina glafs 
jar a foot deep, through which fuch motion might be 
feen. The water within and without the tube being of 
the fame fpecific gravity, balance each other, and both re- 
main atreft. But take out the tube, ftop its bottom with 
a finger and fill it with olive oil, which is lighter than 
water, then ftopping the top, place it as before, its lower 
end under water, its top a very little above. As long as 
you keep the bottom ftopt, the fluids remain at reft, but 
the moment it is unftopt, the heavier enters below, forces 
up the lighter, and takes its place. And the motion then 
ceafes, merely becaufe the new fluid cannot be fucceflively 
made lighter, as air may be by a warm tube, 
In fact, no form of the funnel of a chimney has any 
fhare in its operation or effec refpeCting fmoke, except its 
height. The longer the funnel, if erect, the greater its 
force when filled with heated and rarefied air, to draw in 
below and drive up the fmoke, if one may, in compliance 
with cuftom, ufe the expreflion draw, when in fact it is 
the fuperior weight of the furrounding atmofphere that 
preffes to enter the funnel below, and fo drives up before 
it the {moke and warm air it meets with in its paflage. 
I have been the more particular in explaining thefe firft 
principles, becaufe, for want of clear ideas refpecting them, 
much fruitlefs expence has been occafioned; not only 
fingle chimneys, but in fome inflances, within my know- 
ledge, whole ftacks having been pulled down and rebuilt 
with funnels of different forms, imagined more powerful 
in drawing {moke; but having ftill the fame height and 
the fame opening below, have performed no better than 
their predeceflors. 
What is it then which makes a /izoky chimney, that is, 
a chimney which inftead of conveying up. all the fmoke, 
difcharges 
