The Complete Works in Philosophy, Politics and Morals of the late Dr. Benjamin Franklin, Vol. 2 [of 3]

By Benjamin Franklin

Page 205

the quill. (Plate IX. fig. 1.) If there
were any motion of air through the tube, it would manifest itself
by its effect on the silk; but if the tube and the air in it are
of the same temperature with the surrounding air, there will be no
such motion, whatever may be the form of the tube, whether crooked
or strait, narrow below and widening upwards, or the contrary; the
air in it will be quiescent. Warm the tube, and you will find, as
long as it continues warm, a constant current of air entering below
and passing up through it, till discharged at the top; because the
warmth of the tube being communicated to the air it contains rarefies
that air and makes it lighter than the air without, which therefore
presses in below, forces it upwards, and follows and takes its place,
and is rarefied in its turn. And, without warming the tube, if you
hold under it a knob of hot iron, the air thereby heated will rise
and fill the tube, going out at its top, and this motion in the
tube will continue as long as the knob remains hot, because the air
entering the tube below is heated and rarefied by passing near and
over that knob.

That this motion is produced merely by the difference of specific
gravity between the fluid within and that without the tube, and not
by any fancied form of the tube itself, may appear by plunging it
into water contained in a glass jar a foot deep, through which such
motion might be seen. The water within and without the tube being
of the same specific gravity, balance each other, and both remain
at rest. But take out the tube, stop its bottom with a finger and
fill it with olive oil, which is lighter than water, then stopping
the top, place it as before, its lower end under water, its top a
very little above. As long as you keep the bottom stopt, the fluids
remain at rest, but the moment it is unstopt, the heavier enters
below, forces up the lighter, and takes its place. And the motion
then ceases, merely because the new fluid cannot be successively made
lighter, as air may be by a warm tube.

In fact, no form of the funnel of a chimney has any share in its
operation or effect respecting smoke, 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 smoke, if one
may, in

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