ed air above the opening AB,
and the superior levity of this ought likewise to be taken into
consideration; but, as this portion is continually coolled and carried
off by the external air, it cannot produce any great effect.

But, if we add to this furnace a large hollow tube GHAB of the same
diameter, which preserves the air which has been heated by the burning
coals from being coolled and dispersed by the surrounding air, the
difference of specific gravity which causes the circulation will then be
between two columns equal to GC. Hence, if GC be three times the length
of AC, the circulation will have treble force. This is upon the
supposition that the air in GHCD is as much heated as what is contained
in ABCD, which is not strictly the case, because the heat must decrease
between AB and GH; but, as the air in GHAB is much warmer than the
external air, it follows, that the addition of the tube must increase
the rapidity of the stream of air, that a larger quantity must pass
through the coals, and consequently that a greater degree of combustion
must take place.

We must not, however, conclude from these principles, that the length of
this tube ought to be indefinitely prolonged; for, since the heat of the
air gradually diminishes in passing from AB to GH, even from the contact
of the sides of the tube, if the tube were prolonged to a certain
degree, we would at last come to a point where the specific gravity of
the included air would be equal to the air without; and, in this case,
as the cool air would no longer tend to rise upwards, it would become a
gravitating mass, resisting the ascension of the air below. Besides, as
this air, which has served for combustion, is necessarily mixed with
carbonic acid gas, which is considerably heavier than common air, if the
tube were made long enough, the air might at last approach so near to
the temperature of the external air as even to gravitate downwards;
hence we must conclude, that the length of the tube added to a furnace
must have some limit beyond which it weakens, instead of strengthening
the force of the fire.

From these reflections it follows, that the first foot of tube added to
a furnace produces more effect than the sixth, and the sixth more than
the tenth; but we have no data to ascertain at what height we ought to
stop. This limit of useful addition is so much the farther in proportion
as the materials of the tube are weaker conductors of heat, because the
air