CHIMNEYS,. 3 
recede from each other and take up more fpace, fo that 
the fame weight of air heated will have more bulk, than 
equal weights of cold air which may furround it, and 
ia that cafe muft rife, being forced upwards by fuch cold- 
er and heavier air, which prefles to get under it and take its 
place. That air is fo raretied or expanded by heat, may be 
proved to their comprehenfion by a lank blown bladder, 
which laid before a fire will foon fwell, grow tight and 
burft. 
Another experiment may be to take a glafs tube about 
an inch in diameter, and twelve inches long, open at both 
ends and fixed upright on legs fo that it need not be han- 
dled, for the hands might warm it. At the end of a quill 
faften five or fix inches of the finett light filament of filk, 
fo that it may be held either above the upper end of the 
tube or under the lower end, your warm hand being at a 
diftance by the length of the quill. If there were 
any motion of air through the tube, it would ma- 
nifeft itfelf by its effet on the filk; but if the tube 
and the air in it are of the fame temperature with the fur- 
rounding air, there will be no fuch motion, whatever may 
be the form of the tube, whether crooked or ftrait, narrow 
below and widening upwards, or the contrary ; the air in it 
will be quiefcent. Warm the tube, and you will find as 
long as it continues warm, a conftant current of air enter- 
ing below and pafling up through it, till difcharged at the 
top; becaufe 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 prefles in below, 
forces it upwards, follows and takes its place, and is rare- 
fied in its turn. And, without warming the tube, if you 
hold under it a knob of hot iron, the air thereby heated will 
rife and fill the tube, going out at its top, and this motion in 
the tube will continue as long as the knob remains hot, 
becaufe the air entering the tube below is heated and ra- 
refied by pafling near and over that knob, That 
Plate I. 
Figure t. 
