FURNACE. 
the air, and for a certain length of time the heat will con- 
tinue to increafe till it arrives at its maxim 
he i. air leaned is as the 
here and the heated air. i 
motioa arifing from a change of fpecific ravity may 
referred to the cafe of two weights hanging over a pulley. If 
y of equal weight no n is produced, but if one 
ncreafed, or 
the ot 
de fcend and the ia one oe end, with a velocity propor- 
onate to their differe 
f the fum of the encicre be W and their difference P, 
then, according to Mr. Atwood’s theorem, whichhe confirmed 
by experiment, the velocity at any point will be = multi- 
plied by the velocity which a body would acquire by falling 
freely through the fame fpace. 
The rifing or falling of bodies in different fluids may be 
referred to the fame calculation, by fubftituting their f{pecific 
gravities for the abfolute gravity of the we igh ts 
Let 1 = the dentity of the furrounding ai 
e difference of tempe ae ‘between the 
fur rounding and heated air in degrees of Fahren 
e number of degrees of ae ne to 
the {pace a body will fall "through in a fecond of 
time, 
v = the velocity of the afcending air: then the increafe 
of bulk in the heated air will be 1 + a and the denfity, 
- compared with the furrounding air, will be 
P 
Pag’ The 
: difference of denfity will ber — 
2P 
, and the 
fum of their denfities a 3 then ao == ares 
This fraction, coeds to Mr. Atwood’s 
ae ae 
a ae 
a d j 
~2P+d° 
theorem, multiplied into the velocity a body would acquire 
Py fling through 4, will give the velocity of the afcending 
‘8: ee 4 Sh; then 2S: 4: will be the velo- 
eity a boy seul acquire by falling through 4. 
we ¥ yt 
Hence 5 x 2575 
afcending current. 
_in fnding dy let the ienpeaeee at the bottom of the 
himn that at top T’; and let ee sempertne of 
he air Paes enters at the grate be M: then Je M. — 
B+ T 
v, the velocity e the 
The above theorem will require-an equation for the fric- 
tion of the tube, oe will be very great in chimneys made 
of brick. It is plain that if it were not for the friction of 
the tube, the locity would be as the fquare root of. t 
height, all other things being ji al. 
Ta order to eatin how the friction of the tube 
affects ‘the ratio of the fquare root t of = height, I he (fays 
the writer of this article) an iron tu one foot long and 
two inches in diameter, having a faa fhaped a at the 
lower end, four inches wide, ‘and-about the fame length. At 
the top of the tube was placed a fmall paper fly, with its 
in the brick work at r 
axis = fo that it might be turned by the a curs 
rent, in a manner fimilar to “the fly of a fmoke jac 
I oa that a very fmall change of temperature was 
capable of putting the fly in motion. And that keeping 
both my hands upon the outfide of the funnel, at the fame 
time that peak was free accefs of air into its mouth, I had 
Pahren- 
by co 
theorem oe 
econd, wh ch was nea rut 
e fi t moved and placed at the top of t 
tube, ened . four feet. The velocity in this hes 
y the theorem fhould have been 75, but did not exceed 
five, fo that is defect produced b 
ratio, that is 
- ae boule or as d, then th ie friction will eS aye 
ratio; fort are of t 
fies 
increafing d, mes as great.. 
Hence the height of a chimney cannot ae the power of 
a furnace only to a certain extent, fince the friGtion, after a 
certain limit, would prevent the acceleration, and bring the- 
velocity to an uniform motion 
By other experiments I found that the manner’in which : 
the air enters the mouth of the tube is of fome confequences. 
about two ee 
he tube w: 
ef 
ha: aint ficient to put t motion, and it would ee 
eg till the none was elevated four or five: 
ae “Tf, ioe: one-half the funnel refted upon the 
table, and the other over the edge of it, the fly went at the 
rate of 45 revolutions in a minute. When the funnel’s mouth. 
= covered with paper, having a hole in the cue ale 
o the area of the tube, 47 revolutions were produced ; 
when the whole mouth was expofed about the om ere 
uced 49 revolut 
rhen ‘heal air can afcend freely - 
o the entrance of the furnace, it is ta: 
reateft advantage. 
Since, agreeably to the nature of our theorem, the ve-- 
locity of the-eurrent muft depend upon the difference of tem-- 
perature between the internal and external air, the air entering: 
the furnace ought to be as cool as poflible. furnace, - 
therefore, fhould be fo fituated, that its air may be fupplied 
fi ar below, into which its.afh-pit fhould terminate. 
This cellar diel be as fpacious-as poffible, and the afh-pit: 
fh fiance? width of. the furnace,, 
g fro e grate to the floor ; but the higher the 
becaufe ae air, as we ce before ftated, is fupplied.. 
with fo much more facility in a perpendicular dire@tion. 
The grate of an air-furnace confifts of a number of fingle- 
bars, having fhoulders: at each end, fo that when they are- 
laid fide by fide, bringing their fhoulders together, the 
interftices between U 
. een the bar erK 
n common ufe, but for producing great. 
jill be found to 'anfiwer 4 
bearers being loofe for the purpofe of fliding backwards to 
let all the bars fall down with the fire.. Without this con- 
vi trivance,. 
