CHIMNEYS. 7 
tween the edge of the door and the rabbit it fhould fhut 
into. Suppofe the diftance to be half an inch, and the 
door eight feet high, you find thence that your room re- 
quires an entrance for air equal in area to ninety fix half 
inches, or forty eight fquare inches, or a paflage of fix 
inches by eight. ‘This however is a large fuppofition, 
there being few chimneys, that, having a moderate open- 
ing and a tolerable height of funnel, will not be fatisfied 
with fuch a crevice of a quarter of an inch; and I have 
found a fquare of fix by fix, or thirty fix fquare inches, 
to be a pretty good medium, that will ferve for moft 
chimneys. High funnels with fmall and low openings, 
may indeed be fupplied through a lefs fpace, becaufe, for 
reafons that will appear hereafter, the force of levity, if 
one may fo fpeak, being greater in fuch funnels, the cool 
air enters the room with greater velocity, and confequent= 
ly more enters in the fame time. This however has its. 
limits, for experience {hows that no increafed velocity fo 
occafidned, has made the admiffion of air through the 
key-ho'le equal in quantity to that through an open door; 
though through the door the current moves flowly, and 
through the key-hole with great rapidity. 
It remains then to be confidered how and where this 
neceflary quantity of air from without is to be admitted fo 
as to be leaft inconvenient. For, if at the door, left fo. 
much open, the air thence proceeds directly to the chimney, 
and in its way comes cold to your back and heels as you 
fit before your fire. If you keep the door fhut, and raife 
a little the fath of your window, you feel the fame incon- 
venience. Various have been the contrivances to avoid 
this, fuch as bringing in frefh air through pipes in the 
jams of the chimney, which pointing upwards fhould blow 
the fmoke up the funnel ; opening paflages into the fun- 
nel above, to let in air for the fame purpofe. But thefe 
produce an effect contrary to that intended: For as it is 
the conftant current of air pafling from the room through 
the 
