10 LETTER concernine 
mits the outward air, and-by the continual whirling of 
the vanes, does in fome degree difperfe it. The noife 
only, is a little inconvenient. , 
2. A fecond caufe of the fmoking of chimneys is, their 
openings in the room being too large ; that is, too wide, too 
high or both, Architects in general have no other ideas. 
of proportion in the opening of a chimney, than what 
relate to fymmetry and beauty, refpeCting the dimenfions. 
of the room*; while its true proportion, refpecting its 
funtion and utility depends on quite other principles ; 
and they might as properly proportion the ftep in a ftair- 
eafe to the height of the ftory, inftead of the natural ele 
vation of men’s legs in mounting. The proportion then 
to be regarded, is what relates to the height of the funnel. 
For as the funnels in the different {tories of a houfe are 
neceflarily of different heights or lengths, that from the 
loweft floor being the higheft or longeft, and thofe of the 
other floors fhorter and fhorter, till we come to thofe in 
the garrets, which are of courfe the fhorteft; and the force 
of draft being, as already faid, in proportion to the height 
of funnel filled with rarefied air; and a current of air from 
the room into the chimney, fufficient to fill the opening, 
being neceflary to oppofe and prevent the {moke coming 
out into the room; it follows that the openings of the 
longeft funnels may be larger, and that thofe of the fhorter 
funnels fhould be {maller. For if there be a large open- 
ing to a chimney that does not draw ftrongly, the funnel 
may happen to be furnifhed with the air it demands by a 
partial current entering on one fide of the opening, and 
leaving the other fide free of any oppofing current, may, 
permit the {moke to iffue there into the room. Much too 
of the force of draft in a funnel depends on the degree of 
rarefaCtion in the air it contains, and that depends on the 
nearnefs. to the fire of its paflage in entering the funnel: 
If it can enter far from the fire on each fide, or far above 
the fire, in a wide or high opening, it receives little heat 
Im. 
* Sce Appendix, NO I. 
