32 APPENDIX. 
tinues till towards evening, when it again hefitates and 
changes its courfe, from the change of warmth in the 
outward air, and the nearly remaining fame middle tem- 
perature in the funnel. 
Upon this principle, ifa houfe were built behind Beacon- 
hill, an adit carried from one of the doors into the hill ho- 
rizontally, till it met with a perpendicular fhaft funk from 
its top, it feems probable to me, that thofe who lived in the 
houfe, would conftantly, in the heat even of the calmeft 
day, have as much cool air paffing through the houfe, as 
they fhould chufe; and the fame, though reverfed in its 
current, during the ftilleft night. 
I think, too, this property might be made of ufe to mi- 
ners; as where feveral fhafts or pits are funk perpendicu- 
larly into the earth, communicating at bottom by horizon- 
tal paflages, which is a common cafe, if a chimney of thirty 
or forty feet high were built over one of the fhafts, or fo 
near the fhaft, that the chimney might communicate with 
the top ‘of the fhaft, all air being excluded but what fhould 
pafs up or down by the fhaft, a conftant change of air 
would, by this means, be produced in the paflages below, 
tending to fecure the workmen from thofe damps which 
fo frequently incommode them. For the frefh air would 
be almoft always going down the open fhaft, to go up the 
chimney, or down the chimney to go up the fhaft. Let 
me add one obfervation more, which is, that if that part 
of the funnel of a chimney, which appears above the 
roof of a houfe, be pretty long, and have three of its fides 
expofed to the heat of the fun fucceflively, viz. when he is 
in the eaft, in the fouth, and in the weft, while the north 
fide is fheltered by the building from the cool northerly 
winds; fuch a chimney will often be fo heated by the 
fun, as to continue the draft ftrongly upwards, through the 
whole twenty four hours, and often for many days toge- 
ther. If the outfide of fuch a chimney be painted black, 
the effect will be ftill greater, and the current ftronger. 
N° Il. 
