APPENDIX. 31 
four fold, I am confident that if the linen is kept wet, by 
{prinkling it once a day, the meat would be ‘0 cooled by the 
evaporation, carried on continually by means of the pafling 
air, that it would keep a week or more in the hotteft wea- 
ther. Butter and milk might likewife be kept cool, in vef- 
fels or bottles covered with wet cloths. A fhallow tray, or 
keeler, fhould be under the frame to receive any water that 
might drip from the wetted cloths. I think, too, that this 
property of chimneys might, by means of {moke-jack vanes, 
be applied to fome mechanical purpofes, where a {mall but 
pretty conftant power only is wanted. 
If you would have my opinion of the caufe of this chang- 
ing current of air in chimneys, it is, in fhort, as follows. 
In fummer time there is generally a great difference-in the 
warmth of the air at mid-day and midnight, and, of courfe, 
a difference of fpecific gravity in the air, as the more it is 
warmed the more it is rarefied. ‘The funnel of a chimney 
being for the moft part furrounded by the houfe, is pro- 
tected, ina great meafure, from the direét ation of the 
fun’s rays, and alfo from the coldnefs of the night air. It 
thence preferves a middle temperature between the heat of 
the day, and the coldnefs of the night. This middle tem- 
perature it communicates to the air contained init. If 
the ftate of the outward air be cooler than that in the fun- 
nel of the chimney, it will, by being heavier, force it to 
rife, and go out at the top. What fupplies its place from 
below, being warmed, in its turn, by the warmer funnel, 
is likewife forced up by the colder and weightier air below, 
and fo the current is continued till the next day, when the 
fun gradually changes the ftate of the outward air, makes 
it firft as warm as the funnel of the chimney can make it, 
(when the current begins to hefitate) and afterwards warm- 
er. Then the funnel being cooler than the air that comes 
into it, cools that air, makes it heavier than the outward 
air, of courfe it defcends; and what fucceeds it from 2- 
bove, being cooled in its turn, the defcending current con- 
tinues: 
