ioS Mr, Six’s Experiments 
upper one, although they were placed fo near to each other. 
See Tab. V. 
From the foregoing experiments it appears, that a greater 
diminution of heat frequently takes place near the earth in 
the night-time, than at any elevation in the atmofphere 
within the limits of my inquiry ; and that the greateft degrees 
of cold are at fuch times always found neared to the luiface 
of the earth ; that this is a conftant and regular operation of 
nature, under certain circumftances and difpofitions of the 
atmofphere, and takes place at all leafons of the year ; that 
this difference never happens in any confiderable degree but 
when the air is dill, and the Iky perfectly unclouded*; but 
the moided vapour, fuch as dews and fogs -f-> did not, as far 
as I could perceive, at all impede, but rather increafe the refri- 
geration. In very fevere frods, when the air frequently depo- 
sits a great quantity of frozen vapour, I have generally found 
it greated J ; but the excefs of heat, which in day-time, in the 
fummer feafon, was found at the lower dation, in the winter 
diminhhed almod to nothing. 
The foregoing experiments related to the difference of heat, 
which, at certain times, is found at different altitudes; the 
* Free from clouds which float higher in the air than fogs, and are fufHciently 
opaque to hide the ftars from our view even in the zenith. 
+ When fogs covered the valley, the refrigeration below was confiderable; hut 
they very feldom reached fo high as the upper thermometer, and the Iky was dear 
above. No variation during rain, except mingled with hail. 
* This remarkable diminution of heat near the furface of the earth in clear 
weather, after the fun is fet, and in the night, proceeds, I apprehend, partly 
from the coolnefs which the dews or vapours poflibly may acquire in their defeent, 
and partly from the evaporation which takes place at the fame time from all moi i\ 
or frozen bodies expofed to the open air, particularly from thole on which the dew 
•r hoar froft is depofited. 
7 following 
