AT NIGHT, FROM THE EARTH, ETC. 
151 
Three inches above long grass fully exposed to the sky the mean reading would be 4-39 
Six inches above long grass fully exposed to the sky the mean reading would be 6’02 
One foot above long grass fully exposed to the sky the mean reading would be 7'31 
Two feet above long grass fully exposed to the sky the mean reading would be 7‘6 7 
Four feet above long grass fully exposed to the sky the mean reading would be 7*81 
Six feet above long grass fully exposed to the sky the mean reading would be 7'96 
Eight feet above long grass fully exposed to the sky the mean reading would be 8'26 
Twelve feet above long grass fully exposed to the sky the mean reading would be 8'27 
And the one in air at the height of four feet, protected from six-tenths of the 
sky, would be 8‘39 
The numbers in the last column of Table XLV., opposite to the respective heights 
above long grass, are factors deduced by considering the radiation from long- 
grass to be represented by unity, and therefore the numbers represent the factors 
corresponding to the cooling effect of a body cooled by radiation upon the air at 
different heights above it. They may be considered to apply to all bodies whose 
radiating powers are known, the cooling effect of which upon the air at any particular 
distance above them, less than twelve feet, will be known by multiplying the mean 
radiating power into the factor corresponding to that distance. 
The greater coldness of grass than that of the air in clear and calm weather, in 
places sheltered from the sun but open to a considerable portion of the sky, may 
continue all the day as well as night, and from many unrecorded observations, this 
appears to occur frequently. On September 22 d , 1843, this was observed to be the 
case; the day was fine, the sky was nearly cloudless, and the reading of the baro- 
meter was high. A thermometer on the grass \Vas placed in the shade, and exposed 
to as much of the sky as possible, and the following observations were taken : — 
1843. 
Reading of thermometer 
Excess of reading of 
Month, day, and 
hour. 
thermometer in air 
above that on grass. 
Remarks. 
In air. 
On the grass. 
h 
Sept. 21. 21 
m 
30 
62-8 
58-5 
O 
4-3 
Cloudless ; 
dew abundant. 
22 
15 
60-0 
54-3 
5-7 
Cloudless ; 
dew in globules on grass. 
23 
20 
65-5 
64-0 
1-5 
Large clouds to the north ; dew abundant. 
23 
50 
66*5 
61-5 
5-0 
Cloudless ; 
dew abundant. 
Sept. 22. 0 
0 
69-0 
62-0 
7-0 
A few clouds to the north. 
1 
0 
68-9 
61*5 
7-4 
Thin clouds to the north. 
1 
40 
68-7 
59*0 
9*7 
Cloudless.' 
1 
53 
68*7 
58-8 
9-9 
Cloudless. 
The thermometer was frequently 
3 
30 
67*0 
58-4 
8-6 . 
Cloudless. 
moved so as to keep it in the 
4 
0 
66-6 
59-4 
7-2 
Cloudless. 
> shade, and on grass upon which 
4 
20 
66-3 
58-7 
7-6 
Cloudless. 
the sun had not shone for a long 
4 
35 
65-8 
57*5 
8-3 
Cloudless. 
time. 
5 
0 
65-2 
56-8 
8-4 
Cloudless.^ 
