214 
MR. GLAISHER ON THE RADIATION OF HEAT, 
The power of radiation, as exhibited in this Table, has evidently no tendency to 
increase with the heat, and in this respect does not serve even as an approximation to 
the law of radiation established by the experiments of MM. Dulong and Petit, viz. 
that the velocity of cooling in vacuo (or force of radiation) increases in the terms of 
a geometrical progression for excess of temperature in arithmetical progression ; it 
is most probable that the effect is masked by too many disturbing causes to enable 
us from mere inspection to discover the law of its progression. 
Table CVXI1. — Monthly mean of the minimum temperature of the Air. 
The monthly mean reading of the minimum self-registering thermometer placed in air at the height of 4 feet, 
according to the state of the sky. 
- 
State of the sky. 
Year. 
Month. 
Cloudy. 
Broken clouds. 
Half-cloudy. 
Principally clear. 
Cloudless. 
Number 
Mean of the 
Number 
Mean of the 
Number 
Mean of the 
Number 
Mean of the 
Number 
Mean of the 
of 
minimum 
of 
minimum 
of 
minimum 
of 
minimum 
of 
minimum 
nights. 
temperatures. 
nights. 
temperatures. 
nights. 
temperatures. 
nights. 
temperatures. 
nights. 
temperatures. 
1843. 
April. 
2 
43*9 
1 
37-4 
3 
42-8 
2 
38-3 
3 
40-5 
May. 
12 
46-9 
6 
46-0 
4 
47-0 
7 
43-0 
2 
43-1 
June. 
6 
51-4 
9 
49-8 
1 
47-0 
7 
48-0 
7 
47*2 
July. 
5 
53-3 
6 
55*2 
7 
54-1 
10 
49-7 
2 
54-2 
August. 
2 
57-5 
2 
59-4 
7 
59-2 
12 
53-5 
6 
52*5 
September. 
3 
58-2 
3 
54-3 
7 
55-2 
6 
48*1 
11 
50-8 
October. 
6 
44-9 
3 
44-4 
6 
50-0 
8 
40-4 
8 
34-6 
November. 
9 
42-5 
2 
49*2 
7 
41-6 
7 
39*8 
6 
31*0 
December. 
17 
40-5 
7 
42-8 
l 
40-1 
5 
37-6 
1844. 
January. 
8 
33*0 
7 
37-6 
5 
34*1 
7 
34-1 
2 
22-0 
February. 
7 
32*3 
6 
27“9 
6 
35-3 
6 
29-4 
4 
29-4 
March. 
8 
37-8 
5 
38-9 
9 
33-5 
2 
41-9 
7 
30-5 
April. 
3 
47-1 
2 
46-6 
3 
45-4 
4 
43-0 
18 
39-6 
May. 
9 
47-1 
1 
46-2 
6 
42-4 
5 
42-5 
10 
44*1 
June. 
2 
50-3 
10 
51-4 
7 
52-5 
5 
50*4 
6 
53*0 
July. 
4 
56*2 
9 
55*4 
5 
54-6 
1 
53-8 
12 
52-8 
August. 
7 
54-8 
1 
51-6 
ft 
51*1 
6 
49-3 
12 
47-7 
September. 
6 
55*3 
5 
54-1 
2 
59-7 
5 
50-8 
12 
44-3 
October. 
10 
47-0 
1 
45-3 
6 
45-3 
6 
46-0 
8 
40*1 
November. 
12 
42-6 
3 
42-0 
6 
38-9 
7 
33-5 
2 
41-6 
December. 
23 
31-9 
l 
30-1 
6 
24-6 
By taking the mean of these numbers according to the number of nights from which each result is deduced, 
we find that the mean of the lowest readings of the temperature of the air during 
161 cloudy nights was 46‘2 
89 broken cloudy nights was 46’9 
1 03 half-cloudy nights was 46'0 
119 principally clear nights was 43-9 
144 cloudless nights was 42'4 
