378 
NATURE 
[/eb. 15, 1883 
resting on the opposite ridge of Blackdown, virtually retarded 
his rising. 
8.20 a.m. Air 27° Wool 12° 
SasONes Se ne | AS set ra 
8.40 ,, ae my 25 des LO. 
8.45 55 te x 27 xy Il 
8.50 ,, i 2) ve ney Le 
During the last two observations, the newly-risen sun shone 
upon the air thermometer. As the day advanced, the difference 
between air and wool became gradually less. From 18° at 8.50 
a.m., it had sunk at 9.25 to 15°, at 9.50 to 13,, while at 10.25, 
the sun being unclouded at the time, the difference was 11° ; the 
air at that hour being 31° and the wool 20°. 
In the celebrated experiments of Patrick Wilson, the greatest 
difference observed between a surface of snow and the air 2 feet 
above the snow, was 16°; while the greatest difference noticed 
by Wells during his long-continued observations fell short of 
this amount. Had Wilson employed swansdown or cotton wool, 
and had he placed his thermometer 4 feet instead of 2 feet above 
the surface, his difference would probably have surpassed mine, 
for his temperatures were much lower than those observed by 
me. There is, however, considerable similarity in the conditions 
under which we operated. Snow in both cases was on the 
ground, and with him there was a light movement of the air 
from the east, while with me the motion was from the north- 
east. The great differences of temperature between earth and 
air, which both his observations and mine reveal, are due toa 
common cause, namely, the withdrawal of the check to 
terrestrial radiation which is imposed by the presence of aqueous 
vapour. 
Let us now compare these results with others obtained at a 
time of extreme atmospheric serenity, when the air was almost a 
dead calm, and the sky without a cloud. At 3.30 p.m., 
January 16, the thermometers were placed in position, and 
observed afterwards with the following results :— : 
3-40 p.m. Air 43° Wool 37° 
3:59 55 229 ” 2 00 » 35 
4 ” .* » 40 ore ” 35 
4.15 ? 29 22 ae 329 » 34 
4-39 5, IG ” 3 see » 32 
5 ” 000 » 37 G99 ” 28 
5-39 5, as Oh Sv 000 239° 
6 ” i 30 s» 32 
These observations, and especially the last of them, merit our 
attention. There was no visible impediment to terrestrial radi- 
ation. The sky was extremely clear, the moon was shining ; 
Orion, the Pleiades, Charles’s Wain, including the small com- 
panion star at the bend of the shaft, the north star, and many 
others, were clearly visible. On no previous occasion during 
these observations had I seen the firmament purer; and still, 
under these favourable conditions, the difference between air and 
wool at 6 p.m. was only 4°, or less than one-fourth of that 
observed on the morning of December Io. 
We have here, I submit, a very striking illustration of the 
action of that invisible constituent of the atmosphere, to the 
influence of which I drew attention more than twenty-two years 
ago. On December Io the wind was light from the north-east, 
with a low temperature. On January 16 it was very light from 
the south-west, with a higher temperature. The one was a dry 
air, the other was a humid air; the latter, therefore, though of 
great optical transparency, proved competent to arrest the 
invisible heat of the earth. 
The variations in the temperatures of the wool recorded in the 
last column of figures are, moreover, not without a cause. The 
advance of temperature from 28° at 5 p.m. to 32° at 6 p.m., is 
not to be accounted for by any visible change in the atmosphere, 
or by any alteration in the motion of the air. The advance was 
due to the inirusion at 6 p.m, of an invisible screen between the 
earth and firmament. . 
As the night advanced the serenity of the air became, if pos- 
sible, more perfect, and the observations were continued with 
the following results :— 
6.30 p.m, Air 36° ‘ Wool 31° 
7 53 ae »» 36 0 », 28 
7-30 ” see ” 354 Bas ” 28 
8 ” sea pH 25 an », 26 
8.30 ” one » 34 eee » 25 
9 ” see » 35 Jo} 9 «27 
we ” yd ” 28 
10.30 3), ates UNE mae ee Ite » 29 
After this last observation, my notes contain the remark, 
“* Atmosphere exquisitely clear. From zenith to horizon cloud- 
less all round.” 
Here, again, the difference of 4° between the temperature of 
the wool at 8.30 p.m., and its temperature at 10.30 p.m., is 
not to be referred to any sensible change in the condition of the 
atmosphere. 
The observations were continued on January 17, 23, 24, 25, 
and 30 ; but I will confine myself to the results obtained on the 
evening of the day last-mentioned. The thermometers were 
exposed at 6.45 p.m., and by aid of a lamp read off from time 
to time afterwards. 
7.15 p.m. Air 32° Wool 26° 
8 7 = rp eee + ne) 
O30), re et des pe 27) 
During these observations the atmosphere was very serene. 
There was no moon, but the firmament was powdered with 
stars. The serenity, however, had been preceded by heavy 
rain, which doubtless had left the atmosphere charged with 
aqueous vapour. The movement of the air was from the south- 
westand light. Here again, with an atmosphere at least as clear as 
that on December 10, the difference between air and wool did 
not amount to one-fourth of that observed on the latter 
occasion, 
The results obtained on February 3 were corroborative. 
thermometers were exposed at 6.15 p.m. 
7.15 p.m. Air 34° Wool 28° 
8.25 ” see » 34 con op 342) 
Here again, the difference between air and wool is only 4°, 
although the sky was cloudless, and the stars were bright, The 
movement of the air was from the south-west and light. 
On the forenoon of this day there had been a heavy and 
persistent rain storm. Heavy rain and high wind also occurred 
on the night following. The serene interval during which the 
The 
| observations were made lay, therefore, between the two storms. 
| Doubtless the gap was well filled with pure aqueous vapour. 
Further observations were made in considerable numbers, but 
| they need not here be dwelt upon, my object being to illustrate 
a principle rather than to add to the multitudinous records of 
meteorology. It will be sufficient to say that, with atmospheric 
conditions sensibly alike, the waste of heat from the earth varies 
from day to day; a result due to the action of a body which 
escapes the sense of virion. It is hardly necessary for me to 
repeat here my references to the observations of Leslie, Hen- 
nessey, and others, which revealed variations in the earth’s 
emission for which the observers could not account. A close 
inspection of the observations of the late Principal Forbes on the 
Faulhorn proves, I think, that the action of aqueous vapour 
came there into play, and his detection of this action, while un- 
acquainted with its cause, is in my opinion a cogent proof of the 
accuracy of his work as a meteorologist. 
Postscript.—In the Philosophical Transactions for 1882, Part I. 
p- 348, I refer to certain experiments executed by Prof. Soret of 
Geneva. My friend has recently drawn my attention to a com- 
munication made by him to the French Association for the 
Advancement of Science in 1872. It gives me great pleasure 
to cite here the conclusions at which he has arrived. 
‘The influence of humidity is shown by the whole of the 
observations ; and it may be stated generally that, other circum- 
stances being equal, the greater the tension of aqueous vapour 
the less intense is the radiation. 
‘In winter, when the air is drier, the radiation ismuch more 
intense than in summer, for the same height of the sun above the 
horizon. 2 
“On several occasions a more intense radiation has been 
observed in dry than in humid weather, although the atmo- 
sphere was incontestably purer and more transparent in the 
second case than in the first. 
‘*The maximum intensity of radiation, particularly in the 
summer, corresponds habitually to days exceptionally cold and 
dry.” 
Sack are the results of experiments, executed by a most 
excellent observer, on the radiation of the sun. ‘They apply 
word for word to terrestrial radiation. They are, moreover, in 
complete harmony with the results published by General 
Strachey in the Philosophical Magazine for 1866. Meteoro- 
logists will not, I trust, be offended with me if I say that from 
such outsiders, fresh to the work and equipped with the neces- 
