RADIANT HEAT, AND ITS CONVERSION THEREBY INTO SOUND. 
347 
reason be housed in the manner supposed by Wells. * The true explanation I hold to 
be that already indicated—the checking of radiation by vapour, the abundance of which 
was indicated by the copious deposition of dew. 
If the experiments of Wilson could be made in an atmosphere still colder than that 
in which he worked,—on a large plain, for instance, and in a country remarkable for 
the dryness of its air— Wells considered that a difference of at least 30° would be 
observed on serene nights between the air and a downy substance placed on the earth. 
And as Six had found the air temperature at an elevation of 220 feet to be 10° higher 
than at 7 feet, these 10° being added to the 30° would make the surface at least 40° 
colder than the air at the height of 200 or 300 feet. With all this I agree. I would 
go even further, and reiterate here a statement made by me nineteen years ago, that 
the withdrawal of the aqueous vapour of our atmosphere, for a single calm night, would 
kill every plant in England capable of being killed by a freezing temperature. 
Pictet, I believe, was the first to notice that the temperature of the air near the 
earth’s surface on serene nights diminished as the surface was approached, the 
sequence of the day temperatures being thus inverted. To account for the chilling of 
the air say at 10 feet above the earth’s surface beyond that at 100 feet above the 
surface, Wells invoked the radiant power of the air itself. It is chilled, he thought, 
by its own emission against the cold earth underneath. Wells takes great pains to 
prove that the air possesses this power: and if not the air, the floating matter of the 
air will, he contends, exert the necessary radiation. Difficulties of this nature not 
unfrequently crop up in works on meteorology, but they disappear in presence of the 
fact, that mixed with the air is a gaseous constituent, small in quantity, but capable 
of producing the effects needing explanation. 
As an example of such difficulties, I have already referred to Sir John Leslie’s 
paper “ On certain impressions of cold transmitted from the Higher Atmosphere.”! 
* Wells himself observed in grass a fall of temperature of 7° in twenty minutes. This gives us some 
notion of the rapidity with which a radiant so powerful as water would dispose of its heat.—Essays, p. 157. 
At the instance of my friend Mr. Francis Galton, and with the kind sanction of the Meteorological 
Council, the following instructive observations, showing the temperatures recorded by two thermo- 
meters, the one placed on cotton-wool resting on the earth, and the other hung at a height of four 
feet in the air, w T ere recently made by Mr. Whipple at Kew:— 
Time. 
Air. 
Wool. 
4.20 P.M. . 
.... 34-8° 
. . . 33-2° 
„25 „ 
.... 32-5 
. . . 27-6 
30 „ 
.... 32-4 
. . . 25-7 
„35 „ . 
.... 32-4 . 
. . . 23-4 
„40 „ 
.... 32-2 
. . . 21-7 
„ 45 „ . 
.... 32-2 . 
. . . 207 
The rapidity of radiation is well shown by these observations, an exposure of twenty-five minutes 
sufficing to establish a difference of 1T5°. 
t Transactions Roy. Soc. of Edinburgh, vol. viii., p. 483. 
2 Y 2 
