Sutron— Influence of Water-Vapour upon Nocturnal Radiation. 23 
“In the light of these data, there can no longer be any doubt 
that an increasing amount of water-vapour in the air diminishes 
the extremes of temperature.’’! 
Unfortunately these authors, with the exception of the last, do 
not give any references to the results they state; so that we are 
quite in the dark as to the class of facts and the lines of investiga- 
tion from which they are derived. It is not unlikely that Poynting 
and Thomson refer to some such observations as those of General 
Strachey mentioned above; while Davis and Very may have in 
mind more general considerations, such as, for example, that while 
the average amount of moisture in the air at Durban is double 
what it is at Greenwich, the range of temperature is considerably 
greater at the former place. At any rate, two main ideas seem to 
be afloat: one, that the fall of temperature at night is regulated 
by the quantity of moisture in the air; the other, that moisture has, 
of itself, no influence, but that nearly saturated air begins to 
exert a strong absorption, laboratory experiment and meteorological 
observations being relied upon to support both views. With regard 
to the second, Very has remarked that “the experiments which 
have been interpreted in favour of the diathermancy of water- 
vapour have been refuted long ago; and Professor Davis, since the 
publication of his book, has given evidence that he no longer 
adheres to the erroneous doctrine there enunciated. (See his 
‘ Absorption of Terrestrial Radiation by the Atmosphere,’ Science, 
N.S., vol. ii., p. 485, October 11, 1895).”? Whether this correctly 
represents Davis’s present attitude is, perhaps, open to question ; 
but it is certainly difficult to read any such renunciation in the 
article cited. In fact, there is no mention in it, from beginning to 
end, of the action of aqueous vapour upon solar, lunar, or terrestrial 
radiations. My own impression is that Davis’s argument is dead 
against Very’s contention. 
In the following Table are given the monthly normals of dew- 
point, relative humidity, and cloud at 8 p.m., of wind-velocity and 
1 « Handbook of Climatology,’’ 1903, p. 145. I have not had the good fortune to: 
read Woeikof’s paper. 
2 F. W. Very, ‘‘ Atmospheric Radiation,’’ 1900, p. 84. 
