512 M. Neumayer on Aqueous Vapour 



effect that the temperature of the surrounding air does not sen- 

 sibly influence the difference of temperature between the ra- 

 diating body and the temperature of the air, I thought one might 

 be; inclined to suspect undue radiation influencing the reading 

 of the parabolic thermometer, and I took for this reason the 

 trouble of classifying the observations merely according to tem- 

 perature of the air. 



The results at which I arrive may be summed up in the fol- 

 lowing lines : — 



(a) The absolute quantity of aqueous vapour in the air is in itself 

 alone not sufficient as a criterion for the degree of radiation ; or 

 if it should be otherwise, the instrument used for the observa- 

 tions is not sufficiently delicate to show the degree of influence. 



(b) The absolute quantity of aqueous vapour, together with a 

 certain temperature, i. e. the relative humidity of air, greatly 

 influence terrestrial radiation, in such a manner that, the greater 

 the degree of relative humidity, the less radiation is noticeable 

 (see Table II.). With respect to the latter remark, I cannot 

 refrain from adding a few words. The law is even notice- 

 able, though less distinctly expressed, in the lower per cents, of 

 humidity in extreme cases of dryness — although in all these cases 

 a thin veil of cirrostratus clouds covers the sky, so delicate as 

 frequently not to be perceptible by the eye. It is further to be 

 remarked that, during hot winds, when the air is extremely dry, 

 quantities of dust are floating in the air, seriously interfering 

 with radiation, although the sky is apparently quite clear. All 

 these cases have been included in deriving the- above results; 

 and considering all, we must be surprised that, notwithstanding 

 all this, an increase of radiation of between 15 and 30 per cent 

 is perceptible.* 



It is scarcely necessary for me to dwell on the apparent con- 

 nexion between the increase of temperature of the air and the 

 amount of radiation shown by Table II., such being fully ac- 

 counted for by the relation between temperature and relative 

 humidity. Tables III. and IV. sufficiently prove that the radia- 

 tion-instrument was not influenced by the surrounding air in 

 any undue manner so as to interfere with the recognition of 

 the general law. 



I hastened to put these facts before you, in order you might be 

 able to convince yourself of the reliability of the observations 

 and the method of discussion ; and further, because I was con- 

 vinced you would find it important to receive proofs that by the 

 aid of meteorological instruments generally in use we are able to 

 recognize a function of aqueous vapour in our atmosphere which 



* I have shown that the smoke of the London air is feeble in comparison 

 with its aqueous vapour. — J. T. 



