and Terrestrial Radiation. 



511 



meter. The reductions were effected by Regnault's Tables, the 

 thermometers being Kew standards. 



(3) The radiation-instrument consisted of a spirit minimum- 

 thermometer of Kew, the bulb of which was placed and carefully 

 adjusted in the focus of a parabolic reflector nicely polished 

 and silvered, 6*4 inches wide and 2'4 inches deep*. This re- 

 flector was put in a box filled with cotton and placed in a little 

 house, keeping out the rays of the sun, but in such a manner 

 that the zenith over the instrument remained perfectly free for 

 a space of some 38°. The focus of the reflector was about 1 \ 

 feet above the surrounding ground (see introduction to ( Results/ 

 page 4.) k x 



(4) The psychrometer was 

 placed close to this radiation- 

 apparatus; and in order to 

 have a control, a thermome- 

 ter was placed within the little 

 house (see annexedfigure) , the 

 same being frequently com- 

 pared with the Kew standard. 



(5) Table I. contains in 

 its first column the number 

 of single simultaneous obser- 

 vations, the second column 

 shows the mean pressure of 

 aqueous vapour, the next the mea ntemperature of the air, and the 

 last the mean difference between temperature of air and radia- 

 tion, obtained by the instruments described. The results have 

 been arranged according to the value of the aqueous tension. 



(6) Table II. shows the relation between the degree of relative 

 humidity of air and the radiation-difference. For every single 

 set of observations the relative humidity was computed. 



(7) It remains yet to be mentioned that the elaborate task of 

 classifying and reducing the 4376 single sets of observations has 

 been twice executed, both computations being quite indepen- 

 dent of each other. 



(8) With a view to show that the decrease in radiation with 

 an increase in humidity, as evident from Table II., is not due 

 to the decrease in temperature, as may be suspected when glan- 

 cing over Table II., two more Tables were constructed illus- 

 trative of the influence the temperature of the air alone exerts on 

 the radiation-instrument (Tables III. and IV.). For, although 

 fully aware of the principles generally adopted with regard to 

 the influence of the temperature of the air on radiation, and also 

 of the results of your investigations (§ XII. p. 406), to the 



* In strict conformity with the instructions of the Royal Society. 



