MR JOHN AITKEN ON DEW. 



37 



Air. 



Substance. 



Radiation. 



Substance. 



Radiation. 



35° 



No. 1, black 



28° 



No. 1, white 



28° 



35° 



„ white 



28°'5 



„ black 



28°-5 



35°-5 



j» jj 



28°-5 



» » 



28°-5 



35°-5 



Paint black 



28°-5 



>> jj 



28°-5 



36° 



No. 1, black 



28°-5 



Paint black 



28°-5 



36° 



No. 2, „ 



29° 



No. 2, white 



29° 



35° 



W 3> 



29°-2 



;) i) 



29°-3 



35° 



„ white 



29° 



„ black 



29° 



35°-5 



>> » 



28°'7 



>) >i 



28°-8 



35°-5 



a >> 



28° 



Paint „ 



28° 



35° 



No. 3, white 



27° 



No. 3, black 



27° 



35° 



„ black 



27° 



„ white 



27° 



34°-5 



Paint „ 



26°-5 



J* ?j 



26°-5 



34°-5 



jj » 



26°-5 



>> >> 



26°-5 



In the above table, the first column shows the temperature of the air at the 

 height of the radiating surfaces. In the second and fourth columns are the 

 substances whose radiating powers are compared, No. 1 being black and 

 white cotton cloths, No. 2 merino cloths, and No. 3 thick woollen cloths. 

 In the third and fifth columns are the temperatures of the radiating surfaces. 

 The following was the manner of conducting the experiments : — Take the first 

 on the table. A black cotton cloth was spread over one thermometer box, 

 and a white cotton one over the other; after a time, when the readings were 

 taken, the temperature of the air was 35°, the black cloth 28°, and the white 

 one 28°. The black cotton was now removed from its box, the white one 

 put in its place, and the black one where the white one previously was. This 

 was done to check any error from difference of exposure to wind or difference 

 in thermometer boxes. After a time the readings were taken, and found to 

 be— air 35°, white cotton 28*5, and black cotton 28 - 5. The radiation of the cloth 

 was then compared with the radiation from the black paint on the surface of 

 the radiation box. This was possible on this night, as there was no wind, and 

 radiation was fairly constant. 



In my first experiments with black and white cloths, they were found to be 

 cooled to an unequal amount ; but as the cloths used on this occasion were 

 what first came to hand, and happened to be of unequal thickness and texture, 

 and as there was wind blowing at the time, the heating effect of the passing 

 air acted unequally on the different cloths, and prevented them from being cooled 

 to the same amount ; hence the necessity of using cloths of equal texture in 

 experiments of the kind, especially when wind is blowing. 



It will be observed that these experiments do not show any difference in 

 the radiating powers of white and black cloths ; nor do they show any differ- 

 ence in the radiating powers of cotton, wool, and paint. All radiate equally well, 



