38 



MR JOHN AITKEN ON DEW. 



and have their surfaces cooled to the same amount when exposed to the same 

 radiation. It will be noticed that the temperature of the radiating surfaces 

 varied during the experiments, and was from 6 to 8 degrees below the 

 temperature of the air. 



These experiments make no claim to any great degree of accuracy ; the 

 conditions under which they are made make it difficult to get correct results, 

 as the readings have to be taken with the aid of a lantern in the open air on 

 cold nights, and as special thermometers had not been prepared for the radia- 

 tion boxes, the thermometers used had to be partly withdrawn from the boxes 

 before reading; there may therefore be a slight inaccuracy in the temperatures 

 given. The error from this cause is not likely to be more than a quarter of a 

 degree, and if there had been any great difference in the radiating powers 

 of the surfaces, it would have shown on a scale of 6 to 8 degrees. 



The following table gives the result of a comparison made between the 

 radiating powers of grass and garden soil, on a calm evening when the air was 

 dry. One of the thermometer boxes was sprinkled over with the soil, and over 

 the other was put a layer of cut grass just sufficient to conceal all the black 

 surface, and pressed down so as to make as flat a surface as possible : — 



Temperature of air. 



Temperature of grass. 



Temperature of soil. 



34° 



25°-5 



25° 



35° 



27° 



26°-5 



35° 



27° 



26° 



35° 



27° 



26°'5 



35° 



27° 



26°5 



From the above it will be seen that the garden soil was colder than the 

 grass on this evening. When the grass was removed from the box and the soil 

 compared with the black paint on the other box, the soil was found to 

 be a little colder than the black paint, but not so much as it was colder than 

 the grass. The reason for the soil being colder than the black paint would 

 appear to be due to the evaporation taking place from its surface; the dew- 

 point at the time was very low, and the top of the soil showed signs of 

 drying. Compared with grass, this was not the reason for the difference, as 

 the grass was slightly damp. The higher temperature of the grass would rather 

 appear to be due to the nature of its surface. The passing air would communi- 

 cate more heat to its irregular surface than it would to the more even one of 

 the soil. Grass and soil were compared on other evenings on which the air was 

 not so dry, and the exposed surfaces had vapour condensed on them; on 

 these occasions the two surfaces radiated almost equally well. 



This comparison of the radiating powers of grass and soil gives no support 



