40 



MR JOHN AITKEN ON DEW. 



It will be observed that the black surface radiated one half more than the 

 sulphur. This experiment suggests that a sprinkling of sulphur might be used 

 as a protection to delicate plants on frosty nights, but whether it would pay or 

 not experience alone can determine. 



Polished tin was also tested, a sheet of tin being placed over one box, and 

 another sheet painted black put over the other, so as to make the conditions of 

 both boxes similar. The amount radiated by the tin was small; when the 

 temperature of the black surface fell 7°, the tin only fell about 1°, more or less, 

 according to the perfection of the polish of its surface. 



For meteorological purposes the following observations made on the radiat- 

 ing power of snow will be useful. I regret that owing to the absence of snow 

 so far this winter, I have only had one opportunity of making observations on 

 this substance. In the following table will be found the readings given by the 

 thermometer boxes, one of which was left bare, and gave the radiation of black 

 paint, while over the other was put a thin layer of snow. This was done on 

 the forenoon of the 10th December, and readings were begun shortly after 

 mid-day, and taken from time to time till evening: — 



Hour. 



Air. 



Black. 



Snow. 



Difference. 



12-30 p.m. 



28° 



24° 



21° 



-3° 



1 



28° 



24°-5 



22° 



-2°-5 



2 



26°-8 



22° 



20° 



-2° 



5 



23° 



15°-5 



16° 



+ 0°-5 



5-30 „ 



21° 



15° 



15°-5 



+ 0°-5 



6-30 „ 



21° 



13° 



13°-5 



+ 0°-5 



8 



19° 



9° 



.10° 



+ 1° 



In the above table, the first column gives the hour at which the temperatures 

 were taken. In the second column are the temperatures of the air; in the 

 third are the temperatures of the black radiating surface; in the fourth are 

 the temperatures of snow surface; and in the fifth column are the differences 

 between the temperature of the snow and the black surface at the hour the 

 readings were taken. The day on which this comparison was made was fine, 

 clear, calm, and frosty, with the sun shining brightly. The radiating surfaces 

 had a clear view of the sky, but were protected from the direct rays of the 

 sun. 



It will be observed that while the sun was high the snow surface was very 

 much colder than the other; while the black surface only fell 4° below "he 

 temperature of the air, the snow fell 7°. As the day advanced, and the sun 

 sunk towards the horizon, this difference decreased to 2°*5 at 1 o'clock and to 

 T at 2 o'clock; and at 5 o'clock, by which time the sun had set, the snow 



