ON DEW, &c. 281 



which it appears, those of only one evening were remarkable 

 for the greatness of their results, the weather upon the other 

 evenings not having favoured much my purpose. I took 

 advantage, therefore, of being in the country, at the distance 

 of a few miles from London, on the 2 1st of the present 

 month, the last day but one of an unusually long tract of 

 dry weather, to expose to the sky, 28 minutes before sunset, 

 weighed parcels of wool and swandown, upon a smooth, 

 unpainted, and perfectly dry fir table, 5 feet long, 3 broad, 

 and nearly 3 in height, which had been placed an hour 

 before, in the sunshine, in a large level grass-field. At this 

 time, and throughout my experiments, the air was very still, 

 and the sky very serene. The atmosphere, too, in all pro- 

 bability, contained but little moisture, in consequence of the 

 long absence of rain ; and the surface of the ground appa- 

 rently contained none. The wool, 12 minutes after sunset, 

 was found to be 14 colder than the air, the temperature of 

 the latter being measured by a naked thermometer suspended 

 4 feet above the ground, and to have acquired no weight. 

 The swandown, the quantity of which was much greater 

 than that of the wool, was at the same time 13 colder than 

 the air, and was also without any additional weight. In 2O 

 minutes more, the swandown was 144- colder than the 

 neighbouring air, and was still without any increase of its 

 weight. My experiments now ceased from a failure of 

 daylight. 



In my former experiments of this kind, the greatest cold 

 observed by me from radiation, without the appearance of 

 dew, was only 9^. 



While making the experiments on wool and swandown, 

 I attended frequently to the temperature of the grass, and 

 found it at one time 15 colder than that of the air 4 feet 

 above the ground. This difference is 1 greater, than any 

 I had ever before seen between the temperatures of the same 



