ON DEW, &c. 



of this effect. Thus, I have always found wool 

 attached to the underside of my raised board, 

 on such a night, to be a little colder than the 

 air ; and it has appeared to me a sufficient rea- 

 son for the fact, that the wool in this situation 

 was, in some degree, exposed to the influence 

 of grass, which had become considerably colder 

 than the atmosphere, by radiating its heat to 

 the sky. 



II. No direct experiments can be made to 

 ascertain the manner, in which clouds prevent, 

 or occasion to be small, the appearance of a 

 cold at night, upon the surface of the earth, 

 greater than that of the atmosphere ; but it 

 may, I think, be firmly concluded, from what 

 has been said in the preceding article, that they 

 produce this effect, almost entirely, by radiating 

 heat to the earth, in return for that which they 

 intercept in its progress from the earth towards 

 the heavens. For although, upon the sky be- 

 coming suddenly cloudy during a calm night, 

 a naked thermometer, suspended in the air, 

 commonly rises 2 or 3 degrees, little of this rise 

 is to be attributed to the heat evolved by the 

 condensation of watery vapour in the atmo- 

 sphere, as was supposed by Mr. Wilson * ; since, 

 in consequence of the ceasing of that part of 

 the cold indicated by the thermometer, which 



* Edin. Phil. Trans. I. 15;. 



